Hi all, I'm working on a hobby project and am trying to interface the ADS1248 to a Raspberry Pi to measure liquid temperatures (0-100 deg C) with a 100ohm RTD. I seem to be communicating with the chip just fine (can set and read the registers), however every time I attempt a conversion I get the same result (exactly mid-range) from the ADC. This is hardly my area of expertise (I mentioned this is a hobby project), and I'm hoping there's an obvious mistake that I can't find that someone here can point me to. I've read the data sheet and am basing the design off of the 'Hardware-Compensated Three-Wire RTD Measurement' design example. I've double-checked the wiring and the schematic and I'll admit I'm stumped -- I just can't get any data from the ADC. Here are the values I'm using: R_comp = 119.4ohm (120) PGA = 128 IDAC = 0.75mA Vref = 2.5V R_bias = 1667ohm Here's the schematic: Here's the code (Python): # Set-up script for ADS1248 ADC Converter # Configures ADS1248 for four three-wire 100ohm RTDs # --- IMPORTS --- import sys import spidev import time import RPi.GPIO as GPIO # --- ASSIGNMENTS --- #Define GPIO Pins (using board numbering) START = 7 RESET = 16 DRDY = 18 # --- VALUES --- #ADS1248 setup values (used for calculating temperature from conversion calculation) PGA = 128 Vref = 2.5 #In Volts IDAC = .00075 #In Amps Ct = 0.385 #PT-100 temp coefficient (ohm / deg C) r_comp = 119.40 class ADS1248: #ADS1248 register addresses MUX0 = 0x00 MUX1 = 0x02 VBIAS = 0x01 SYS0 = 0x03 OFC0 = 0x04 OFC1 = 0x05 OFC2 = 0x06 FSC0 = 0x07 FSC1 = 0x08 FSC2 = 0x09 IDAC0 = 0x0a IDAC1 = 0x0b GPIOCFG = 0x0c GPIODIR = 0x0d GPIODAT = 0x0e #ADS1248 commands WREG = 0x40 #Write to register (0100xxxx) RREG = 0x20 #Read from register (0010xxxx) NOP = 0xff #No operation RDATA = 0x12 #Read conversion result RDATAC = 0x14 #Read conversion continuously def resetADS1248(): """ Resets the ADS1248 by pulsing RESET pin LOW and back to HIGH. Returns nothing.""" #Reset the ADS1248 and re-read register GPIO.output(RESET, False) #RESET pin low -> reset time.sleep(0.025) GPIO.output(RESET, True) #RESET pin high -> run time.sleep(0.025) return def writeReg(reg,data): """ Writes one byte to the given register. Accepts two integer bytes: register and data.""" r = spi.xfer2([ADS1248.WREG+reg,0x00,data]) return r def readReg(reg): """ Reads one byte from the given register. Accepts one integer byte: register.""" spi.xfer2([ADS1248.RREG+reg,0x00]) r = spi.xfer2([ADS1248.NOP]) return r def convert(): """ Returns ADC conversion from ADS1248. """ spi.writebytes([ADS1248.RDATA]) #Issue read data once command raw = spi.readbytes(3) #Read bytes spi.writebytes([ADS1248.NOP]) #Send no operation print(raw) adc = (raw[0] AIN0, IDAC2 -> AIN1 (pg 47) writeReg(ADS1248.VBIAS, 0b00000000); #VBIAS off (pg 43) writeReg(ADS1248.GPIOCFG, 0b00000000); #GPIO pins are analog inputs (GPIO disabled) (pg 48) initGPIO() #Initialize SPI spi = spidev.SpiDev() spi.open(0,0) spi.cshigh=False spi.bits_per_word=8 spi.mode=1 #For ADS1248 clock dwells low (CPOL=0) and data clocked on falling edge (CPHA=1) resetADS1248() print printRegs() time.sleep(0.050) configureADS1248() print printRegs() time.sleep(0.050) while True: GPIO.wait_for_edge(DRDY, GPIO.FALLING) #Wait for DRDY time.sleep(0.050) print("Data Ready") v_rtd_gained = (convert()/16777215.0)*Vref v_rtd = v_rtd_gained/PGA r_rtd = (v_rtd / IDAC) + r_comp temp_C = (r_rtd - 100)/Ct print "%.4f (V*128) %.4f (V) %.4f (ohm) %.2f (C) %.2f (F)" % (v_rtd_gained,v_rtd,r_rtd,temp_C,(temp_C*(9/5)+32)) time.sleep(5) GPIO.cleanup() Here's the result of the above code: --- ADS1248 Register Values: --- dec binary hex GPIODIR: 000 0b00000000 0x00 SYS0: 000 0b00000000 0x00 OFC1: 000 0b00000000 0x00 OFC0: 000 0b00000000 0x00 OFC2: 000 0b00000000 0x00 VBIAS: 000 0b00000000 0x00 FSC2-RO: 064 0b01000000 0x40 FSC0-RO: 192 0b11000000 0xc0 GPIOCFG: 000 0b00000000 0x00 FSC1-RO: 018 0b00010010 0x12 IDAC0: 128 0b10000000 0x80 IDAC1: 255 0b11111111 0xff GPIODAT: 000 0b00000000 0x00 MUX1: 000 0b00000000 0x00 MUX0: 001 0b00000001 0x01 --- ADS1248 Register Values: --- dec binary hex GPIODIR: 000 0b00000000 0x00 SYS0: 113 0b01110001 0x71 OFC1: 000 0b00000000 0x00 OFC0: 000 0b00000000 0x00 OFC2: 000 0b00000000 0x00 VBIAS: 000 0b00000000 0x00 FSC2-RO: 064 0b01000000 0x40 FSC0-RO: 000 0b00000000 0x00 GPIOCFG: 000 0b00000000 0x00 FSC1-RO: 019 0b00010011 0x13 IDAC0: 133 0b10000101 0x85 IDAC1: 001 0b00000001 0x01 GPIODAT: 000 0b00000000 0x00 MUX1: 032 0b00100000 0x20 MUX0: 001 0b00000001 0x01 Data Ready [127, 255, 255] 8388607 1.2500 (V*128) 0.0098 (V) 132.4208 (ohm) 84.21 (C) 116.21 (F) The first set of register reads is upon chip start up, and the second set is after configuring, demonstrating that I can communicate with the chip and read registers. The issue is that the three bytes read out for the conversion are always [127,255,255]. Thanks in advance for any help you can offer!
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