DIGILENT PmodTC1 Cold-Junction Thermocouple-to-Digital Converter Module Owner’s Manual

DIGILENT  PmodTC1 Cold-Junction Thermocouple-to-Digital Converter Module

PmodTC1TM Board Reference Manual

Revised April 12, 2016 This manual applies to the PmodTC1 rev.
A 1300 Henley Court Pullman, WA 99163 509.334.6306 www.digilentinc.com

Overview
The PmodTC1 is a temperature measurement module that utilizes a K-Type thermocouple wire to measure a wide range of temperatures. The module is capable of measuring temperatures ranging from -2700°C up to 18000°C.

Features

• Type 2 Specification
• Library and example code available in resource center

Functional Description
The PmodTC1 communicates with the host board via the SPI protocol. It sends temperature data measured by the thermocouple and the temperature of the cold junction to the host board. The module uses a 6-pin port to connect to the host board.

Interfacing with the Pmod
Temperature data is updated only when the chip select (CS) pin is held high. It is recommended to drive CS high after reading. A complete temperature reading of the thermocouple requires 14 clock cycles, while 32 clock cycles are required to read both the thermocouple and reference junction temperatures.

Pin Mapping of the Output Pins

Digital Temperature Data Format
The digital output received by the host board corresponds to temperature values. The following tables show the temperature values for the bit values received.

Table 1: Bit memory map of thermocouple temperature data

Table 2: Incoming data example

Note: The above information is modified from the MAX31855 datasheet.

Overview
The Digilent PmodTC1 is a cold-junction thermocouple-to-digital converter module designed for a classic K-Type thermocouple wire. With Maxim Integrated’s MAX31855, this module reports the measured temperature in 14- bits with 0.25°C resolution.

The PmodTC1.

Features include:

• K-type thermocouple-to-digital converter
• Wide temperature range of -73°C to 482°C with provided wire
• ±2°C accuracy from -200°C to 700°C
• 14-bit with 0.25°C resolution
• Cold-junction temperature compensation
• Small PCB size for flexible designs 1.0 in × 0.8 in (2.5 cm × 2.0 cm)
• 6-pin Pmod port with SPI interface
• Follows Digilent Pmod Interface Specification Type 2
• Library and example code available in resource center

Functional Description

The PmodTC1 utilizes a K-Type thermocouple wire to measure a wide range of temperatures. The wire provided with the PmodTC1 is capable of measuring temperatures ranging from -730 C to 4820 C, although the module itself is capable of measuring temperatures ranging from -2700 C up to 18000 C.
A thermocouple wire needs to be attached onto the screw terminal. The polarity of the thermocouple matters, therefore it is required to have the wires screwed down into the right orientation for accurate temperature readings. The device measures the difference in temperature between the two ends of the thermocouple, of which one is the internal temperature and serves as the reference junction. Once a temperature reading is established, the data passes onto a 14-bit ADC and then passed out through SPI.

Interfacing with the Pmod

The PmodTC1 communicates with the host board via the SPI protocol. The module sends a variety of information to the host board in 32 clock cycles, including the temperature measured by the thermocouple, the temperature of the “cold junction, as well as signals indicating if there is a fault with the thermocouple. The PmodTC1 uses a 6-pin port to connect to the host board.
To read data, the host board drives the slave select (SS) pin low and drives a clock to the slave device, the PmodTC1. The first bit (and the sign bit) of the 14-bits of data is loaded onto the master-in-slave-out (MISO) line on the falling edge of the chip select (CS) and can be read by the host board on the rising edge of the serial clock
(SCLK) line. Similarly, each consecutive bit of data, starting with the most significant bit (MSB), are placed onto the data line on each falling edge of SCLK and is read on the subsequent rising edge of SCLK. Users can then choose to stop reading information from the PmodTC1 by driving the CS line high.

Temperature data is updated only when the chip select (CS) pin is held high, therefore it is recommended to drive CS high after reading. A complete temperature reading of the thermocouple requires 14 clock cycles. 32 clock cycles are required to read both the thermocouple and reference junction temperatures. Table 1 below shows the bit memory map of the thermocouple temperature data.

| 14-Bit Thermocouple Temperature Data
---|---
Bit| D31| D30| …| D18|
Value| Sign| MSB 210

(10240C)

| …| LSB 2-2

(0.250C)

|

Table 1. Bit memory map of thermocouple temperature data.

| 12-Bit Internal Temperature Data| Res| SCV Bit| SCG Bit| OC Bit
---|---|---|---|---|---
Bit| D15| D14| D13| …| D5| D4| D3| D2| D1| D0|
__

Value

| Sign Bit| MSB 26(64°C)| __

25(32°C)

| __

…

| 2-

3(0.125°C)

| 2-

4(0.0625°C)

| __

Reserved

| 1 =

Short to

Vcc

| 1 = Short to GND| 1 = Open Circuit|

Table 2. Incoming data example.

Note 1 Table information modified from Table 2 in the MAX31855 datasheet.
Note 2 Bits D2, D1, and D0 refer to the thermocouple is shorted to VCC, shorted to GND, and the thermocouple has an open connection, respectively.

Table 3 below is the pin mapping of the output pins on the PmodTC1 that connect to a host board.

Table 3. Pin mapping of the output pins.

Figure 1. Block diagram of MAX31855 interfacing with a microcontroller over SPI.

Digital Temperature Data Format
Two tables (Tables 4 & 5 from the MAX31855 datasheet) are provided below to show the temperature that corresponds to the two’s compliment bit values received by the host board.

Table 4. 14-bit thermocouple temperature data.

Table 5. 12-bit cold-junction temperature data.

The on-board chip on the PmodTC1 requires at least 200 ms to power up before being able to conduct accurate temperature readings. Each temperature conversion takes up 100 ms to complete and outputs a voltage (to then be converted into a digital value) by the following linear equation (assuming a virtual reference point of 0°C):
VOUT = (41.276 μV/°C) × (TR – TAMB)
Where VOUT is the thermocouple output voltage in μV, TR is the temperature of the remote thermocouple junction (the cold-junction) in °C, and TAMB is the temperature of the thermocouple itself in °C.
Any external power applied to the PmodTC1 must be within 3.0V and 3.6V; It is recommended that Pmod is operated at 3.3V.

Physical Dimensions
The pins on the pin header are spaced 100 mil apart. The PCB is 1 inch long on the sides parallel to the pins on the pin header and 0.8 inches long on the sides perpendicular to the pin header.

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References

• Digilent – Start Smart, Build Brilliant.

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