max31865.py

#!/usr/bin/python
#The MIT License (MIT)
#
#Copyright (c) 2015 Stephen P. Smith
#
#Permission is hereby granted, free of charge, to any person obtaining a copy
#of this software and associated documentation files (the "Software"), to deal
#in the Software without restriction, including without limitation the rights
#to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
#copies of the Software, and to permit persons to whom the Software is
#furnished to do so, subject to the following conditions:
#
#The above copyright notice and this permission notice shall be included in all
#copies or substantial portions of the Software.
#
#THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
#IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
#FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
#AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
#LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
#OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
#SOFTWARE.

import time, math
import RPi.GPIO as GPIO
#import numpy

class max31865(object):
	"""Reading Temperature from the MAX31865 with GPIO using 
	   the Raspberry Pi.  Any pins can be used.
	   Numpy can be used to completely solve the Callendar-Van Dusen equation 
	   but it slows the temp reading down.  I commented it out in the code.  
	   Both the quadratic formula using Callendar-Van Dusen equation (ignoring the
	   3rd and 4th degree parts of the polynomial) and the straight line approx.
	   temperature is calculated with the quadratic formula one being the most accurate.
	"""
	def __init__(self, csPin = 8, misoPin = 9, mosiPin = 10, clkPin = 11):
		self.csPin = csPin
		self.misoPin = misoPin
		self.mosiPin = mosiPin
		self.clkPin = clkPin
		self.setupGPIO()
		
	def setupGPIO(self):
		GPIO.setwarnings(False)
		GPIO.setmode(GPIO.BCM)
		GPIO.setup(self.csPin, GPIO.OUT)
		GPIO.setup(self.misoPin, GPIO.IN)
		GPIO.setup(self.mosiPin, GPIO.OUT)
		GPIO.setup(self.clkPin, GPIO.OUT)

		GPIO.output(self.csPin, GPIO.HIGH)
		GPIO.output(self.clkPin, GPIO.LOW)
		GPIO.output(self.mosiPin, GPIO.LOW)

	def readTemp(self):
		#
		# b10000000 = 0x80
		# 0x8x to specify 'write register value'
		# 0xx0 to specify 'configuration register'
		#
		# 0b10110010 = 0xB2
		# Config Register
		# ---------------
		# bit 7: Vbias -> 1 (ON)
		# bit 6: Conversion Mode -> 0 (MANUAL)
		# bit5: 1-shot ->1 (ON)
		# bit4: 3-wire select -> 1 (3 wire config)
		# bits 3-2: fault detection cycle -> 0 (none)
		# bit 1: fault status clear -> 1 (clear any fault)
		# bit 0: 50/60 Hz filter select -> 0 (60Hz)
		#
		# 0b11010010 or 0xD2 for continuous auto conversion 
		# at 60Hz (faster conversion)
		#

		#one shot
		self.writeRegister(0, 0xB2)

		# conversion time is less than 100ms
		time.sleep(.1) #give it 100ms for conversion

		# read all registers
		out = self.readRegisters(0,8)

		conf_reg = out[0]
		# print("config register byte: %x" % conf_reg)

		[rtd_msb, rtd_lsb] = [out[1], out[2]]
		rtd_ADC_Code = (( rtd_msb << 8 ) | rtd_lsb ) >> 1

		temp_C = self.calcPT100Temp(rtd_ADC_Code)

		[hft_msb, hft_lsb] = [out[3], out[4]]
		hft = (( hft_msb << 8 ) | hft_lsb ) >> 1
		# print("high fault threshold: %d" % hft)

		[lft_msb, lft_lsb] = [out[5], out[6]]
		lft = (( lft_msb << 8 ) | lft_lsb ) >> 1
		# print("low fault threshold: %d" % lft)

		status = out[7]
		#
		# 10 Mohm resistor is on breakout board to help
		# detect cable faults
		# bit 7: RTD High Threshold / cable fault open 
		# bit 6: RTD Low Threshold / cable fault short
		# bit 5: REFIN- > 0.85 x VBias -> must be requested
		# bit 4: REFIN- < 0.85 x VBias (FORCE- open) -> must be requested
		# bit 3: RTDIN- < 0.85 x VBias (FORCE- open) -> must be requested
		# bit 2: Overvoltage / undervoltage fault
		# bits 1,0 don't care	
		#print "Status byte: %x" % status

		if ((status & 0x80) == 1):
			raise FaultError("High threshold limit (Cable fault/open)")
		if ((status & 0x40) == 1):
			raise FaultError("Low threshold limit (Cable fault/short)")
		if ((status & 0x04) == 1):
			raise FaultError("Overvoltage or Undervoltage Error") 

		return temp_C
		
	def writeRegister(self, regNum, dataByte):
		GPIO.output(self.csPin, GPIO.LOW)
		
		# 0x8x to specify 'write register value'
		addressByte = 0x80 | regNum;
		
		# first byte is address byte
		self.sendByte(addressByte)
		# the rest are data bytes
		self.sendByte(dataByte)

		GPIO.output(self.csPin, GPIO.HIGH)
		
	def readRegisters(self, regNumStart, numRegisters):
		out = []
		GPIO.output(self.csPin, GPIO.LOW)
		
		# 0x to specify 'read register value'
		self.sendByte(regNumStart)
		
		for byte in range(numRegisters):	
			data = self.recvByte()
			out.append(data)

		GPIO.output(self.csPin, GPIO.HIGH)
		return out

	def sendByte(self,byte):
		for bit in range(8):
			GPIO.output(self.clkPin, GPIO.HIGH)
			if (byte & 0x80):
				GPIO.output(self.mosiPin, GPIO.HIGH)
			else:
				GPIO.output(self.mosiPin, GPIO.LOW)
			byte <<= 1
			GPIO.output(self.clkPin, GPIO.LOW)

	def recvByte(self):
		byte = 0x00
		for bit in range(8):
			GPIO.output(self.clkPin, GPIO.HIGH)
			byte <<= 1
			if GPIO.input(self.misoPin):
				byte |= 0x1
			GPIO.output(self.clkPin, GPIO.LOW)
		return byte	
	
	def calcPT100Temp(self, RTD_ADC_Code):
		R_REF = 430.0 # Reference Resistor
		Res0 = 100.0; # Resistance at 0 degC for 400ohm R_Ref
		a = .00390830
		b = -.000000577500
		# c = -4.18301e-12 # for -200 <= T <= 0 (degC)
		c = -0.00000000000418301
		# c = 0 # for 0 <= T <= 850 (degC)
		#print("RTD ADC Code: %d" % RTD_ADC_Code)
		Res_RTD = (RTD_ADC_Code * R_REF) / 32768.0 # PT100 Resistance
		#print("PT100 Resistance: %f ohms" % Res_RTD)
		#
		# Callendar-Van Dusen equation
		# Res_RTD = Res0 * (1 + a*T + b*T**2 + c*(T-100)*T**3)
		# Res_RTD = Res0 + a*Res0*T + b*Res0*T**2 # c = 0
		# (c*Res0)T**4 - (c*Res0)*100*T**3  
		# + (b*Res0)*T**2 + (a*Res0)*T + (Res0 - Res_RTD) = 0
		#
		# quadratic formula:
		# for 0 <= T <= 850 (degC)
		temp_C = -(a*Res0) + math.sqrt(a*a*Res0*Res0 - 4*(b*Res0)*(Res0 - Res_RTD))
		temp_C = temp_C / (2*(b*Res0))
		temp_C_line = (RTD_ADC_Code/32.0) - 256.0
		# removing numpy.roots will greatly speed things up
		#temp_C_numpy = numpy.roots([c*Res0, -c*Res0*100, b*Res0, a*Res0, (Res0 - Res_RTD)])
		#temp_C_numpy = abs(temp_C_numpy[-1])
		#print("Straight Line Approx. Temp: %f degC" % temp_C_line)
		#print("Callendar-Van Dusen Temp (degC > 0): %f degC" % temp_C)
		#print "Solving Full Callendar-Van Dusen using numpy: %f" %  temp_C_numpy
		if (temp_C < 0): #use straight line approximation if less than 0
			# Can also use python lib numpy to solve cubic
			# Should never get here in this application
			temp_C = temp_C_line
		return temp_C

class FaultError(Exception):
	pass

if __name__ == "__main__":
	try:
		csPin = 5
		misoPin = 19
		mosiPin = 20
		clkPin = 21
		max = max31865(csPin,misoPin,mosiPin,clkPin)
		while True:
			tempC = max.readTemp()
			print(tempC)
			time.sleep(0.1)
	except KeyboardInterrupt:
		pass
	GPIO.cleanup()