GY-63_MS5611/libraries/MAX6675

MAX6675

Arduino library for MAX6675 chip for K type thermocouple.

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

Experimental as not tested with hardware yet.

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 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

Library tested with breakout board -

  TODO

Hardware SPI vs software SPI

Default pin connections. ESP32 can overrule with setGPIOpins().

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

Performance read() function, timing in us. (TODO ESP32 @240MHz)

mode	clock	timing UNO	timing ESP32	Notes
HW SPI	4000000	~32		highest supported.
HW SPI	2000000	~36
HW SPI	1000000	~44
HW SPI	500000	~60
SW SPI	bit bang	~276

Interface

Constructor

• MAX6675() create object.
• void begin(const uint8_t select) set select pin => hardware SPI
• void begin(const uint8_t sclk, const uint8_t select, const uint8_t miso) set clock, select and miso pin => software SPI

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.

ESP32 specific

• void selectHSPI() must be called before begin()
• void selectVSPI() must be called before begin()
• bool usesHSPI()
• bool usesVSPI()
• void setGPIOpins(uint8_t clk, uint8_t miso, uint8_t mosi, uint8_t select) to overrule ESP32 default hardware pins.

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()

value	Description	Action
0	OK
1	Thermocouple open circuit	check wiring
2	Thermocouple short to GND	check wiring
4	Thermocouple short to VCC	check wiring
7	Generic error
128	No read done yet	check wiring
129	No communication	check wiring

There are six functions to check the individual error conditions mentioned above. These make it easier to check them.

• bool openCircuit()
• bool shortToGND()
• bool shortToVCC()
• bool genericError()
• bool noRead()
• bool noCommunication()

After a uint8_t read() you can get the temperature with float getTemperature() and float getInternal() for the internal temperature of the chip / board itself. Normally these are (almost) equal.

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.

Note the offset used is a float, so decimals can be used.

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 (5Volt). 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");
  }

or

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

Operation

See examples

Future

• update and verify documentation (as it is copied from MAX31855 lib)
• keep interface in sync with MAX31855 if possible.