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.
 

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 -73°C to 482°C, although the module itself is capable of measuring temperatures ranging from -270 0 C up to 1800°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 Pmod TC1 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 Pmod TC1 uses a 6-pin port to connect to the host board.

To read data, the system 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 temperature 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 choose to stop reading information from the Pmod TC1 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 just the thermocouple requires 14 clock cycles. 32 clock cycles are required to read both the thermocouple and reference junction temperatures.

The remaining bits of data not associated with the temperatures consist of a reserved bit that always reads low (bit 17) and a fault bit that reads high when one of the fault indicators (bits 2, 1, or 0) are reporting a fault. The faults refer to an issue detected with the thermocouple, such as a short.
 

Pinout Description Table

 

Pin	Signal	Description
1	CS	Active Low Chip Select
2	N/A	N/A
3	MISO	Serial Data Out
4	SCLK	Serial Clock
5	GND	Power Supply Ground
6	VCC	Power Supply (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.