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1300 Henley Court Pullman, WA 99163 509.334.6306 www.digilentinc.com PmodTC1™ Board Reference Manual Revised April 12, 2016 This manual applies to the PmodTC1 rev. A 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 14bits with 0.25°C resolution. Features include:         The PmodTC1. 1  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. DOC#: 502-315 Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners. Page 1 of 3 PmodTC1™ Board Reference Manual 2 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 … LSB 2-2 (0.250C) 10 Value MSB 2 (10240C) Sign Table 1. Bit memory map of thermocouple temperature data. Bit Value 12-Bit Internal Temperature Data D15 D14 D13 … D5 D4 Sign Bit 24(0.0625°C) MSB 26(64°C) 25(32°C) … 23(0.125°C) Res D3 Reserved SCV Bit D2 1= Short to Vcc SCG Bit D1 OC Bit D0 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. Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners. Page 2 of 3 PmodTC1™ Board Reference Manual Table 3 below is the pin mapping of the output pins on the PmodTC1 that connect to a host board. 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) Table 3. Pin mapping of the output pins. 2.1 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. Temperature (°C) +1600.00°C +1000.00°C +100.75°C +25.00°C 0.00°C -0.25°C -1.00°C -250.00°C Digital Output (D31-D18) 0110 0100 0000 00 0011 1110 1000 00 0000 0110 0100 11 0000 0001 1001 00 0000 0000 0000 00 1111 1111 1111 11 1111 1111 1111 00 1111 0000 0110 00 Table 4. 14-bit thermocouple temperature data. Temperature (°C) +127.0000°C +100.5625°C +25.0000°C 0.0000°C -0.0625°C -1.0000°C -20.0000°C -55.0000°C Digital Output (D15-D4) 0111 1111 0000 0110 0100 1001 0001 1001 0000 0000 0000 0000 1111 1111 1111 1111 1111 0000 1110 1100 0000 1100 1001 0000 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. 3 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. Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners. Page 3 of 3