Using SPI on Raspberry PI gives error

Question

I'm trying to read the output of an ADC chip LTC 1867 (need the high precision). I think I understand how it how works. Since it's output is 16 bit I need to send 2 8-bit words to it, the first word starts with a 7 bit input word that controls, what channel, mode and other parameters.

When I ground the channel, it returns 0 as expected but otherwise, it's output is very wrong. A 2.5V reference voltage returns about 1.525. I have tried spidev and pigpio and results seem to be consistent across both.

I decided to use piscope (seems very convenient and useful! especially as I don't have access to an oscilloscope in the evening) to see what was going on with my MISO, MOSI, CLK, CE0. The clock seems to be missing some pulses and I think that is the problem

The returned values are something like this What I would like to know is:

• Could it be that one of my pins is not working well, (My digital output from the ADC was 5V so it could have messed it up). How do I check if that is the issue
• It shows the right value (2.5V) for a like 2 times before returning to the wrong ones (1.524V) and other random values
• How do I fix the missing pulses? -Does it matter that the clock pulses are uneven -Would software SPI/ bit banging be better?

My Code is

from time import sleep
# Import SPI library (for hardware SPI) and MCP3008 library.
import pigpio
from time import sleep
import os

# Open SPI bus
pi = pigpio.pi()
h=pi.spi_open(0,50000,0)

def twos_complement(input_value, num_bits):
    '''Calculates a two's complement integer from the given input value's bits'''
    mask = 2**(num_bits - 1)
    return -(input_value & mask) + (input_value & ~mask)

## http://cds.linear.com/docs/en/datasheet/18637fc.pdf

#Function to read SPI data from LTC1867 chip
# Channel must be an integer 0-7
def ReadChannel(channel):
  #pin 7-GND=244, pin 5-GND=228, pin 1-GND=196
  (count, rx_data) = pi.spi_xfer(h,[228,0])
  print "rx_data"
  #print rx_data
  index=0
  rx_array=[0,0]
  for byte in rx_data:
          rx_array[index]= byte
          index=index+1
  print rx_array
  data = (rx_data[0]<<8) + rx_data[1]
  print "data"
  #print data
  #print "2's complement"
  #data= twos_complement(data, 16)
  print data
  return data

# Converts data to voltage level,
# rounded to specified number of decimal places.
def ConvertVolts(data,places):
  volts = (data * 2.5) / float(pow(2,16))
  volts = round(volts,places)
  return volts

print('Reading LTC1867 values, press Ctrl-C to quit...')

tilt_channel_pos=0
tilt_channel_neg=1
delay=5

while True:
      print "--------------------------------------------"
      tilt_bits_pos = ReadChannel(tilt_channel_pos)
      tilt_volts_pos = ConvertVolts(tilt_bits_pos,3)
      print("Volts: {}V".format(tilt_volts_pos))
      break

      # Wait before repeating loop
      sleep(5)

Answer 1

piscope will not be seeing all the SPI traffic as the SPI bus is running at the same speed as the pigpio sample rate (default 200ksps).

When you are testing drop the SPI sample rate to something like 50kbps to ensure all the GPIO level transitions are captured.

E.g. use h=pi.spi_open(0,50000,0) rather than h=pi.spi_open(0,200000,0).

The code is sending two bytes (228, 0) to the LTC 1867. So the control byte is 0xE4. Is that correct.

Could you show some sample of the returned data?