iC880A Backplane: ADC example script

adc.py

"""
Example script to read the input voltage, measured by the MCP3425 ADC.
Datasheet: http://ww1.microchip.com/downloads/en/DeviceDoc/22072b.pdf
"""
# -*- coding: utf-8 -*-
from __future__ import print_function, division

import time
import smbus

bus = smbus.SMBus(1)

# I2C address
DEVICE_ADDRESS = 0x68

# Voltage dividers
DIVIDER_R1 = 6800
DIVIDER_R2 = 3600
DIVIDER_R3 = 470

# ADC reference
ADC_REF = 2048  # mV

# Config register bits
BIT_G0 = 0
BIT_G1 = 1
BIT_S0 = 2
BIT_S1 = 3
BIT_OC = 4
BIT_C0 = 5
BIT_C1 = 6
BIT_RDY = 7

# Conversion mode
CONVERSION_MODE_ONESHOT = 0
CONVERSION_MODE_CONT = 1 << BIT_OC

# Sample rate / accuracy
SAMPLE_RATE_240SPS = 0  # 12 bits
SAMPLE_RATE_60SPS = 1 << BIT_S0  # 14 bits
SAMPLE_RATE_15SPS = 1 << BIT_S1  # 16 bits

# PGA gain selection
PGA_GAIN_1 = 0
PGA_GAIN_2 = 1 << BIT_G0
PGA_GAIN_4 = 1 << BIT_G1
PGA_GAIN_8 = (1 << BIT_G1) | (1 << BIT_G0)

# Start a conversion in one shot mode
START_CONVERSION = 1 << BIT_RDY

# Write configuration
config = START_CONVERSION | CONVERSION_MODE_ONESHOT | SAMPLE_RATE_15SPS | PGA_GAIN_1
print('Writing data: %s' % bin(config))
bus.write_byte(DEVICE_ADDRESS, config)

# Wait a bit for the measurement to finish
time.sleep(0.15)

# Read measurement
data = bus.read_i2c_block_data(DEVICE_ADDRESS, 0x00, 3)
value = (data[0] << 8) + data[1]

# Calculate voltage for the specified bit of accuracy
def get_voltage(measurement, bit):
    v2 = measurement * ADC_REF / (2**bit)
    total_r = DIVIDER_R1 + DIVIDER_R2 + DIVIDER_R3
    return v2 / (DIVIDER_R2 / total_r) * 2

print(list(map(hex, data)))
print(list(map(bin, data)))
print('Value is %d/%d' % (value, 2**16))
print('Voltage is %.4f V' % (get_voltage(value, 16) / 1000))