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/*
* ThermocoupleSensor31855.cpp
*
* Created on: 8 Jun 2017
* Author: David
*/
// MAX31855 thermocouple chip
//
// The MAX31855 continuously samples a Type K thermocouple. When the MAX31855
// is selected via its chip select (CS) pin, it unconditionally writes a 32 bit
// sequence onto the bus. This sequence is designed such that we need only
// interpret the high 16 bits in order to ascertain the temperature reading and
// whether there a fault condition exists. The low 16 bits provide the
// MAX31855's cold junction temperature (and thus the board temperature) as
// well as finer detail on any existing fault condition.
//
// The temperature read from the chip is a signed, two's-complement integer.
// As it is a 14 bit value (in units of one-quarter degrees Celsius), we
// convert it to a proper signed 16 bit value by adding two high bits. The
// high bits added should both be zero if the value is positive (highest bit
// of the 14 bit value is zero), or both be one if the value is negative
// (highest bit of the 14 bit value is one).
//
// Note Bene: there's a Arduino Due sketch floating about the internet which
// gets this wrong and for negative temperatures generates incorrect values.
// E.g, -2047C for what should be -1C; -1798C for what should be -250C. The
// values of -1C and -250C are shown as examples in Table 4 of the datasheet
// for the MAX21855.) The incorrect Arduino Due sketch appears in, and may be
// from, the book Arduino Sketches: Tools and Techniques for Programming
// Wizardry, James A. Langbridge, January 12, 2015, John Wiley & Sons.
// Bits -- Interpretation
// ----- -----------------------------------------------------------------
// 31:18 -- 14 bit, signed thermocouple temperature data. Units of 0.25 C
// 17 -- Reserved
// 16 -- Fault indicator (1 if fault detected; 0 otherwise)
// 15:04 -- 12 bit, signed cold junction temperature data. Units of 0.0625 C
// 03 -- Reserved
// 02 -- SCV fault; reads 1 if the thermocouple is shorted to Vcc
// 01 -- SCG fault; reads 1 if the thermocouple is shorted to ground
// 00 -- OC fault; reads 1 if the thermocouple is not connected (open)
// For purposes of setting bit transfer widths and timings, we need to use a
// Peripheral Channel Select (PCS). Use channel #3 as it is unlikely to be
// used by anything else as the Arduino Due leaves pin 78 unconnected.
//
#include "ThermocoupleSensor31855.h"
#include "RepRap.h"
#include "Platform.h"
#include "Core.h"
const uint32_t MAX31855_Frequency = 4000000; // maximum for MAX31855 is 5MHz
// SPI modes:
// If the inactive state of SCL is LOW (CPOL = 0) (in the case of the MAX31865, this is sampled on the falling edge of CS):
// The MAX31855 sets up the first data bit after the falling edge of CLK, and changes the data on each falling clock edge.
// So the SAM needs to sample data on the rising clock edge. This requires NCPHA = 1.
const uint8_t MAX31855_SpiMode = SPI_MODE_0;
// Define the minimum interval between readings
const uint32_t MinimumReadInterval = 100; // minimum interval between reads, in milliseconds
ThermocoupleSensor31855::ThermocoupleSensor31855(unsigned int channel)
: SpiTemperatureSensor(channel, "Thermocouple (MAX31855)", channel - FirstThermocoupleChannel, MAX31855_SpiMode, MAX31855_Frequency)
{
}
// Perform the actual hardware initialization for attaching and using this device on the SPI hardware bus.
void ThermocoupleSensor31855::Init()
{
InitSpi();
lastReadingTime = millis();
}
TemperatureError ThermocoupleSensor31855::GetTemperature(float& t)
{
if (inInterrupt() || millis() - lastReadingTime < MinimumReadInterval)
{
t = lastTemperature;
}
else
{
uint32_t rawVal;
TemperatureError sts = DoSpiTransaction(nullptr, 4, rawVal);
if (sts != TemperatureError::success)
{
lastResult = sts;
}
else
{
lastReadingTime = millis();
if ((rawVal & 0x00020008) != 0)
{
// These two bits should always read 0. Likely the entire read was 0xFF 0xFF which is not uncommon when first powering up
lastResult = TemperatureError::ioError;
}
else if ((rawVal & 0x00010007) != 0) // check the fault bits
{
// Check for three more types of bad reads as we set the response code:
// 1. A read in which the fault indicator bit (16) is set but the fault reason bits (0:2) are all clear;
// 2. A read in which the fault indicator bit (16) is clear, but one or more of the fault reason bits (0:2) are set; and,
// 3. A read in which more than one of the fault reason bits (0:1) are set.
if ((rawVal & 0x00010000) == 0)
{
// One or more fault reason bits are set but the fault indicator bit is clear
lastResult = TemperatureError::ioError;
}
else
{
// At this point we are assured that bit 16 (fault indicator) is set and that at least one of the fault reason bits (0:2) are set.
// We now need to ensure that only one fault reason bit is set.
uint8_t nbits = 0;
if (rawVal & 0x01)
{
// Open Circuit
++nbits;
lastResult = TemperatureError::openCircuit;
}
if (rawVal & 0x02)
{
// Short to ground;
++nbits;
lastResult = TemperatureError::shortToGround;
}
if (rawVal && 0x04)
{
// Short to Vcc
++nbits;
lastResult = TemperatureError::shortToVcc;
}
if (nbits != 1)
{
// Fault indicator was set but a fault reason was not set (nbits == 0) or too many fault reason bits were set (nbits > 1).
// Assume that a communication error with the MAX31855 has occurred.
lastResult = TemperatureError::ioError;
}
}
}
else
{
rawVal >>= 18; // shift the 14-bit temperature data to the bottom of the word
rawVal |= (0 - (rawVal & 0x2000)); // sign-extend the sign bit
// And convert to from units of 1/4C to 1C
t = lastTemperature = (float)(0.25 * (float)(int32_t)rawVal);
lastResult = TemperatureError::success;
}
}
}
return lastResult;
}
// End
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