MAX6675 "K-Thermocouple-to-Digital Converter"
Posted: Mon Sep 10, 2012 2:23 pm
Hi,
I have requested by some users to post SPI communication using MAX6675 "K-Thermocouple-to-Digital Converter" to read temperature data. This post uses the linked pdf for device information..
http://www.sparkfun.com/datasheets/IC/MAX6675.pdf
The MAX6675 performs cold-junction compensation and digitizes the signal from a type-K thermocouple. The data is output in a 12-bit resolution, SPI™-compatible, read-only format.
Temperature Conversion:
The MAX6675 includes signal-conditioning hardware to convert the thermocouple’s signal into a voltage compatible with the input channels of the ADC. The T+ and Tinputs connect to internal circuitry that reduces the introduction of noise errors from the thermocouple wires.
Before converting the thermoelectric voltages into equivalent temperature values, it is necessary to compensate for the difference between the thermocouple cold-junction side (MAX6675 ambient temperature) and a 0°C virtual reference. For a type-K thermocouple, the voltage changes by 41µV/°C, which approximates the thermocouple characteristic with the following linear
equation:
Vout = (41µV / °C) ✕ (Tr - Tamb)
Where:
Vout is the thermocouple output voltage (µV).
Tr is the temperature of the remote thermocouple junction (°C).
Tamb is the ambient temperature (°C)
Interface Protocol: Output Format: Data read:
The MAX6675 processes the reading from the thermocouple and transmits the data through a serial interface. Force CS low and apply a clock signal at SCK to read the results at SO. Forcing CS low immediately stops any conversion process. Initiate a new conversion
process by forcing CS high. Force CS low to output the first bit on the SO pin. A complete serial interface read requires 16 clock cycles.
Read the 16 output bits on the falling edge of the clock. The first bit, D15, is a dummy sign bit and is always zero. Bits D14–D3 contain the converted temperature in the order of MSB to LSB. Bit D2 is normally low and goes high when the thermocouple input is open. D1 is low to provide a device ID for the MAX6675 and bit D0 is three-state.
Here I have attached the program considering 16-bit SPI read and output format. But to have accurate temperature conversion user need to calibrate with respective thermocouple.
I have requested by some users to post SPI communication using MAX6675 "K-Thermocouple-to-Digital Converter" to read temperature data. This post uses the linked pdf for device information..
http://www.sparkfun.com/datasheets/IC/MAX6675.pdf
The MAX6675 performs cold-junction compensation and digitizes the signal from a type-K thermocouple. The data is output in a 12-bit resolution, SPI™-compatible, read-only format.
Temperature Conversion:
The MAX6675 includes signal-conditioning hardware to convert the thermocouple’s signal into a voltage compatible with the input channels of the ADC. The T+ and Tinputs connect to internal circuitry that reduces the introduction of noise errors from the thermocouple wires.
Before converting the thermoelectric voltages into equivalent temperature values, it is necessary to compensate for the difference between the thermocouple cold-junction side (MAX6675 ambient temperature) and a 0°C virtual reference. For a type-K thermocouple, the voltage changes by 41µV/°C, which approximates the thermocouple characteristic with the following linear
equation:
Vout = (41µV / °C) ✕ (Tr - Tamb)
Where:
Vout is the thermocouple output voltage (µV).
Tr is the temperature of the remote thermocouple junction (°C).
Tamb is the ambient temperature (°C)
Interface Protocol: Output Format: Data read:
The MAX6675 processes the reading from the thermocouple and transmits the data through a serial interface. Force CS low and apply a clock signal at SCK to read the results at SO. Forcing CS low immediately stops any conversion process. Initiate a new conversion
process by forcing CS high. Force CS low to output the first bit on the SO pin. A complete serial interface read requires 16 clock cycles.
Read the 16 output bits on the falling edge of the clock. The first bit, D15, is a dummy sign bit and is always zero. Bits D14–D3 contain the converted temperature in the order of MSB to LSB. Bit D2 is normally low and goes high when the thermocouple input is open. D1 is low to provide a device ID for the MAX6675 and bit D0 is three-state.
Here I have attached the program considering 16-bit SPI read and output format. But to have accurate temperature conversion user need to calibrate with respective thermocouple.
