class MAX31865

MAX31865

Thanks to github.com/steve71/MAX31865/blob/master/max31865.py github.com/hackenbergstefan/MAX31865/blob/master/max31865.py

Constants

A

Constants for Callendar-Van-Dusen equation

B
C

C = -4.18301e-12 # for -200 <= T <= 0 (degC)

CHIPS

C = 0 # for 0 <= T <= 850 (degC)

FAULTS
R0

Resistance at 0 degC for 400ohm R_Ref

VERSION

Attributes

chip[RW]
clock[RW]
hz[RW]
ref[RW]
type[RW]
wires[RW]

Public Class Methods

new(chip: 0, ref: 430.0, clock: 2_000_000) click to toggle source 
# File lib/max31865.rb, line 36
def initialize(chip: 0, ref: 430.0, clock: 2_000_000)
  @chip  = CHIPS[chip]
  @ref   = ref
  @clock = clock
end

Public Instance Methods

config(byte = 0b11010010) click to toggle source

Run once config

Optionally set samples

0x8x to specify 'write register value' 0xx0 to specify 'configuration register'

Config Register

bit 7 : Vbias -> 1 (ON) bit 6 : Conversion Mode -> 0 (MANUAL) bit 5 : 1-shot ->1 (ON) bit 4 : 3-wire select -> 1 (3 wire config) bit 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) 0b10110010 = 0xB2

# File lib/max31865.rb, line 85
def config(byte = 0b11010010)
  spi_work do |spi|
    spi.write(0x80, byte)
  end
  sleep 0.2 # give it 200ms for conversion
end
read() click to toggle source

Read temperature!

# File lib/max31865.rb, line 95
def read
  spi_work do |spi|
    read_temp(spi.write(Array.new(8, 0x01)))
  end
end
spi_work() { |spi| ... } click to toggle source 
# File lib/max31865.rb, line 42
def spi_work
  PiPiper::Spi.begin do |spi|
    # Set cpol, cpha
    PiPiper::Spi.set_mode(1, 1)

    # Setup the chip select behavior
    spi.chip_select_active_low(true)

    # Set the bit order to MSB
    spi.bit_order PiPiper::Spi::MSBFIRST

    # Set the clock divider to get a clock speed of 2MHz
    spi.clock clock

    spi.chip_select(chip) do
      yield spi
    end
  end
end

Private Instance Methods

quadratic(adc, rtd) click to toggle source

Callendar-Van Dusen equations

Convert 0 <= raw_temp < 2^l5 to temperature in oC using quadratic polynom:

R(T)=R(0)(1+A*T+B*T^{2})

> T = (-A*R(0) + sqrt(A^2 R(0)^2 - 4*B*R(0)*(R(0) - R(T)))) / (2*B*R(0)) ↑

quadratic formula: for 0 <= T <= 850 (degC)

# File lib/max31865.rb, line 115
def quadratic(adc, rtd)
  temp = -(A * R0)
  temp += Math.sqrt(A * A * R0 * R0 - 4 * B * R0 * (R0 - rtd))
  temp /= 2 * B * R0
  # puts "Callendar-Van Dusen Temp (degC > 0): #{temp}C"
  temp < 0 ? (adc / 32) - 256 : temp
end
quartic(rtd) click to toggle source

“”“ Convert 0 <= raw_temp < 2^15 to temperature in oC using quartic polynom:

R(T) = R(0)[1+A*T+B*T^{2}+(T-100)C*T^{3}]

> 0 = R(0)[1 + A*T + B*T^2 - 100*C*T^3 + C*T^4] - R(T) ↑

Solution is calculated numerically.

# File lib/max31865.rb, line 132
def quartic(rtd)
  temp = rtd * R0 / (2**15 - 1)
  roots = [C * R0, -100 * R0 * C, B * R0, A * R0, R0 - temp]
          .map { |n| Math.sqrt(n) }
  Math.abs(roots[-1])
end
read_fault(byte) click to toggle source

Read register faults

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: #{status}”

# File lib/max31865.rb, line 168
def read_fault(byte)
  FAULTS.each do |code, fault|
    raise fault if byte & code == 1
  end
end
read_temp(raw) click to toggle source

Read temperature

# File lib/max31865.rb, line 142
def read_temp(raw)
  read_fault(raw[7])
  rtd_msb, rtd_lsb = raw[1], raw[2]
  adc = ((rtd_msb << 8) | rtd_lsb) >> 1
  # puts "RTD ADC Code: #{adc}"
  # temp_line = (adc / 32.0) - 256.0
  # puts "Straight Line Approx. Temp: #{temp_line}C"
  rtd = (adc * ref) / 32_768.0 # PT100 Resistance
  # puts "PT100 Resistance: #{rtd} ohms"
  quadratic(adc, rtd)
end