I have very basic test setup with an MCP3002 where I measure the voltage on Channel 0. For some bizarre reason however, the read and calculated values are 0.6V lower than the real voltage (I use a precise multimeter) measured from pin_2 on the MCP3002.
I've tried two different ways for retrieving and calculating the value from the MCP3002 (commented out), and both return the same result. I even added a 5 measurement cycle to work out the average value, but that made no difference. The Python script is this:
Code: Select all
#!/usr/bin/python
import spidev
import string
import time
import os
from time import gmtime, strftime
# Definitions
channel_0 = 0 # ADC Channel 0
channel_1 = 1 # ADC Channel 1
delay = 5 # Delay between readings
measurements = 5 # Number of readings for average value
# Open SPI bus
spi = spidev.SpiDev()
spi.open(0,0)
# Function to read SPI data from MCP3002 chip
# Channel must be an integer 0|1
def ReadChannel(channel):
data = 0
for i in range(0,measurements):
adc = spi.xfer2([104,0])
#adc = spi.xfer2([1,(2+channel)<<6,0])
data += int(((adc[0]&3) << 8) + adc[1])
#data += ((adc[1]&31) << 6) + (adc[2] >> 2)
time.sleep(0.2)
data = float(data) / measurements
return data
while True:
# Read the light sensor data
level_0 = ReadChannel(channel_0)
volts_0 = round((level_0 * 3.33) / float(1024), 2)
# Print out results
timenow = str(strftime("%Y-%m-%d %H:%M:%S", gmtime()))
print("{} | {} | {}".format(timenow, level_0, volts_0))
# Wait before repeating loop
time.sleep(delay)
Thanks,
Pimmy
