Texas Instruments | Inter-Operation of the DS14C335 with 5V UARTs | Application notes | Texas Instruments Inter-Operation of the DS14C335 with 5V UARTs Application notes

Texas Instruments Inter-Operation of the DS14C335 with  5V UARTs Application notes
DS14C335
Inter-Operation of the DS14C335 with +5V UARTs
Literature Number: SNLA163
National Semiconductor
Application Note 876
John Goldie
Joe Vo
January 1993
This application brief describes the inter-operation between
the DS14C335 (+3.3V supply TIA/EIA-232 3 x 5 Driver/
Receiver) and a +5V UART. The DS14C335, illustrated in
Figure 1, is ideally suited for notebook and laptop computer
applications which either employ one uniform +3.3V supply
for all internal components or mixed +3.3V and +5V power
supplies. In mixed supply applications, the DS14C335 does
NOT require a +5V to +3.3V translator device between it and
the UART. This application brief describes how this is accomplished.
and the RS-232 port. The drivers provide translation from
TTL/CMOS voltage levels on the driver input pins to RS-232
compliant driver output voltage levels ( > |5V|), while the receivers accept standard RS-232 input levels and translate
them back to TTL/CMOS compatible output voltage levels.
Figure 2 illustrates a typical application where the
DS14C335 provides the interface between the +5V UART
Because this application specifies a +5V UART, care must
be taken to consider the characteristics of three pins on the
DS14C335. They are the:
DIN
Driver Input,
SD
ROUT
Shutdown,
Receiver Output
Inter-Operation of the DS14C335 with +5V UARTs
Inter-Operation of the
DS14C335 with +5V UARTs
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FIGURE 1. DS14C335 Functional Diagram
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© 1998 National Semiconductor Corporation
AN011787
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AN011787-2
FIGURE 2. Typical Mixed Supply (3V/5V) DTE Application
mum). One receiver remains active to monitor the Ring Indicator (RI) modem control line, to inform the CPU that a call is
coming in from a remote site. In the shutdown mode, the
charge pump is disabled, and the charge on C4 eventually
drops to one diode below VCC, or the input voltage, whichever is greater. If C4 has discharged to one diode below
VCC, and an input voltage is applied that is greater than VCC,
C4 will charge up to one diode below that level. However, no
DC current flows between the input pin and the +3.3V power
supply. The DIN and SD pins still present standard DC loading to the driving logic. Blocking diode D3 prevents a large
DC current from flowing between the input pins and the
+3.3V supply when the input pin is taken above the device’s
+3.3V (VCC) power supply pin. This is the classical problem
that can occur when directly interfacing a +5V device to
some +3.3V devices. A minimal amount of noise is coupled
onto the VCC (+3.3V) supply rail if the driver input pin is
switched (0V to 5V) while the DS14C335 is in the shutdown
mode. However, the magnitude is small, and power supply
bypassing capacitors effectively filter out the noise. To prevent noise from coupling onto the VCC rail to begin with, simply hold the driver inputs at a VIL (voltage input low), since
with a VIL applied both diodes (D1 and D2) will remain off.
Let us first examine the input structures of the DINand SD input pins, as these structures are very similar. The common
circuitry is illustrated in Figure 3 and is composed of two input protection diodes (D1 and D2). In addition, a third diode
(D3) exists between the VCC and V+ pins and is normally reversed biased. Diode D1 is situated between the input (DIN
or SD) pin and GND to clamp negative input voltages. Diode
D2 is situated between the input pin and the V+ pin. When
the DS14C335 is active (ON), the V+ pin is typically greater
than +9V. External charge pump capacitor C4, holds 6V of
charge, and is referenced to the VCC (+3.3V) pin. This creates a potential of greater than +9V on the V+ pin, and is
used to power the driver outputs. The input pins (DIN and
SD) present standard input current loading to the driving device (UART) since D1 and D2 remain reversed biased between −0.3V and one diode above the V+ pin potential (typically greater than +9V).
The DS14C335 supports another unique feature that allows
the CPU to disable the device to save power when RS-232
communication is not required. The DS14C335 is put into
shutdown mode, by asserting the SD pin high. This disables
the internal charge pump circuit, the drivers, and also 4 of
the 5 receivers, droping ICC to typically 1.0 µA (10 µA maxi-
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2
This unique input structure allows the driver input pins and
shutdown pin to accept any standard TTL/CMOS levels regardless of the DS14C335 mode (active or shutdown) or the
fact that the DS14C335 is powered from a +3.3V power supply. The input pins (DIN and SD) present standard loading to
the driving logic with input voltages ranging from 0V to
+5.5V, in magnitude.
The last pin of concern is the receiver output (ROUT) pin. The
ROUT pin must have the drive capability to meet standard
TTL/CMOS requirements. The ROUT VOH is specified to be
greater than 2.4V at 1 mA. This drive capability should meet
all standard TTL/CMOS requirements.
SUMMARY
The DS14C335’s unique input structure allows the driver input (DIN) and shutdown (SD) pins to present standard steady
state input loading to the driving logic. Valid input voltages
can range from −0.3V to greater than +5.5V, thereby enabling the device to be driven by a +5V UART in applications
that employ mixed power supplies. The high drive capability
of the receiver output meets the requirements of +5V logic
levels, or CMOS compliant JEDEC +3.3V levels. These features make the DS14C335 the optimal single chip solution
for RS-232 serial ports in +3.3V/+5V or pure +3.3V power
supply laptop and notebook computer applications.
AN011787-3
FIGURE 3. Input Protection Circuitry
3
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Inter-Operation of the DS14C335 with +5V UARTs
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