230 Thermocouple-to-temperature converter (digital backend)
230 : Thermocouple-to-temperature converter (digital backend)
- Author: Aidan Medcalf
- Description: Converts digitized thermocouple voltage into temperature.
- GitHub repository
- Clock: 10000000 Hz
How it works
Converts 10-bit thermocouple ADC counts into temperature by approximating the transfer function with piecewise linear segments and interpolating.
- Interface: SPI (16-bit word)
- ADC interface: SPI (16-bit word, 10 bits used)
- Output: Temperature in "centi-celsius", predivided by 4; 16-bit over full positive range of thermocouple type
- ADC passthrough: When enabled, directly connects SPI master to ADC for configuration
- Type-J and type-K thermocouples supported
ADC range: 0 counts = 0 mV = 0 C, max counts (1023) = max mV = max C. Example: For type-K thermocouple, 1023 counts = 54.886 mV = 1372 C
Temperature output: Output is in "centi-Celsius", or hundredths of degrees C, pre-divided by 4, with a granularity of 0.4C.
T = A / 25.0
Where T is in degrees C, and A is the value read from SPI. For example, for a type-K thermocouple
at 415.06C, A = 10376 (0x2888), and T = 415.04. Note the error of 0.02C.
Configuration: There are two configuration bits. Write to cfg[1:0] by issuing a SPI transaction with the high bit set (i.e. write 0x800X).
- cfg[1]: Thermocouple type: 0 = J, 1 = K
- cfg[0]: ADC passthrough enable
How to test
Requires a J or K thermocouple analog front-end with compatible ranging. Wait 20 clocks after reset, then read 16-bit temperaute from device.
External hardware
Thermocouple AFE with compatible ranging, for chosen thermocouple type
IO
| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | SCE | ||
| 1 | SIN | ||
| 2 | SOUT | ||
| 3 | SCK | ||
| 4 | ADC_SCE | ||
| 5 | ADC_SOUT | ||
| 6 | ADC_SIN | ||
| 7 | ADC_SCK |