MAX6675

Max6675 is an Arduino library for MAX6675 chip with a K type thermocouple.

The library is based upon (stripped and adapted version of) the https://github.com/RobTillaart/MAX31855_RT library.

Currently the library is experimental, so use with care.

Hardware has finally arrived (April 2022) and I had time to do my first round of tests with an UNO @ 16 MHz. The library works and it reads temperatures well, both with HW SPI and SW SPI.

Description

The MAX6675 is a chip to convert the reading of a K-type thermocouple to a temperature. The MAX6675 only supports positive degrees Celsius.

The values are read with an precision of 0.25°C. Typical noise seen during usage are ± 0.5°C, so using a low pass filter on the temperature might be a good idea.

The working of thermocouples (TC) is based upon Seebeck effect. Different TC's have a different Seebeck Coefficient (SC) expressed in µV/°C. See http://www.analog.com/library/analogDialogue/archives/44-10/thermocouple.html

Breakout

The library is tested with a breakout board with following pins:

             +---------------------+
             |          signal out |  -->  MISO
             | -       chip select |  <--  SELECT
    TC here  |               clock |  <--  CLOCK    processor side
             | +               VCC |  ---  VCC
             |                 GND |  ---  GND
             +---------------------+

0.3.0 Breaking change

Version 0.3.0 introduced a breaking change to improve handling the SPI dependency. The user has to call SPI.begin() or equivalent before calling AD.begin(). Optionally the user can provide parameters to the SPI.begin(...)

0.2.0 Breaking change

The version 0.2.0 has breaking changes in the interface. The essence is removal of ESP32 specific code from the library. This makes it possible to support the ESP32-S3 and other processors in the future. Also it makes the library a bit simpler to maintain.

Note the order of the parameters of the software SPI constructor has changed in 0.2.0.

Hardware SPI vs software SPI

Pins

Default pin connections.

HW SPI	UNO	ESP32 VSPI	ESP32 HSPI	Notes
CLOCK	13	18	14
MISO	12	19	12
MOSI	11	23	13	not used...
SELECT	eg. 4	5	15	can be others too.

Performance

Performance read() function, timing in us.

UNO @ 16 MHz
TODO ESP32 @ 240 MHz

mode	clock	timing UNO	Notes
HW SPI	4000000	36	highest supported.
HW SPI	3500000	40
HW SPI	3000000	40
HW SPI	2500000	40
HW SPI	2000000	40-44
HW SPI	1500000	48
HW SPI	1000000	48-52
HW SPI	500000	64-68
SW SPI	bit bang	276

Note the UNO micros() has a 4 us precision, but it is clear that 4 Mb is not even twice the speed of 0.5 Mb.
Tested with MAX6675_test_HWSPI.ino

Interface

#include "MAX6675.h"

Constructor

MAX6675(uint8_t select, SPIClassRP2040 * mySPI) hardware SPI R2040
MAX6675(uint8_t select, SPIClass * mySPI) hardware SPI other
MAX6675(uint8_t select, uint8_t miso, uint8_t clock) software SPI
void begin() initialize internals

Hardware SPI

To be used only if one needs a specific speed.

void setSPIspeed(uint32_t speed) set SPI transfer rate.
uint32_t getSPIspeed() returns SPI transfer rate.
void setSWSPIdelay(uint16_t del = 0) for tuning SW SPI signal quality. Del is the time in micros added per bit. Even numbers keep the duty cycle of the clock around 50%.
uint16_t getSWSPIdelay() get set value in micros.

Reading

To make a temperature reading call read(). It returns the status of the read which is a value between 0..7 The function getStatus() returns the same status value.

Table: values returned from uint8_t read() and uint8_t getStatus()

Note: this list is a subset of MAX31855 errors.

value	Description	Action
0	OK
4	Thermocouple short to VCC	check wiring
128	No read done yet	check wiring
129	No communication	check wiring

After a uint8_t read() you can get the temperature with float getTemperature().

Repeated calls to getTemperature() will give the same value until a new read(). The latter fetches a new value from the sensor. Note that if the read() fails the value of getTemperature() can become incorrect. So it is important to check the return value of read().

Offset

The library supports a fixed offset to calibrate the thermocouple. For this the functions float getOffset() and void setOffset(float offset) are available. This offset is "added" in the getTemperature() function.

Notes

the offset can be positive or negative.
the offset used is a float, so decimals can be used. A typical usage is to call setOffset(273.15) to get ° Kelvin.
the offset can cause negative temperatures.

Delta analysis

As the tc object holds its last known temperature it is easy to determine the delta with the last known temperature, e.g. for trend analysis.

  float last = tc.getTemperature();
  int state  = tc.read();
  if (state == STATUS_OK)
  {
    float new  = tc.getTemperature();
    float delta = new - last;
    // process data
  }

Last time read

The tc object keeps track of the last time read() is called in the function uint32_t lastRead(). The time is tracked in millis(). This makes it easy to read the sensor at certain intervals.

if (millis() - tc.lastRead() >= interval)
{
  int state = tc.read();
  if (state == STATUS_OK)
  {
    float new = tc.getTemperature();
    // process read value.
  }
  else
  {
    // handle error
  }
}

GetRawData

The function uint32_t getRawData() allows you to get all the 32 bits raw data from the board, after the standard uint8_t tc.read() call.

Example code can be found in the examples folder.

  int state = thermocouple.read();              
  uint32_t value = thermocouple.getRawData();  // Read the raw Data value from the module

This allows one to compact the measurement e.g. for storage or sending over a network.

Pull Up Resistor

To have proper working of the MAX6675 board, you need to add a pull-up resistor (e.g. 4K7 - 1K depending on wire length) between the MISO pin (from constructor call) and the VCC (5 Volt). This improves the signal quality and will allow you to detect if there is proper communication with the board. Without pull-up one might get random noise that could look like real data.

Note: the MISO pin can be different from each board, please refer to your board datasheet.

If the MAX6675 board is not connected tc.read() will return STATUS_NO_COMMUNICATION.

You can verify this by tc.getRawData() which will give 16 HIGH bits or 0xFFFF).

You can use a simple code to detect connection error board:

  uint8_t status = thermocouple.read();
  if (status == STATUS_NO_COMMUNICATION)
  {
    Serial.println("NO COMMUNICATION");
  }

  uint8_t status = thermocouple.read();
  if (thermocouple.getRawData() == 0xFFFF)
  {
    Serial.println("NO COMMUNICATION");
  }

Operation

See examples

Future

Must

update and verify documentation (as it is copied from MAX31855 lib)

Should

keep interface in sync with MAX31855 if possible.

Could

create example to distinguish between MAX6675 and MAX31855
- #5

Wont

Support

If you appreciate my libraries, you can support the development and maintenance. Improve the quality of the libraries by providing issues and Pull Requests, or donate through PayPal or GitHub sponsors.

Thank you,

RobTillaart/MAX6675