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255 lines
8.2 KiB
Markdown
255 lines
8.2 KiB
Markdown
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[](https://github.com/marketplace/actions/arduino_ci)
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[](https://github.com/RobTillaart/MAX6675/actions/workflows/arduino-lint.yml)
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[](https://github.com/RobTillaart/MAX6675/actions/workflows/jsoncheck.yml)
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[](https://github.com/RobTillaart/MAX6675/blob/master/LICENSE)
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[](https://github.com/RobTillaart/MAX6675/releases)
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# MAX6675
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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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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.
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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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See http://www.analog.com/library/analogDialogue/archives/44-10/thermocouple.html
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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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+---------------------+
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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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| 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
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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 | 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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### Constructor
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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)**
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set CLOCK, SELECT and MISO pin => software SPI
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### Hardware SPI
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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.
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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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### ESP32 specific
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- **void selectHSPI()** must be called before **begin()**
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- **void selectVSPI()** must be called before **begin()**
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- **bool usesHSPI()**
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- **bool usesVSPI()**
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- **void setGPIOpins(uint8_t clk, uint8_t miso, uint8_t mosi, uint8_t select)** to overrule ESP32 default hardware pins.
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### Reading
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To make a temperature reading call **read()**.
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It returns the status of the read which is a value between 0..7
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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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| 4 | Thermocouple short to VCC | check wiring |
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| 128 | No read done yet | check wiring |
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| 129 | No communication | check wiring |
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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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the value of **getTemperature()** can become incorrect. So it is important to check
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the return value of **read()**.
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### Offset
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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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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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As the **tc** object holds its last known temperature it is easy to determine the delta
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with the last known temperature, e.g. for trend analysis.
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```cpp
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float last = tc.getTemperature();
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int state = tc.read();
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if (state == STATUS_OK)
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{
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float new = tc.getTemperature();
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float delta = new - last;
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// process data
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}
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```
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### Last time read
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The **tc** object keeps track of the last time **read()** is called in the function **uint32_t lastRead()**.
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The time is tracked in **millis()**. This makes it easy to read the sensor at certain intervals.
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```cpp
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if (millis() - tc.lastRead() >= interval)
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{
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int state = tc.read();
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if (state == STATUS_OK)
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{
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float new = tc.getTemperature();
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// process read value.
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}
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else
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{
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// handle error
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}
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}
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```
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### GetRawData
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The function **uint32_t getRawData()** allows you to get all the 32 bits raw data from the board,
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after the standard **uint8_t tc.read()** call.
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Example code can be found in the examples folder.
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```cpp
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int state = thermocouple.read();
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uint32_t value = thermocouple.getRawData(); // Read the raw Data value from the module
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```
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This allows one to compact the measurement e.g. for storage or sending over a network.
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## Pull Up Resistor
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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 (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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**Note:** the MISO pin can be different from each board, please refer to your board datasheet.
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If the MAX6675 board is not connected **tc.read()** will return **STATUS_NO_COMMUNICATION**.
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You can verify this by **tc.getRawData()** which will give 16 HIGH bits or 0xFFFF).
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You can use a simple code to detect connection error board:
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```cpp
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uint8_t status = thermocouple.read();
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if (status == STATUS_NO_COMMUNICATION)
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{
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Serial.println("NO COMMUNICATION");
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}
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```
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or
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```cpp
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uint8_t status = thermocouple.read();
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if (thermocouple.getRawData() == 0xFFFF)
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{
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Serial.println("NO COMMUNICATION");
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}
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```
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## Operation
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See examples
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## Future
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- update and verify documentation (as it is copied from MAX31855 lib)
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- keep interface in sync with MAX31855 if possible.
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-
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