CN0281: Automotive SENT Interface-Based Thermocouple Temperature Sensor with

CN0281: Automotive SENT Interface-Based Thermocouple Temperature Sensor with
Circuit Note
CN-0281
Devices Connected/Referenced
Circuits from the Lab™ reference circuits are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support visit www.analog.com/CN0281.
ADuC7060/
ADuC7061
ARM7-Based Microcontroller with Dual
24-Bit Σ-Δ ADCs
ADT7311
±0.5°C Accurate ,16-Bit Digital SPI
Temperature Sensor
AD8628
Precision, Low Power, Zero Drift Op Amp
ADP7102
Low Dropout Linear Regulator
Automotive SENT Interface-Based Thermocouple Temperature Sensor with Cold Junction
Compensation Using the ADuC7060/ADuC7061 Precision Analog Microcontroller
EVALUATION AND DESIGN SUPPORT
Design and Integration Files
Schematic, Layout Files, Source Code
CIRCUIT FUNCTION AND BENEFITS
This circuit uses the ADuC7060 or the ADuC7061 precision
analog microcontroller in an accurate thermocouple temperature
monitoring application. The ADuC7060/ADuC7061 integrate
dual 24-bit sigma-delta (Σ-Δ) analog-to-digital converters (ADCs),
dual programmable current sources, a 14-bit digital-to-analog
converter (DAC), and a 1.2 V internal reference, as well as an
ARM7 core, 32 kB flash, 4 kB SRAM, and various digital peripherals
such as UART, timers, serial peripheral interface (SPI), and I2C
interfaces.
In the circuit, the ADuC7060/ADuC7061 are connected to a
thermocouple and a 100 Ω platinum resistance temperature
detector (RTD). The RTD is used for cold junction compensation.
As an extra option, the ADT7311 digital temperature sensor
can be used to measure the cold junction temperature instead of
the RTD.
In the source code, an ADC sampling rate of 4 Hz was chosen.
When the ADC input programmable gain amplifier (PGA) is
configured for a gain of 32, the noise-free code resolution of the
ADuC7060/ADuC7061 is greater than 18 bits.
The single edge nibble transmission (SENT) interface to the host
is implemented by using a timer to control a digital output pin.
This digital output pin is then level shifted externally to 5 V using
an external NPN transistor. An EMC filter is provided on the
SENT output circuit as recommended in Section 6.3.1 of the SENT
protocol (SAE J2716 Standard). The data is measured as falling
edge to falling edge, and the duration of each pulse is related to the
number of system clock ticks. The system clock rate is determined
by measuring the SYNC pulse. The SYNC pulse is transmitted
at the start of every packet. More details are provided in the
SENT Interface section.
CIRCUIT DESCRIPTION
The following features of the ADuC7060/ADuC7061 are used
in this application:
•
•
•
•
•
•
•
•
A primary, 24-bit Σ-Δ ADC with a PGA. The PGA is set
for a gain of 32 in the software for this application. The
primary ADC was switched continuously between sampling
the thermocouple and the RTD voltages.
If an RTD is used for measuring the cold junction temperature,
programmable excitation current sources force a controlled
current through the RTD. The dual current sources are
configurable in 200 µA steps from 0 µA to 2 mA. For this
example, a 200 µA setting was used to minimize the error
introduced by the RTD self-heating.
If the ADT7311 is used for measuring the cold junction
temperature, the SPI interface is used in master mode to
connect the ADT7311 slave device.
An internal 1.2 V reference for the ADC in the ADuC7060/
ADuC7061. For measuring the thermocouple voltage, the
internal voltage reference was used due to its precision.
An external voltage reference for the ADC in the ADuC7060/
ADuC7061. For measuring the RTD resistance, a ratiometric
setup was used where an external reference resistor (RREF)
was connected across the external VREF+ and VREF− pins.
The AD8628 single supply op amp was used to buffer the
high impedance reference voltage across RREF to the ADC.
The OP193 is another option instead of the AD8628.
A DAC that was used to set the thermocouple commonmode voltage to 850 mV above ground.
Rev. A
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room temperature. However, you are solely responsible for testing the circuit and determining its
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CN-0281
•
•
•
•
Both the thermocouple and the RTD generate very small signals;
therefore, a PGA is required to amplify those signals. As the
auxiliary ADC on the ADuC7060/ADuC7061 does not have a
PGA, both were connected to the primary ADC, and switching
between the two was done in software.
An ARM7TDMI® core. The powerful 16-/32-bit ARM7 core
with integrated 32 kB flash and SRAM memory runs the
user code that configures and controls the ADC, processes
the ADC conversions from the RTD, and controls the
communications over the SPI interface.
Timer1 and a digital output pin are used to generate the
SENT output signal.
An optional PESDLIN protection diode for suppression of
ESD, electrical fast transients (EFT), and surge transients
up to 23 kV.
An EMC filter on the SENT output as recommended in
Section 6.3.1 of the SAE J2716 standard (SENT Protocol).
Two external switches are used to force the part into its
flash boot mode. By holding S1 low and toggling S2, the
ADuC7060/ADuC7061 enter boot mode instead of normal
user mode. In boot mode, the internal flash can be
reprogrammed through the UART interface.
5V
PWRIN
DVDD
ADP7102
10Ω
VIN
VOUT
GND
0.1µF
The RTD was used for cold junction compensation. In this circuit,
a platinum, 100 Ω RTD, Enercorp PCS 1.1503.1 was used. It is
available in a 0805 surface-mount package. This RTD has a
temperature variation of 0.385 Ω/°C.
Note that the reference resistor, RREF, should be a precision
5.6 kΩ (±0.1%).
AVDD
AVDD
BEAD
(ADJ)
The thermocouple used in this application is a Type T (copperconstantan), which has a temperature range of −200°C to +350°C.
Its sensitivity is approximately 40 µV/°C; therefore, in bipolar
mode, the ADC, with a PGA gain of 32, can cover the entire
temperature range of the thermocouple.
0.1µF
1.6Ω
5V
PWRIN
0.1µF
0.1µF
2.5V
DVDD
10kΩ
4.7µF
AVDD
SENT
R1
DVDD
AVDD
IEXC0
100Ω
Pt RTD
ONLY REQUIRED FOR ADT7311
TEMPERATURE SENSOR OPTION
PWRIN
(ADJ)
10Ω
PESD1LIN
P0.0
ADuC7061
PWRIN
0.1µF
P0.1
AD8628
SPI
VREF+
RREF
5.6kΩ
(0.1%)
P0.2
P0.3
VREF–
2.7V
P0.0
SCLK
P0.1
DOUT
VDD
ADT7311
SPI
P0.2
DIN
P0.3
CS
THERMOCOUPLE
JUNCTION
C2
ADC1
0.1µF
VOUT
GND
0.1µF
C1
ADC0
ADP7102
VIN
2N3904
0.1µF
2.7V
10Ω
P0.4
10Ω
10Ω
ADC2
0.1µF
RESET
S2
10Ω
ADC3
0.1µF
NTRST
S1
DAC0
P1.0/SIN P1.1/SOUT GND
5V
PROGRAMMABLE
INTERFACE
Figure 1. ADuC7060/ADuC7061 as a Temperature Monitor Controller with a Thermocouple Interface (Simplified Schematic, All Connections Not Shown)
Rev. A | Page 2 of 6
10889-001
•
Circuit Note
Circuit Note
CN-0281
SENT Interface
Code Description
The SENT interface is a single pin, unidirectional (sensor to host),
time modulated signal used mainly in automotive systems for
interfacing distributed sensors to host CPUs.
Timer1 is used to control the SENT output pin, P0.4. When a
temperature result is been calculated from the ADC results for
the thermocouple and the cold junction temperature, the SENT
packet structure, SENT_PACKET, is updated, and Timer1 started.
The fields of the structure are clocked out on the P0.4 pin, as
shown in Figure 2. The first stage of the packet is the sync sequence.
The host determines the clock period from this pulse, and it
determines the subsequent nibble values from this.
The main requirements for SENT include the following:
•
•
•
It must have a 0 V to 5 V signal swing with EMC filtering.
The clock used for the SENT signal must be accurate to ±20%.
The SENT output circuit must be robust enough to tolerate
short circuit to ground and short circuit to supply voltages.
The associated source code uses the P0.4 digital pin of the
ADuC7061 as the SENT output pin. The packet format used is
the single sensor packet format as detailed in A.4 section of the
SAE J2716 standard (SENT Protocol) document. The source
code, particularly the SENT.h and Sent.c files can be modified to
support other packet formats. The overall temperature result is
returned in /°C format in Data Nibble 1 to Data Nibble 3. In
summary, the returned output packet is
•
•
•
•
•
•
•
•
First, the voltage between the two wires of the thermocouple (V1)
is measured. Then, the RTD voltage is measured and converted to
a temperature via a lookup table. This temperature is then
converted to its equivalent thermocouple voltage (V2). V1 and
V2 are then added to give the overall thermocouple voltage, and
this value is then converted to the final temperature measurement.
56 clock ticks for sync pulse
Status pulse (7 ticks to 15 ticks)
Data Nibble 1 (Bit 11 to Bit 8 of the temperature result)
Data Nibble 2 (Bit 7 to Bit 4 of the temperature result)
Data Nibble 3 (Bit 3 to Bit 0 of the temperature result)
Data Nibble 4 (Bit 7 to Bit 4 of the counter)
Data Nibble 5 (Bit 3 to Bit 0 of the counter)
Data Nibble 6 (inverse of Nibble 1)
A piecewise linearization scheme is used to calculate the final
temperature value. The respective temperatures for a fixed number
of voltages are stored in an array, and values in between are
calculated using a linear interpolation between the adjacent points.
Figure 3 gives the algorithm error using the ideal thermocouple
voltages. Figure 4 shows the error obtained when using the
ADC0 pin on the ADuC7060/ADuC7061 to measure the
52 thermocouple voltages over the full thermocouple
operating range. The overall worst-case error is <1°C.
An example packet is shown in Figure 2.
56
CLOCK TICKS
To get a temperature reading, measure the temperature of the
thermocouple and the RTD. The RTD temperature is converted
to its equivalent thermocouple voltage via a lookup table. These
two voltages are added together to give the absolute value of the
voltage at the thermocouple.
0.30
0
1
0
7
1
2
8
0
0.25
ERROR (°C)
0.20
0.15
0.10
0.05
D1 TO D6
CRC
PULSE
–0.05
–210
–140
–70
0
70
140
210
280
350
TEMPERATURE (°C)
Figure 2. Example SENT Packet Measured at the P0.4 Pin
The circuit must be constructed on a multilayer printed circuit
board (PCB) with a large area ground plane. Proper layout,
grounding, and decoupling techniques must be used to achieve
optimum performance (see Tutorial MT-031, Grounding Data
Converters and Solving the Mystery of "AGND" and "DGND,"
Tutorial MT-101, Decoupling Techniques, and the ADuC7060/
ADuC7061 evaluation board layouts).
Rev. A | Page 3 of 6
Figure 3. Error When Using Piecewise Linear Approximation Using
52 Calibration Points and Ideal Measurements
10889-003
SYNC STATUS
PULSE PULSE
10889-002
0
CN-0281
Circuit Note
For a wider temperature range, a different thermocouple can be
used, such as a Type J. To minimize the cold junction compensation
error, a thermistor can be placed in contact with the actual cold
junction instead of on the PCB.
1.0
0.9
0.8
ERROR (°C)
0.7
Instead of using the RTD and external reference resistor for
measuring the cold junction temperature, an external digital
temperature sensor can be used. For example, the ADT7311
can connect to the ADuC7060/ADuC7061 via the I2C interface.
0.6
0.5
0.4
0.3
For additional information on cold junction compensation, see
the Sensor Signal Conditioning, Analog Devices, Chapter 7,
"Temperature Sensors."
0.2
0.1
–140
–70
0
70
140
210
280
350
TEMPERATURE (°C)
CIRCUIT EVALUATION AND TEST
10889-004
0
–210
SENT Interface Tests
Figure 4. Error When Using Piecewise Linear Approximation Using
52 Calibration Points Measured by the ADC0 Pin of the
ADuC7060/ADuC7061
The RTD temperature is calculated using lookup tables and is
implemented for the RTD the same way as for the thermocouple.
Note that the RTD has a different polynomial describing its
temperatures as a function of resistance.
The SENT interface output was evaluated using a digital oscilloscope and a logic analyzer. The SENT interface was only tested
with a SENT clock period of 100 µs, which is the maximum
allowed by the SENT standard. The limiting factors in this
implementation include the following:
•
The source code in the design support package was generated using
KEIL µVision V3.90.
For details on linearization and maximizing the performance of
the RTD, refer to Application Note AN-0970, RTD Interfacing
and Linearization Using an ADuC706x Microcontroller.
COMMON VARIATIONS
Instead of using an external RTD, the ADT7311 temperature
sensor can be used to measure the cold junction temperature.
•
Source code using the ADT7311 is provided with the software
package in the design support package:
www.analog.com/CN0281-DesignSupport.
Variation in FIQ interrupt latency for the timer. The interrupt
latency of an ARM7 can be up to 45 CPU clocks. With a
10.24 MHz CPU clock, this is up to 4.4 µs. The minimum
is 5× CPU clocks (0.5 µs). The worst-case latency occurs
when the ARM7 core is executing a LDMIA or STMIA (load
or store multiple values to/from memory). Select the Split
Load and Store Multiple compiler option to minimize this,
and see Figure 5 to see how to do this.
The 10.24 MHz oscillator on the ADuC7060/ADuC7061
has a worst-case accuracy of ±3% over the −40°C to +125°C
operating temperature range.
An extra external regulator is required to power the ADT7311.
When using the external reference resistor as the reference source
for the RTD measurement, it is recommended to buffer the input
to the VREF+ pin using an op amp in unity-gain mode. This is to
ensure that the input leakage current to the VREF+ pin does
not distort the measurement accuracy. In Figure 9, the OP193
was used for this purpose. Figure 1 shows the AD8628 which is
characterized to work in the −40°C to +150°C operating range.
10889-005
If more GPIO pins are required on the microcontroller, the
ADuC7060, which comes in a 48-lead LFCSP or a 48-lead LQFP,
is another option. Note that the ADuC7060/ADuC7061 can be
programmed or debugged via a standard JTAG interface.
Figure 5. Selecting Split Load and Store Multiple Option
Despite these limitations, this implementation is well within the
required ±20% timing accuracy of the SENT specification when
a SENT clock period of 100 µs is selected.
Rev. A | Page 4 of 6
Circuit Note
CN-0281
An EMC filter has been catered for on the SENT output in Figure 1.
Chose values for this filter (R1, C1, C2) to meet the targeted
transmitter output waveform, as shown in Figure 6, to reduce
EMC emissions due to the SENT output.
Only limited testing of this filter has been done; therefore, the
resistor and capacitor values for the output filter were omitted,
as is shown in Figure 1 (R1, C1, and C2).
4.5
To evaluate the accuracy of the lookup algorithm, 551 voltage
readings, equivalent to temperatures in the range of −200°C to
+350°C spaced at +1°C, were passed onto the temperature
calculation functions. Errors were calculated for the linear method
and the piecewise linear approximation method as can be seen
in Figure 3 and Figure 4.
4.0
3.5
AMPLITUDE (V)
A Wavetek 4808 multifunction calibrator was used to fully
evaluate the error, as shown in Figure 3 and Figure 4. In this
mode, the thermocouple was replaced with the calibrator as the
voltage source, as shown in Figure 7. To evaluate the entire range
of a Type T thermocouple, the calibrator was used to set the
equivalent thermocouple voltage at 52 points between −200°C
to +350°C for the negative and positive ranges of the Type T
thermocouple (see the ITS-90 Table for Type T thermocouple).
3.0
2.5
2.0
RTD Measurement Test
1.5
To evaluate the RTD circuit and linearization source code, the
RTD on the board was replaced with an accurate, adjustable
resistance source. The instrument used was the 1433-Z decade
resistor. RTD values from 90 Ω to 140 Ω, which represent an
RTD temperature range of −25°C to +114°C, was the range of
the RTD values tested.
1.0
0
0
5
10
15
20
25
TIME (µs)
30
35
40
10889-006
0.5
Figure 6. Example SENT Shaped Waveform Transmitter Output
The error results for the RTD tests are shown in Figure 8, and
the setup circuit is shown in Figure 9.
To further test and evaluate the circuit, the thermocouple
measurements and the RTD measurements were evaluated
separately.
0
–0.01
Thermocouple Measurement Test
–0.02
The basic test setup is shown in Figure 7. Two methods were
used to evaluate the performance of the circuit. Initially, the
circuit was tested with the thermocouple attached to the board
and used to measure the temperature of an ice bucket, and then
the temperature of boiling water.
ERROR (°C)
–0.03
–0.04
–0.05
–0.06
–0.07
EVAL-ADuC7061MKZ
–0.08
THERMOCOUPLE
JUNCTION
J2-9
–0.09
–0.10
–25
SEE TEXT
J2-5
–5
15
35
55
75
TEMPERATURE (°C)
95
115
10889-009
J2-8
Figure 8. Error in °C of the RTD Measurement Using Piecewise Linearization
Code and ADC0 Measurements
USB
CABLE
WAVTEK 4808
MULTIFUNCTION
CALIBRATOR
10889-007
PC
Figure 7. Test Setup Used to Calibrate and Test the Circuit Over Full
Thermocouple Output Voltage Range
Rev. A | Page 5 of 6
CN-0281
Circuit Note
AVDD
DVDD
USB
HEADER
5V
D–
D+
GND
SHIELD
BEAD
ADP3333-2.5
IN
BEAD
OUT
4.7µF
0.1µF
FT232QN
BEAD
10Ω
AVDD
DVDD
4.7µF
0.1µF
0.1µF
AVDD
RxD
TxD
DVDD
IEXC0
1433-Z DECADE
RESISTOR
10Ω
ADuC7061
ADC0
0.01µF
10Ω
RREF
5.6kΩ
(0.1%)
+
–
ADC1
0.01µF
AVDD
OP193
VREF+
P1.0/SIN
P1.1/SOUT
RxD
TxD
10889-008
VREF–
Figure 9. Test Setup Used to Calibrate and Test the RTD Output Voltage Range from −25°C to +114°C
LEARN MORE
Data Sheets and Evaluation Boards
CN0281 Design Support Package:
http://www.analog.com/CN0281-DesignSupport.
ADuC7060/ADuC7061 Data Sheet
ADuC7060/ADuC7061 Evaluation Kit
ADIsimPower Design Tool.
Kester, Walt. 1999. Sensor Signal Conditioning. Analog Devices.
Chapter 7, "Temperature Sensors."
ADM3202 Data Sheet
ADT7311 Data Sheet
Kester, Walt. 1999. Sensor Signal Conditioning. Analog Devices.
Chapter 8, "ADCs for Signal Conditioning."
AD8628 Data Sheet
Looney, Mike. RTD Interfacing and Linearization Using an
ADuC706x Microcontroller. AN-0970 Application Note.
Analog Devices.
REVISION HISTORY
MT-022 Tutorial, ADC Architectures III: Sigma-Delta ADC
Basics. Analog Devices.
MT-023 Tutorial, ADC Architectures IV: Sigma-Delta ADC
Advanced Concepts and Applications. Analog Devices.
ADP7102 Data Sheet
7/13—Rev. 0 to Rev. A
Change to Figure 1 ............................................................................2
Changes to Common Variations Section .......................................4
7/12—Revision 0: Initial Version
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of "AGND" and "DGND." Analog Devices.
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
ITS-90 Table for Type T Thermocouple.
SAE J2716 Standard (SENT Protocol).
(Continued from first page) Circuits from the Lab circuits are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you
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CN10889-0-7/13(A)
Rev. A | Page 6 of 6
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