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Texas Instruments Live Insertion Application notes
Live Insertion
SDYA012
October 1996
1
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2
Contents
Title
Page
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 Internal Construction of Integrated Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3 Operating Conditions When Changing Modules With the Supply Voltage On . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1 GND and the signal line (SL) make contact first; the output of circuit N1 is low . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 VCC and the signal line (SL) make contact first; the output of circuit N1 is low . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 GND and the signal line (SL) make contact first; the output of circuit N1 is high . . . . . . . . . . . . . . . . . . . . . . . . .
3.4 VCC and the signal line (SL) make contact first; the output of circuit N1 is high . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5 VCC and GND make contact first . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
4
4
4
5
5
4 Simple Circuit Modifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5 Avoidance of Bus Conflicts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6 Avoidance of Disturbances on the System Bus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7 Special Bus-Interface Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8 Avoidance of Disturbances to the Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
List of Illustrations
Figure
Title
Page
1
Diodes in the Inputs and Outputs of Integrated Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2
Voltage-Monitoring Circuit in BiCMOS ICs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3
Simplified Circuit to Show Current Paths When Connecting a Module to a System . . . . . . . . . . . . . . . . . . . . . . . . . 4
4
Improved Circuit Using Bipolar or BiCMOS Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5
Curve for Calculating the Short-Circuit Currents That Occur With Bus Conflicts . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6
Monitoring Bus-Interface Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7
Connecting a Capacitor to a Line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8
Voltage Peaks When Making Contacts to Signal Lines (Without Voltage Bias) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9
Connection of a Capacitor to a Line (With Voltage Bias) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
10
Voltage Peaks When Making Contacts to Signal Lines (With Voltage Bias) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
11
Generation of the Voltage Bias and Control of the Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
12
Generation of the Voltage Bias and Control in the SN74ABTE16245 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
13
Bus-Interface Control in ETL Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
14
Bus-Interface Control With BTL Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Widebus is a trademark of Texas Instruments Incorporated.
iii
List of Illustrations (Continued)
iv
15
Equivalent Circuit of the Current Supply to a Module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
16
Transient Behavior of the Supply Voltage at Switch On, With Various Degrees of Damping . . . . . . . . . . . . . . . . . 15
17
Switch-On Delay Circuit Using a MOS Transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
18
Decentralized Power Supply With Limiting of Switch-On Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Abstract
In many current applications, there is a requirement to exchange modules in electronic systems while the supply voltage
remains on. This procedure is commonly known as live insertion. To understand this requirement, consider the case of an
electronic telephone exchange in which replacing modules for maintenance or repair must be possible at any time without
interrupting the operation of the system. To avoid damage to components, and any interruption of operation when changing
modules in this way, additional circuitry modifications are necessary.
This report describes the phenomena that occur during live insertion, then presents circuit proposals to solve the potential
problems that might otherwise arise.
1 Introduction
For many years, diverse functions and processes have been monitored and controlled successfully by electronic systems. The
advantages of electronic controls in such cases (compared with manual or mechanical controls) are lower operating costs and
the improved reliability that electronics now provide. However, faults also can arise in such electronic systems, which then
require repair. By using modular construction for the equipment, it is possible to quickly exchange a defective module and clear
the fault.
With most equipment, it is necessary to switch off the supply voltage during the exchange of the modules to avoid incorrect
operation or destruction of components. In many cases–an obvious example is the computer in an office–the exchange of
modules while the equipment is turned off is permissible.
In many electronic systems, switching off the equipment to exchange a defective module is unacceptable. Examples include
an electronic telephone exchange, the switching control center of an electric utility, or the computer that processes data in the
air-traffic control center of an airport. In all of these cases, when a fault is found in a module, exchanging this module in a
running system without disturbing or otherwise compromising the rest of the system functions must be possible.
The engineer planning and developing electronic systems for such applications must consider the operating states that can arise
and meet these requirements by choosing appropriate components and circuitry layout.
A distinction must be made at this point between two different cases. In the first, it is necessary only to ensure that a module
or a part of the installation can be exchanged during operation without switching off the system and without damaging it. In
such cases, it can be acceptable (for reasons described later) that the operation of the equipment is disturbed. An example is
the installation or exchange of a printer for an operating office computer. The primary requirement is the simple and safe
operation of the entire system; in this case, the requirements can be fulfilled by choosing appropriate components for
the interfaces.
In the second case, the additional requirement must be met that, during exchange of the modules, the components involved
should neither be damaged, nor should the operation be disturbed. To ensure this, beside choosing the appropriate components,
the development engineer also must incorporate additional circuit modifications to allow continuous and reliable operation
of the system.
This applications report provides the development engineer with suggestions for fulfilling these requirements. The report
begins with the choice of the most appropriate components for the application, and continues with a description of circuit
modifications that are necessary under the operating conditions described.
1
2 Internal Construction of Integrated Circuits
Some knowledge of the internal construction of integrated circuits is necessary to develop a system in which successful live
insertion during operation is possible. It is less important to have a detailed knowledge of the internal circuit construction of
the device than to know what information, in addition to that contained in data sheets, must be considered. In this case,
protection circuits at the inputs and outputs are particularly important (for example, those for protection against destruction
as a result of electrostatic discharge), together with the parasitic diodes in these parts of the circuit. The latter are determined
by the internal construction of the circuits and by the characteristics that result from the production process.
In general, the structure of integrated circuits can be represented as shown in Figure 1. This simplified representation shows
basic features of integrated circuits. An understanding of these features is necessary for proper understanding of the problems
discussed in this report.
D1
D3
Input
Output
Internal Circuit
D2
D4
Figure 1. Diodes in the Inputs and Outputs of Integrated Circuits
The diodes shown in Figure 1 have the following functions:
•
•
•
•
2
D1: This diode is integrated into most CMOS circuits for ESD protection. It is intended to limit positive voltages
at the input of the circuit. This diode is not included in ABT, LVT, LVC, or AHC/AHCT devices.
D2: This is a parasitic diode that is predetermined as a result of the internal construction of the semiconductor circuit.
With digital circuits, an additional (lower-resistance) diode is intentionally integrated into the chip to limit
undershoot of the input signals arising from line reflections. This diode also provides some protection from an
electrostatic discharge.
D3: This diode protects CMOS circuits against destruction as a result of electrostatic discharges. Most bipolar
devices have a parasitic diode at this point as a result of the internal construction of the semiconductor. An exception
is bipolar devices with an open-collector or 3-state output. With these devices, special modifications to the circuit
ensure that this diode is not present and that this current path always remains at a high resistance.
D4: This diode is present in all digital circuits. In most cases, it is the collector-substrate or drain-substrate diode
of the lower-output transistor. With bipolar devices, an additional diode (a Schottky diode) is often integrated into
the chip to limit undershoot arising from line reflections. With CMOS circuits, additional ESD-protection diodes
are often incorporated.
Table 1. Internal Diodes and Withstanding Voltages of Logic Circuit Inputs and Outputs
INPUT
D1
TOTEM-POLE
OUTPUT
D3
OPEN-CIRCUIT
COLLECTOR
OUTPUT
D3
5.5 V
7V
7V
SN74LS
7V
SN74S
5.5 V
VCC + 0.5 V
VCC + 0.5 V
SN74F
7V
SN74ALS
7V
SN74AS
7V
SN74BCT
3-STATE
OUTPUT
D3
POWER-UP
3-STATE
CIRCUIT
No
7V
VCC + 0.5 V
5.5 V
7V
5.5 V
No
VCC + 0.5 V
VCC + 0.5 V
7V
5.5 V
No
7V
5.5 V
No
7V
5.5 V
No
7V
VCC + 0.5 V
–
5.5 V
5.5 V
Yes
SN74ABT
7V
–
–
5.5 V
Yes
SN74LVT
7V
–
5.5 V
Yes
SN74HC
VCC + 0.5 V
VCC + 0.5 V
VCC + 0.5 V
VCC + 0.5 V
VCC + 0.5 V
–
VCC + 0.5 V
VCC + 0.5 V
No
VCC + 0.5 V
7V
VCC + 0.5 V
VCC + 0.5 V
VCC + 0.5 V
–
VCC + 0.5 V
5.5 V
No
SN7400
SN74AC
SN74LV
SN74LVC
No
No
No
SN74ALVC
4.6 V
–
–
7V
–
SN74AHCT
7V
VCC + 0.5 V
VCC + 0.5 V
VCC + 0.5 V
VCC + 0.5 V
No
SN74AHC
–
VCC + 0.5 V
No
No
When changing modules with the supply voltage switched on, the voltages that are permissible to apply to the input and output
components of the interfaces are of particular interest, as well as voltages at the interfaces with the supply voltage switched
off. There are a large number of circuit variants and diverse logic functions. Table 1 provides development engineers with an
overview of the voltages that can be applied to the terminals of various integrated circuits.
With devices that have diodes D1 and/or D3, a maximum voltage can be applied to the inputs and outputs that are 0.5 V more
positive than the instantaneous VCC. In Table 1, this is given as a maximum value of VCC + 0.5 V. At higher voltages, these
diodes become conducting. In this case, if there is no current limiting, the possibility of an overload or the destruction of the
device must be accepted. If there are no diodes at this point, a voltage can be applied to the appropriate terminal, depending
on the breakdown voltage of the transistors in the corresponding part of the circuit, independently of the instantaneous supply
voltage. That is, when VCC = 0. The permissible values of voltage are given in Table 1.
BiCMOS circuits also incorporate a power-up 3-state circuit. This circuit consists of a simple voltage-monitoring circuit (see
Figure 2) in which VCC is measured by diodes D1 and D2 and with the base-emitter voltage of transistor Q1. Below a certain
supply voltage VCC(off) (ABT/BCT: VCC(off) ≈ 2.5 V; LVT: VCC(off) ≈ 1.8 V), the output of this circuit is at a high logic state.
This signal switches all transistors in the output stage into a high-resistance blocked state (3-state), independently of signals
applied to the control inputs of the circuits.
This property of BiCMOS bus-interface devices ensures that a defined behavior can be predicted, even at very low supply
voltages where, usually, component behavior is not predictable. Because of power-up 3-state, BiCMOS devices have an
advantage over CMOS devices in live-insertion applications.
3
VCC
D1
To the Output Stage
D2
Q1
Figure 2. Voltage-Monitoring Circuit in BiCMOS ICs
3 Operating Conditions When Changing Modules With the Supply Voltage On
The previous section describes certain peculiarities of the inputs and outputs of integrated circuits. The operating conditions
that can arise when inserting a module in a large system are now examined. The basis for this discussion is the circuit shown
in Figure 3. Module 1 is assumed to be connected to the backplane wiring and supplied with voltage from source V1 via the
backplane wiring.
Backplane
Module 1
P1
D11
Cb1
VCC
Module 2
P2
D21
Signal
Line (SL)
N1
N2
D12
Cs1
V1
GND
Cs2
Cb2
D22
Figure 3. Simplified Circuit to Show Current Paths When Connecting a Module to a System
If Module 2 is inserted into a slot, the contacts of connector P2 make contact in random order as a result of the mechanical
tolerances of the guide rails and of the connector. Thus, it is necessary to analyze the various logic states that result.
3.1 GND and the signal line (SL) make contact first; the output of circuit N1 is low
When the module is inserted, a switching capacitance (Cs2 ≈ 20 pF), consisting of the capacitance of the connector P2, the
connection to the module, and the input and output capacitance of circuit N2, is charged to the instantaneous logic level
(USL ≈ 0.4 V) of the signal line. Diodes D21 and D22 remain blocking. The signal line is not undesirably influenced. After
the connection to VCC, capacitor Cb2 (consisting of the blocking capacitors and the capacitance of the entire circuit) becomes
charged, and the circuit on the module that has been inserted commences operation. The disturbances that arise during this
switching-on phase, and which result from the initially undefined operation of circuit N2 and from disturbances on VCC, are
discussed later in more detail.
3.2 VCC and the signal line (SL) make contact first; the output of circuit N1 is low
When the module is inserted, a connection between the signal line and GND is made via diode D22. The output of circuit N1
first supplies current via this route to the second module until GND also makes contact. This produces severe distortion of the
signal on the signal line. As a result of the large equalizing currents that can be expected, an overload of both the output of
circuit N1 and diode D22 is likely.
3.3 GND and the signal line (SL) make contact first; the output of circuit N1 is high
When the module is inserted, conditions arise similar to those described in the previous case. The output of circuit N1 supplies
current via diode D21, causing an overload of both the output of circuit N1 and of diode D21. Under these circumstances, a
defined signal on the signal line cannot be expected.
4
3.4 VCC and the signal line (SL) make contact first; the output of circuit N1 is high
Assuming that capacitor Cs2 is discharged and that the high level at the output of circuit N1 < VCC , diode D22 conducts and
charges capacitor Cb2 to the difference between VCC and the high level at the output of circuit N1. Under these circumstances,
the current that flows is negligible as a result of the low level of current supplied by circuit N1. Signal distortion on the signal
line is also very small, although the high level is somewhat elevated. This situation is not critical until, after a short time, the
output of circuit N1 switches to low, when the state described in section 3.2 is reached.
3.5 VCC and GND make contact first
Under these circumstances, VCC goes to Module 2 before the signal line makes contact. Assuming that the supply voltage to
Module 2 has reached an adequate level before the signal line makes contact, and that at the output of circuit N2 a defined
(high-resistance) state already has been reached, any disturbance on the signal line can be largely avoided. However, it is
possible that the output of circuit N1 is also at a high level, and the switching capacitance Cs2 also must be charged to this level.
As shown later, under these circumstances a disturbance of the signal also must be expected.
4 Simple Circuit Modifications
The circuit shown in Figure 3 is so arbitrary that, under the conditions described above, it is impossible to attain definite
operating states when inserting a module while the system is operating. The undefined order in which the VCC and GND
connections are made is unfavorable. Diodes D21 and D22 also are detrimental, because, in most cases, they open up
undesirable current paths, which can then result in faulty operation, if not destruction of components.
The behavior of the circuit can be predicted if the following two requirements are met:
•
•
The connector has one or more leading ground (GND) contacts that make contact before any other contacts
are closed.
For the interface, only integrated circuits without diodes (D11 and D21 in Figure 3) connected to the supply voltage
rail, either at the inputs or at the outputs, are used. This requirement applies to all bipolar and BiCMOS circuits with
3-state or open-collector outputs.
The five possible operating states described previously have been reduced to those described in subsections 3.1 and 3.3.
Figure 4 shows the changed circuit.
Backplane
Module 1
P1
VCC
Module 2
P2
Signal
Line (SL)
Cb1
N1
N2
D12
Cs1
V1
GND
Cs2
Cb2
D22
Leading GND Pin
Figure 4. Improved Circuit Using Bipolar or BiCMOS Circuits
When inserting a module into a running system, the ground connection GND makes contact first. This ensures that definite
potentials exist between the system and the new module. Diode D22, between the inputs and outputs of the integrated circuits
and the ground line, is now operating only in a blocking state. Equalizing currents no longer occur along this path and, to this
extent, destruction of the components is prevented.
If contact is made with the signal line before VCC, outputs of the integrated circuits first remain in a high-resistance state. Only
after the positive VCC has been connected and the supply voltage to the module has slowly risen, can the outputs of the circuits
N2 at the interfaces switch in a more-or-less undefined fashion. At the first instance (charging time of capacitor Cb2), with
supply voltages that are still low, neither a definite operation of the enable logic in these bus-interface circuits can be expected,
nor is it certain that valid signals will be provided by the controlling circuit to the module that has been inserted.
5
Using the simple precautions discussed previously can, to a large extent, ensure that a module inserted in an operating system
is not damaged. More comprehensive (and costly) precautions are not necessary.
However, there are a number of effects caused by inserting a module that can result in faulty system operation or, in the worst
case, can destroy the integrated circuits. For such possibilities, the following phenomena must be considered:
•
•
When inserting a module and before making contact to the signal line, capacitor Cb2 in Figure 4 must be charged.
The resulting currents inevitably cause distortion of the signals transmitted through the backplane wiring.
As mentioned previously, when the supply voltage to the module that has just been inserted ramps up, the outputs
of the interface circuits (N2 in Figure 4) can switch on in an uncontrolled fashion. In the simplest case, this can result
in short circuiting of the signal, which interferes with the transmission of data in progress. If these bus conflicts
continue long enough, thermal overloading of the circuits and destruction of the components can follow, involving
not only the module that has just been inserted, but also circuits N1 and N2 in Figure 4.
5 Avoidance of Bus Conflicts
Bus conflicts arise when two or more interface circuits attempt to drive the bus simultaneously, one circuit delivering a high
level and the other circuit a low level. Figure 5 shows the simplified typical output characteristics of bipolar bus-interface
circuits. The currents that result when a bus conflict occurs are about Ik = 120 mA per output (see Figure 5), determined by
the short-circuit current of the circuit that is attempting to supply the high level. The circuit that is trying to generate the low
level takes precedence in such a situation. With this circuit, the overload is within acceptable limits. With a low output voltage,
VOL < 0.5 V, the total power dissipation Pdl of a circuit with n = 8 outputs (for example, the SN74ABT240) can be calculated
as follows:
P dl
+n
+8
V OL I k
0.5 V 120 mA
+ 480 mW
(1)
VO – V
4
3
2
1
High-Output Curve
IOH
Low-Output Curve
IOL
IS
0
IOS
IO – mA
Figure 5. Curve for Calculating the Short-Circuit Currents That Occur With Bus Conflicts
Even with 16-bit Widebus circuits, reliability should not be detrimentally affected. The situation is different in the case of
the circuit that should deliver the high level. As a result of the short circuit, the voltage drop across the output is about 5 V. With
the currents assumed previously, the power dissipation increases to Pdh = 5 W (with Widebus circuits, to more than 10 W).
Assuming a thermal resistance between the chip and ambient temperature of RθJA ≈ 100 K/W, and a thermal time constant for
the silicon chip of tϑ ≈ 1 ms, in a very short time unacceptably high chip temperatures are reached, and destruction of the
integrated circuit (or at least a significant deterioration of its reliability) is likely.
Additional circuit modifications are necessary to prevent an overload of the interface circuits. The simplest way to make the
situation less critical consists of using only bus-interface circuits, which are provided with the power-up 3-state circuit,
mentioned previously, at any questionable points in the circuit. Assuming that, in the system in question, the other digital
circuits on the module that is to be exchanged have been implemented in CMOS technology (which at VCC > 2 V are known
to supply defined logic levels), bus conflicts should be avoided.
6
Unfortunately, in many cases it cannot be assumed that the logic circuits mentioned will also supply defined levels at higher
supply voltages, keeping the bus-interface circuits in an inactive state. These logic circuits usually are controlled by VLSI
circuits, e.g., by the microprocessor on this module. Last, control circuits do not necessarily supply valid output signals at very
low supply voltages and during the initialization phase. The development engineer must use additional circuits that ensure,
under all circumstances, that the circuits switch into 3-state.
A circuit proposal is shown in Figure 6. The heart of this circuit consists of the supply voltage supervision and initialization
integrated circuit TLC7705. A component of this kind in the module is usually necessary to ensure correct initialization of the
circuit (RESET) when switching on the supply voltage. Additionally, the outputs of this circuit are taken to the enable inputs
of the bus-interface circuit in order, during the initialization phase, to keep the bus-interface circuits, under every condition,
in an inactive high-resistance state. For this purpose, it is advantageous to use bus-interface devices having two enable inputs
of equal priority, such as the SN74ABT541. With this device, one of the inputs is connected to the output of the monitor circuit,
while the other input is connected to the part of the circuit that normally releases the outputs of the bus-interface circuits. If
two independent enable inputs are not available, as with the SN74ABT245, the required function must be performed with an
additional gate, for example, the SN74AC32. In some cases, the resulting increase in delay time is unacceptable. Faster bipolar
circuits, such as the gate SN74F00, can remedy the delay-time problem. With these single-stage logic circuits, it is possible
to ensure, even at very low supply voltages of 2 to 4.5 V, that a low level at an input will result in a high state at the output.
However, under these circumstances, an adequate voltage level cannot be guaranteed. For this reason, as shown in Figure 6,
a resistor should be provided at the output of this gate to pull the output up to a sufficiently high level.
TLC7705
SENSE
+5V
≥1
COMP
(7)
RESIN
S
S<Vt
Vt
Microprocessor and Control Logic
Z1
(2)
CT
Ct
CONTROL
1
CX
(3)
1
2
(1)
3
Z2
≥1
≥1
Z3
RESET
(5)
RESET
(6)
+5 V
10 kW
G1
G2
DIR
G
≥1
SN74AC32
SN74ABT541
SN74ABT245
&
DIR
G
SN74F00
SN74ABT245
Figure 6. Monitoring Bus-Interface Circuits
6 Avoidance of Disturbances on the System Bus
When inserting a new module in an operating system, the switching capacitance (Cs2 in Figure 3) of the individual signal lines
at the interface must be charged to the instantaneous voltage on the corresponding bus line. These additional currents distort
the signal that is being transmitted at that instant. Figure 7 shows the equivalent circuit that describes this situation. This
involves switching a capacitor (Cs2 = 20 pF) into the middle of a bus line. The inductance of the connector contact (Lc = 30 nH)
is in series with this capacitor. The line has an impedance (ZO = 30 Ω) with a signal propagation time (tp = 10 ns). It is terminated
at both ends with resistors (Rt = 100 Ω) such that, in the quiescent state, a high level (Vh = 3 V) results. The circuit arrangement
shown here conforms to conditions typically found with backplane wiring systems that are driven by TTL levels.
7
Vb = 3 V
Vb = 3 V
Rt = 100 W
Rt = 100 W
B
A
S
Lc = 30 nH
Cs2 = 20 pF
Figure 7. Connecting a Capacitor to a Line
Using the SPICE simulation program, the voltage waveform on the line was simulated at points A and B when closing switch S.
Figure 8 shows the result. As expected, connecting capacitor Cs2 to the line at point A generates a voltage peak, which is already
close to the threshold voltage (Vt = 1.5 V) of the receiver to which it is connected. The situation is even more critical at B (the
end of the line). As a result of intentional mismatching at this point by the terminating resistors (Rt = 100 Ω), the amplitude
of the disturbance increases such that the threshold voltage of the circuits at this point may be significantly exceeded.
Considering the short response time of modern logic circuits (the delay time of the inverter in the bus-interface circuits is only
a few hundred picoseconds), faulty operation of the circuits connected to the bus must be expected.
4V
3.5 V
3V
2.5 V
2V
1.5 V
A
B
1V
0.5 V
Switch
0V
–0.5 V
0 ns
10 ns
20 ns
30 ns
40 ns
50 ns
60 ns
Figure 8. Voltage Peaks When Making Contacts to Signal Lines (Without Voltage Bias)
The amplitude of the interference voltage peaks can be reduced considerably if capacitor Cs2 is charged to the threshold voltage
of the circuit via a high-value series resistor Rp (in the simulated circuit, Rp = 10 kΩ) before making contact. Cs2 is charged
from an auxiliary voltage source (Vp) that corresponds approximately to the threshold voltage of the receiving circuits, i.e.,
typically Vp = 1.5 V. Figure 9 shows the corresponding circuit and Figure 10 shows that the interference voltage peaks have
significantly reduced amplitude.
8
Vb = 3 V
Vb = 3 V
Rt = 100 W
Rt = 100 W
B
A
S
Ls = 30 nH
Rp = 10 kW
Vp = 1.5 V
Figure 9. Connection of a Capacitor to a Line (With Voltage Bias)
3.5 V
3V
A
2.5 V
B
2V
1.5 V
1V
Switch
0.5 V
0V
–0.5 V
0 ns
10 ns
20 ns
30 ns
40 ns
50 ns
60 ns
Figure 10. Voltage Peaks When Making Contacts to Signal Lines (With Voltage Bias)
Resistor Rp should have as high a value as possible to keep the loading of the bus during the module-insertion process as low
as possible. However, the maximum value of the resistance is determined by the leakage current Ioff of the circuit to be charged.
This leakage current flowing through resistor Rp causes a voltage drop that must be added to the value of the voltage bias.
9
In practice, in addition to the leading ground contact on the connector, a leading supply-voltage contact must be provided.
Figure 11 shows a circuit proposal for such an interface. The following parts of the circuit in the module are initially supplied
via the leading ground connection (GND) and the supply voltage connection (VCC1):
•
•
The reference voltage supply D1 for generating the voltage bias on the signal lines
Switch U3 (for example, SN74CBT3244), which switches the voltage bias to the signal lines during the switch-on
phase via series resistor Rp
Voltage-monitoring circuit U1, which keeps bus-interface circuits U2 in an inactive state during the switch-on phase
and, as previously described, controls the initialization of the module.
•
To avoid a voltage collapse on the supply line of the backplane wiring, which could arise from the sudden charging of blocking
capacitor C1, a current-limiting resistor R3 should be connected in series with the VCC1 contact on the connector. Because the
current consumption of the part of the circuit supplied via this contact is only a few milliamperes, and because it places no
excessive demands on the stability of the voltage, the current limiting can be implemented with a simple resistor. A similar
filter must also be provided in the VCC2 line. However, because this line has considerably higher currents, a significantly more
complicated arrangement is necessary. A suitable circuit is described in the next section.
VCC2
Filter
Connector
VCC1
R3
100 W
U2
SN74BCTxxx
SN74ABTxxx
Enable Signal
U1
TLC7705
EN1
VCC
Signal Line
≥1
COMP
SENSE
R2
10 kW
(7)
S
S<Vt
Vt
Rp
10 kW
Z1
RESIN
S
C1
0.1m
VCC
EN1
U3
SN74CBT3244
(2)
CT
Ct
CONTROL
1
CX
(3)
1
2
(1)
3
R1
VCC
Z2
≥1
≥1
Z3
RESET
(5)
RESET
(6)
#
R4
2.2 kW
R5
10 kW
D1
TLV431
GND
Figure 11. Generation of the Voltage Bias and Control of the Interface
Because supply voltage VCC2, which must power the bus-interface circuits and all other circuits on the module, has not been
applied at this instant, outputs of monitor circuit U1 are active, i.e., the voltage at its SENSE input is ≈ 0 V. The RESET output
holds the bus-interface circuit U2 in an inactive state via enable input EN1. Simultaneously, the RESET signal closes the switch
in circuit U3, so that the voltage bias supplied by the zener diode D1 via the decoupling resistor is applied to the signal lines.
In addition, the RESET signal and, if necessary, the noninverting RESET signal, as well, are supplied to the other parts of the
circuit on the module that need to be initialized when switching on. When the signal lines make contact as the module is inserted
further, signal distortion on the system bus remains very low (see Figure 10). After VCC2 makes contact, the other parts of the
circuit on the module are supplied with current. In the same way the potential at the SENSE input of circuit U1 rises to 5 V.
After the elapsed time determined by timing capacitor Ct, the logic circuits on this module are in a defined initial state and can
start operating. At this moment, switch U3 is again opened to avoid an additional loading of the signal lines by the resistors (Rs).
10
It is advantageous to make the signals delivered by voltage-monitoring circuit U1 independent of VCC1 and VCC2. As a result
of the contacts bouncing on the connector when inserting a module, undefined operating states can arise. For this reason, switch
S at the inverting RESIN input of circuit U1 is provided, which is coupled with the mechanical interlock of the module. When
a module is removed, unlatching this interlock automatically switches off logic circuits in the module. In this way, bus-interface
circuits U2 also are immediately switched into an inactive state. Conversely, these circuits are held in an inactive state when
inserting a new module until the interlock has been mechanically latched, and thus the correct mechanical insertion and defined
contacting of the connector is ensured.
7 Special Bus-Interface Circuits
Integrating voltage bias, switches, and resistors into bus-interface circuits allows a significant reduction in the circuit
complexity. This feature has been implemented in the enhanced transceiver logic (ETL), series SN74ABTE, and backplane
transceiver logic (BTL), series SN74FB. The ETL circuits have been specified by the VITA working group, which has been
concerned with the VME bus (ANSI/IEEE Standard 1014), while the BTL circuits are used in the FutureBus+
(IEEE Standard 896), among other applications.
VCCBIAS
1
+
VCC
+
A
1
Figure 12. Generation of the Voltage Bias and Control in the SN74ABTE16245
The ETL circuits (for example, SN74ABTE16245) have an additional supply voltage connection (VCCBIAS). This feeds the
circuit, which generates the voltage bias mentioned above and, together with the VCC connection, controls the switching on
and off of the voltage bias. Figure 12 shows the simplified circuit diagram of this part of the circuit. It does not include the
power-up 3-state circuit (see Figure 2), which also is contained in these bus-interface circuits, and which switches all outputs
into the high-impedance state (3-state) at a supply voltage below about 2.5 V.
Figure 13 shows a bus interface with ETL circuits. At the moment the module is inserted, and after the connection of the VCC1–
and GND contacts, the generation of the voltage bias in the bus interface circuit U3 becomes effective. Voltage-monitoring
circuit U1 is simultaneously switched on, which, via OR gate U2, switches the OE input of bus driver U3 to high; when the
outputs of this circuit are inactive, this is the 3-state. Thus, the circuit remains in an inactive high-impedance state until
initialization of the module has been completed. This also is the case after the VCC2 connector makes contact and this supply
voltage has increased to the point that the power-up 3-state circuit in bus-interface circuit U3 is no longer effective. It also is
advisable to control the end of the initialization phase by means of switch S, which should be coupled to the mechanical
interlock of the module.
Before removing the module from a running system, this circuit must be brought to a definite inactive state. This also is
performed with switch S, which closes when the module interlock is released. Thus, the outputs of bus-interface circuit U3
are automatically switched into the 3-state by circuits U1 and U2.
11
VCC2
Filter
Connector
VCC1
R3
100 W
U1
TLC7705
C1
0.1m
VCC
COMP
SENSE
S
S<Vt
R2
(7)
V
t
10 kW
RESIN
(2)
S
1
CT
CX
(3)
1
≥1
Ct
2
CONTROL
≥1
(1) 3
≥1
Z1
GND
Z2
Z3
Enable Signal
Direction
RESET
(5)
RESET
(6)
U2
SN74AC32
VCC
≥1
U3
SN74ABTE16245
VCC
VCCBIAS
OE
DIR
Signal Line
Figure 13. Bus-Interface Control in ETL Circuits
Additional circuit features are found in BTL circuits, which support the use of these components in the applications discussed
here. As shown in Figure 12, these bus-interface circuits contain a voltage-bias generator, which is supplied with voltage via
the BIASVCC terminal. When inserting the module, this part of the circuit must thus be supplied with voltage (VCC1 in
Figure 14). In addition, a power-up 3-state circuit also has been integrated, which holds the A and B outputs in an inactive
high-impedance state (3-state), until the voltage at the VCC– connections is < 2.5 V. The circuit design is simplified in that
several of the BTL circuits for the control of the B outputs, which are taken to the backplane wiring, are provided with two
enable inputs. One of the two inputs is monitored by the bus control, while the other input is controlled by the
voltage-monitoring circuit. By eliminating an additional OR gate (U2 in Figure 13) the component complexity is reduced. It
is more important that, because of the delay time of this gate, about 5 ns are saved.
12
VCC2
Filter
Connector
VCC1
R3
100 W
U1
TLC7705
C1
0.1m
VCC
≥1
COMP
SENSE
R2
10 kW
(7)
RESIN
S
S
S<Vt
Vt
Z1
GND
(2)
CT
Ct
CONTROL
1
CX
(3)
1
2
(1) 3
Z2
≥1
Z3
≥1
RESET
(5)
RESET
(6)
U2
SN74FB2033
VCC
BIASVCC
&
Enable Signal
Direction
OEB
OEA
Signal Line
Signal Line
Figure 14. Bus-Interface Control With BTL Circuits
8 Avoidance of Disturbances to the Supply Voltage
Disturbances on the supply voltage lines of a system can occur as a result of charging the line capacitance in the same way as
when switching the signal lines to the bus. When a module is inserted in a slot, the large capacitance of this circuit must first
be charged. This capacitance is dictated primarily by the blocking capacitor on the circuit board where values of several tens
to hundreds of µF are used. Another source of interference is the load on the module, which causes a considerable change of
current, and hence voltage, when switching on the supply voltage. This problem is easily solved with the leading
supply-voltage connector (VCC1 in Figures 13 and 14). Because the part of the circuit fed via this connector has a current
consumption of only a few milliamperes, it is easy to implement current limiting at switch on with resistor R3 in Figures 13
and 14. Minor reductions of voltage are acceptable because the CMOS circuits in question can operate over a wide range of
supply voltage.
The only question to be answered here is, what is the time constant (td = R3 × C1)?, for example, see Figure 14. This time
constant determines the rise time of VCC1, and this voltage must have reached its nominal value before the lagging connections
are made to VCC2. The time that is available for the build up of VCC1 should be estimated. For this purpose, assume that the
insertion distance of a module in a module carrier is Dm = 15 cm, that the leading contacts protrude by Dc = 1.5 mm, and that
the time during which the module is being inserted is ti = 0.1 s. Under these conditions, the leading contacts make contact about
1 ms before the subsequent ones. As shown by the applicable mechanical factors, these times are much longer than the time
constant of the previously mentioned R/C network.
13
For the filter of the connection to VCC2, a significantly more complicated arrangement is necessary. The current of several
amperes, which flows at this point, makes it impossible to achieve current limiting with a simple resistor. A solution is an
inductance in the VCC2 line. These components have a high ac impedance, but a very low dc resistance. When designing such
a filter, care must be taken that the resonant circuit formed by the L/C element is damped sufficiently to ensure that oscillation
does not occur when switching on the supply voltage (the meeting of the contacts on the connector) or when the load changes.
A simplified representation of the current supply to a module is shown in Figure 15. After switch S closes (the connector makes
contact), capacitor Cb (blocking capacitor in the module) is charged via inductance L. Resistor RL, together with diodes D1
and D2 connected in series, represents the load that is formed by the electronic circuit in the module. Semiconductors have
notoriously nonlinear characteristics, and these are also evident in the current supply path. To account for nonlinearity, the
equivalent circuit of the integrated circuits in the module is represented by a resistor and two diodes.
S
L
RL
VCC
Cb
D1
D2
Figure 15. Equivalent Circuit of the Current Supply to a Module
Optimum results (short ramp-up time, no overshoot of the supply voltage) occur when this network is dimensioned such that
critical damping is achieved. In this case, the following applies:
2
RL
+
Ǹ
L
Cb
(2)
The inductance can be calculated as follows:
L
+4
R L2
Cb
(3)
With a load resistor of RL = 1 Ω and a capacitor Cb = 50 mF, this expression applies:
L
+4
1 2W 2
50 mF
+ 200 mH
(4)
Figure 16 shows the waveform of the supply voltage with various degrees of damping. It shows the following situations:
•
•
•
u
t
+
+
+
Overshoot (Q
1) : L
4 R L 2 C b (R L
1 W, C b
50 mF, L
50 mH)
Critical damping (Q
1) : L
4 R L2 C b (R L
1 W, C b
50 mF, L
200 mH)
Overdamping (Q
1) : L
4 R L2 C b, (R L
1 W, C b
50 mF, L
800 mH)
t
+
u
+
+
+
+
+
+
+
In the above calculations, the diodes in series with resistor RL are ignored. This results in the curve (shown in Figure 16 as a
case of critical damping) still showing a slight overshoot. In practice, this simplification of the calculations is permissible
because the differential resistance of the diodes is usually small compared to resistor RL. In addition, to avoid an overshoot
of the supply voltage, inductance L is made sufficiently large to ensure that, in the worst case, overdamping occurs. In many
applications, a problem is then presented by the mechanical dimensions of inductance L, which must be designed to carry high
currents.
14
7
Oscillation
6
Supply Voltage – V
5
4
Critical Damping
3
Overdamping
2
1
0
0
0.2
0.4
0.6
0.8
1
Time – ms
Figure 16. Transient Behavior of the Supply Voltage at Switch On, With Various Degrees of Damping
Another way to limit the switch-on current when inserting a module consists of inserting a semiconductor switch, which
switches on slowly, in the supply line. Figure 17 shows such an arrangement. In this case, P-channel MOS transistor Q is used
as a switch, an R/C network (R1, Cd) in the gate circuit of this transistor, ensuring that the latter switches on slowly. Diode D
ensures that the capacitor discharges rapidly when the module is withdrawn.
VCC2
Q
Cd
0.01mF
RL
D
1N4148
GND
Cb
R1
10 kW
Figure 17. Switch-On Delay Circuit Using a MOS Transistor
When considering transistor Q, one with a low on-resistance (RDSon) should be chosen to avoid an excessive voltage drop
across this component. It may be necessary to use several transistors in parallel. Because the power dissipation is exceptionally
low when in operation (as a result of the low voltage drop), transistors in the SO package, such as the TPS1101PW, also are
suitable at this point in the circuit.
15
L1
100 mH
Q1
IRF9Z14
R1
1W
C1
47 m
+
C2
1, 0 m
D4
IRF
6CWQ06F
Q2
2N2222
D1
1N4148
C7
220 p
VCC
RT
R3 43 k
DTC
1
2
C3 0, 1 m
C4 1, 0 m
SCP
D2
1N4148
5V, 5A
R8
7k5
1%
R10
820
C9
220 m
C8
1, 2 n
C10
10 n
R9
1k87
1%
R6 2k2
U1
TL5001
R2 43 k
R7
43 W
R5 10k
Vi = 20 – 40 V
L2
27 mH
D3
1N4148
0V
Z2
Q3
2N2907
C5 470p
Comp
R4
3, 0 k
C6
27 n
Z1
–
Figure 18. Decentralized Power Supply With Limiting of Switch-On Current
In equipment where it is required to change modules with the supply voltage switched on, a decentralized power supply is often
used. An unstabilized voltage of several tens of volts is applied to all modules. Stabilization of the operating voltage is
performed by a switching regulator or voltage converter on each module. However, even in this case, limiting of the switch-on
current also should be provided to avoid unacceptable voltage drops on the common ground line (GND). The requirements
of these parts of the circuit can, however, be achieved more simply. Voltage drops across inductances are not so significant,
because these can be compensated by subsequent voltage regulators; therefore, a considerably smaller inductance can be used.
Operating the filter in a region in which overshoot of the voltage at the input to the regulator occurs is acceptable.
Figure 18 shows a proposed circuit for a decentralized current supply of this kind. The switch-on current limiting is again
performed with the help of the inductance L1 at the input. Damping of the switch-on process is implemented with resistor R1
in series with the inductor. Integrated circuit TL5001 is used as the switching regulator. More detailed information regarding
the use of this component can be found in the corresponding data sheets and application reports.
16
9 Summary
When exchanging modules in electronic equipment whose supply voltage must remain switched on, the development engineer
must make circuit modifications to avoid damage to the circuit and to prevent disturbances in the operation of the equipment.
The first requirement (avoiding damage) can be met comparatively easily. In this case, it is necessary only to ensure that, when
a module is inserted, no undefined currents flow into the integrated circuits at the interfaces. In addition, the destruction of the
interface circuits as a result of undefined states when the circuits are switched on (bus conflicts) must be prevented and can
be achieved with the following precautions:
•
•
•
Leading ground contacts (GND) on the connector
Use of interface circuits that remain at a high resistance when the supply voltage is switched off. All circuits in the
bipolar and BiCMOS logic families meet this requirement (see Table 1).
Appropriate circuit modifications must be made to prevent undefined states at the outputs of the interface circuits
(bus conflicts) when the supply voltage is switched on. Bus-interface circuits from the series ABT, BCT, and LVT
are provided with a power-up 3-state circuit, which, in many cases, provides adequate protection. It is safer to switch
the interface circuits during the start-up phase so that they are forced into an inactive state (see Figure 6), by means
of an additional voltage-monitoring circuit on the module being inserted. Experience has shown that it is advisable
to take this precaution, not only when modules must be inserted while the supply voltage is switched on, but because
bus conflicts also arise when the system is switched on under normal conditions.
If there is the additional requirement that an operating system not be disturbed when a module is removed or inserted (live
insertion), circuit design is much more complicated and the following requirements must be met:
•
•
•
•
•
•
Leading supply voltage contacts (GND and VCC) on the connector
Exclusive use of interface circuits that are provided with a power-up 3-state circuit [series ABT, BCT, and LVT (see
Table 1)]. These circuits ensure that, even with supply voltages < 2 V, the bus-interface circuits remain in an inactive
high-resistance state.
With suitable circuit modifications, undefined states of the outputs of the interface circuits (bus conflicts) must be
avoided when the supply voltage is further increased. A supply-voltage monitoring circuit on the module being
inserted is essential to maintain the interface circuits in an inactive high-impedance state during the switch-on phase.
Control both the enabling and the deactivation of the circuit on the module (including the bus-interface circuits) with
a switch that is coupled to the mechanical interlocking of the module.
Ensure by suitable circuit modifications that, before making contact, the signal lines of the module being inserted
are charged to a voltage (pre-charge) that lies approximately halfway between the high and low logic levels on the
backplane wiring. This avoids disturbances on the system bus. Bus-interface circuits from the series BTL and ETL
are provided with such integrated pre-charge circuits.
Ensure a slow rise of the supply current when a module is inserted to prevent the rapid charging (giving the effect
of a short-circuit) of the capacitors in a module when the module is inserted. This precaution ensures that no
unacceptable interference voltages (ground bounce) arise on the ground line (GND) of the backplane wiring and
a collapse of the voltage on the power-supply line of the system also is avoided.
If modules must be exchanged while the supply voltage remains on (live insertion), the engineer developing the system must
account for a number of operating states, particularly during insertion. If suitable precautions are taken and the correct
components are chosen, it is possible to not only prevent damage to parts of the circuit, but, more significantly, operation of
the system is not adversely affected by removing or inserting modules.
The precautions that isolate a module from the system logic when the module is removed and reinserted in the system have
not been covered in this report. Results of these tasks must be controlled by the operating system of the computer, a discussion
that is far beyond the scope of this application report.
Acknowledgment
Eilhard Haseloff is the author of this application report.
17
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