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0.1.1 MAX6675
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rob tillaart
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//
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// FILE: MAX6675.cpp
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// AUTHOR: Rob Tillaart
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// VERSION: 0.1.0
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// VERSION: 0.1.1
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// PURPOSE: Arduino library for MAX6675 chip for K type thermocouple
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// DATE: 2022-01-11
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// URL: https://github.com/RobTillaart/MAX6675
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//
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// HISTORY:
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// 0.1.0 2022-01-11 initial version.
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//
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// 0.1.1 2022-04-20 updated documentation after tests with UNO.
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// added examples.
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#include "MAX6675.h"
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@ -2,7 +2,7 @@
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//
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// FILE: MAX6675.h
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// AUTHOR: Rob Tillaart
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// VERSION: 0.1.0
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// VERSION: 0.1.1
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// PURPOSE: Arduino library for MAX6675 chip for K type thermocouple
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// DATE: 2022-01-12
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// URL: https://github.com/RobTillaart/MAX6675
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@ -23,7 +23,7 @@
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#include "SPI.h"
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#define MAX6675_LIB_VERSION (F("0.1.0"))
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#define MAX6675_LIB_VERSION (F("0.1.1"))
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#define MAX6675_NO_TEMPERATURE -999
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@ -55,9 +55,9 @@ public:
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// returns state - bit field: 0 = STATUS_OK
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uint8_t read();
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float getTemperature(void) { return _temperature + _offset; };
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float getTemperature(void) { return _temperature + _offset; };
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uint8_t getStatus(void) const { return _status; };
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uint8_t getStatus(void) const { return _status; };
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// use offset to calibrate the TC.
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void setOffset(const float t) { _offset = t; };
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@ -8,50 +8,79 @@
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# MAX6675
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Arduino library for MAX6675 chip for K type thermocouple.
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Max6675 is an Arduino library for MAX6675 chip with a K type thermocouple.
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The library is based upon (stripped and adapted version of) the https://github.com/RobTillaart/MAX31855_RT library.
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Experimental as not tested with hardware yet.
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Currently the library is experimental, so use with care.
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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.
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## Description
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The MAX6675 is a chip to convert the reading of a K-type thermocouple to a temperature. The MAX6675 only supports positive degrees Celsius.
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The MAX6675 is a chip to convert the reading of a K-type thermocouple to a temperature.
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The MAX6675 only supports positive degrees Celsius.
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The values are read with an precision of **0.25°C.**
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Typical noise seen during usage are **± 0.5°C**, so using a low pass filter on the temperature might be a good idea.
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The working of thermocouples (TC) is based upon Seebeck effect.
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Different TC's have a different Seebeck Coefficient (SC) expressed in µV/°C.
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Different TC's have a different Seebeck Coefficient (SC) expressed in µV/°C.
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See http://www.analog.com/library/analogDialogue/archives/44-10/thermocouple.html
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Library tested with breakout board -
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### Breakout
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The library is tested with a breakout board with following pins:
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```
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TODO
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+---------------------+
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| signal out | --> MISO
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| - chip select | <-- SELECT
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TC here | clock | <-- CLOCK processor side
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| + VCC | --- VCC
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| GND | --- GND
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+---------------------+
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```
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## Hardware SPI vs software SPI
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### Pins
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Default pin connections. ESP32 can overrule with **setGPIOpins()**.
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| HW SPI | UNO | ESP32 VSPI | ESP32 HSPI | Notes
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|:---------|:-----:|:-----------:|:-----------:|:----------|
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| CLOCKPIN | 13 | 18 | 14 |
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| MISO | 12 | 19 | 12 |
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| MOSI | 11 | 23 | 13 | *not used...*
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| CLOCK | 13 | 18 | 14 |
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| MISO | 12 | 19 | 12 |
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| MOSI | 11 | 23 | 13 | *not used...*
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| SELECT | eg. 4 | 5 | 15 | *can be others too.*
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Performance read() function, timing in us. (TODO ESP32 @240MHz)
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### Performance
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| mode | clock | timing UNO | timing ESP32 | Notes
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Performance read() function, timing in us.
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- UNO @ 16 MHz
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- TODO ESP32 @ 240 MHz
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| mode | clock | timing UNO | timing ESP32 | Notes
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|:-------|---------:|-----------:|-------------:|:----------|
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| HW SPI | 4000000 | ~32 | | highest supported.
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| HW SPI | 2000000 | ~36 | |
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| HW SPI | 1000000 | ~44 | |
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| HW SPI | 500000 | ~60 | |
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| SW SPI | bit bang | ~276 | |
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| HW SPI | 4000000 | 36 | | highest supported.
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| HW SPI | 3500000 | 40 | |
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| HW SPI | 3000000 | 40 | |
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| HW SPI | 2500000 | 40 | |
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| HW SPI | 2000000 | 40-44 | |
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| HW SPI | 1500000 | 48 | |
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| HW SPI | 1000000 | 48-52 | |
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| HW SPI | 500000 | 64-68 | |
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| SW SPI | bit bang | 276 | |
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Note the UNO micros() has a 4 us precision, but it is clear that
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4 Mb is not even twice the speed of 0.5 Mb.
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Tested with **MAX6675_test_HWSPI.ino**
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## Interface
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@ -61,7 +90,8 @@ Performance read() function, timing in us. (TODO ESP32 @240MHz)
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- **MAX6675()** create object.
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- **void begin(const uint8_t select)** set select pin => hardware SPI
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- **void begin(const uint8_t sclk, const uint8_t select, const uint8_t miso)** set clock, select and miso pin => software SPI
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- **void begin(const uint8_t sclk, const uint8_t select, const uint8_t miso)**
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set CLOCK, SELECT and MISO pin => software SPI
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### Hardware SPI
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@ -70,7 +100,8 @@ To be used only if one needs a specific speed.
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- **void setSPIspeed(uint32_t speed)** set SPI transfer rate.
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- **uint32_t getSPIspeed()** returns SPI transfer rate.
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- **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%.
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- **void setSWSPIdelay(uint16_t del = 0)** for tuning SW SPI signal quality.
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Del is the time in micros added per bit. Even numbers keep the duty cycle of the clock around 50%.
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- **uint16_t getSWSPIdelay()** get set value in micros.
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@ -91,29 +122,17 @@ The function **getStatus()** returns the same status value.
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Table: values returned from **uint8_t read()** and **uint8_t getStatus()**
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Note: this list is a subset of MAX31855 errors.
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| value | Description | Action |
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|:-----:|:--------------------------|:-------------|
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| 0 | OK | |
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| 1 | Thermocouple open circuit | check wiring |
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| 2 | Thermocouple short to GND | check wiring |
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| 4 | Thermocouple short to VCC | check wiring |
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| 7 | Generic error | |
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| 128 | No read done yet | check wiring |
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| 129 | No communication | check wiring |
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There are six functions to check the individual error conditions mentioned above.
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These make it easier to check them.
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- **bool openCircuit()**
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- **bool shortToGND()**
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- **bool shortToVCC()**
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- **bool genericError()**
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- **bool noRead()**
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- **bool noCommunication()**
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After a **uint8_t read()** you can get the temperature with **float getTemperature()**
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and **float getInternal()** for the internal temperature of the chip / board itself.
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Normally these are (almost) equal.
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After a **uint8_t read()** you can get the temperature with **float getTemperature()**.
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Repeated calls to **getTemperature()** will give the same value until a new **read()**.
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The latter fetches a new value from the sensor. Note that if the **read()** fails
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@ -127,7 +146,10 @@ The library supports a fixed offset to calibrate the thermocouple.
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For this the functions **float getOffset()** and **void setOffset(float offset)** are available.
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This offset is "added" in the **getTemperature()** function.
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Note the offset used is a float, so decimals can be used.
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Notes
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- the offset used is a float, so decimals can be used.
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A typical usage is to call **setOffset(273.15)** to get ° Kelvin.
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- the offset can cause negative temperatures.
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### Delta analysis
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@ -188,7 +210,7 @@ This allows one to compact the measurement e.g. for storage or sending over a ne
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To have proper working of the MAX6675 board, you need to add a pull-up resistor
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(e.g. 4K7 - 1K depending on wire length) between the MISO pin (from constructor call) and the
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VCC (5Volt). This improves the signal quality and will allow you to detect if there is
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VCC (5 Volt). This improves the signal quality and will allow you to detect if there is
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proper communication with the board. Without pull-up one might get random noise that could
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look like real data.
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@ -0,0 +1,67 @@
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//
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// FILE: MAX6675_test_HWSPI.ino
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// AUTHOR: Rob Tillaart
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// PURPOSE: demo
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// DATE: 2022-01-12
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// URL: https://github.com/RobTillaart/MAX6675
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#include "MAX6675.h"
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const int selectPin = 5;
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MAX6675 thermoCouple;
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uint32_t start, stop;
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void setup()
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{
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Serial.begin(115200);
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Serial.println(__FILE__);
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Serial.println(MAX6675_LIB_VERSION);
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Serial.println();
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thermoCouple.begin(selectPin); // HW SPI
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}
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void loop()
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{
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for (uint32_t s = 500; s < 4500; s += 500)
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{
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testPerformance(s * 1000);
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delay(500);
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}
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Serial.println();
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}
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void testPerformance(uint32_t speed)
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{
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thermoCouple.setSPIspeed(speed);
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start = micros();
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int status = thermoCouple.read();
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stop = micros();
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float temp = thermoCouple.getTemperature();
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Serial.print(millis());
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Serial.print("\tspeed: ");
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Serial.print(speed / 1000);
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Serial.print("\tstatus: ");
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Serial.print(status);
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Serial.print("\ttemp: ");
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Serial.print(temp);
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Serial.print("\tus: ");
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Serial.println(stop - start);
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delay(100);
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}
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// -- END OF FILE --
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@ -0,0 +1,64 @@
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//
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// FILE: MAX6675_test_plotter.ino
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// AUTHOR: Rob Tillaart
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// PURPOSE: demo
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// DATE: 2022-04-20
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// URL: https://github.com/RobTillaart/MAX6675
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#include "MAX6675.h"
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// note: pins are slightly different than other examples!
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const int dataPin = 7;
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const int clockPin = 6;
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const int selectPin = 5;
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MAX6675 thermoCouple;
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uint32_t start, stop;
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float temp = 0;
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void setup()
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{
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Serial.begin(115200);
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// Serial.println(__FILE__);
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// Serial.println(MAX6675_LIB_VERSION);
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// Serial.println();
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// thermoCouple.begin(clockPin, selectPin, dataPin);
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thermoCouple.begin(selectPin); // HW SPI
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thermoCouple.setSPIspeed(4000000);
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thermoCouple.setOffset(273);
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int status = thermoCouple.read();
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temp = thermoCouple.getTemperature();
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}
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void loop()
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{
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delay(100);
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start = micros();
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int status = thermoCouple.read();
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stop = micros();
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float newValue = thermoCouple.getTemperature();
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// 0.2 is low pass filter
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temp += 0.2 * (newValue - temp);
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// temp = newValue;
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Serial.print(status);
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Serial.print("\t");
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Serial.print(temp);
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// Serial.print("\t");
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// Serial.print(stop - start);
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Serial.println();
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delay(100);
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}
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// -- END OF FILE --
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@ -15,7 +15,7 @@
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"type": "git",
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"url": "https://github.com/RobTillaart/MAX6675"
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},
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"version": "0.1.0",
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"version": "0.1.1",
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"license": "MIT",
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"frameworks": "arduino",
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"platforms": "*",
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@ -1,5 +1,5 @@
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name=MAX6675
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version=0.1.0
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version=0.1.1
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author=Rob Tillaart <rob.tillaart@gmail.com>
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maintainer=Rob Tillaart <rob.tillaart@gmail.com>
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sentence=Arduino library for MAX6675 chip for K type thermocouple.
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