PCF8591-read

Why

The PCF8591 is a fairly inexpensive I2C ADC/DAC chip, with four 8-bit ADC inputs and one 8-bit DAC output.

It often comes on breakout boards. The boards, like eg. this one, sometimes come with example devices, here eg. a trimpot, a thermistor, and a photoresistor, on the inputs, and a red LED on the output. The LED doesn't light up below about half of the output range.

Python library is available.

For reading from shell scripts, especially on underpowered boards like Raspberry Pi 1, python is less suitable due to the overheads at process startup. Insane amount of files has to be read and interpreted. A frequently executed task will bring a significant load on the machine.

How

A simple C code was derived from the sw_chip_PCF8591 one.

As the sensor doesn't do conventional registers, and only bare reads and writes are used (so eg. i2cdump will show just all zeroes and confuse the register settings), the barest of interfacing will do. File and IOCTL access to /dev/i2c-something was therefore chosen.

chip

The chip operates in a simple way. Send a one-byte configuration, optionally with another byte (or bytes) to be written to the DAC output. Read raw bytes from the ADC, from a fixed channel or from autoincrementing sequence of individual inputs.

code

The code first opens the I2C device (or fails when device doesn't exist or is inaccessible). IOCTL I2C_SLAVE is executed on the resulting file handle, with the chip's address as parameter.

The config byte is sent (code fails here if the chip is not on the given address), optionally followed with DAC value.

Then several reads are issued. First byte is an old value from previous read.

Usage

options

If executed without options, it queries first BH1750 sensor on the /dev/i2c-1 bus. This is a default configuration on raspberry pi boards.

PCF8591 sensor read
Usage: pcf8591 [-v] [-x] [-O] [-o <DACval>] [-c <chan>] [-a <addr>] [-d <dev>]
ADC:
  -c <num>  display ADC channel (0..3), all if not specified
  -m <num>  set the input mux (0..3) (bits 4,5 of config; default 0, four separate inputs)
  -x        hex output
  -j        json format
DAC:
  -O        force DAC output (do not unset when DAC value not specified)
  -o <num>  set DAC value (0..255)
I2C:
  -a <num>  address, 0..7, default=0; adds to base address 0x48
  -d <dev>  specify I2C device, default /dev/i2c-1
general:
  -v        verbose mode
  -h,--help this help

• ADC
• -x - hexadecimal mode, output values in hex (default decadic integer)
• -j - JSON format (all four values, decadic integers)
• -c <n> - output only channel n (default all four channels)
• -m <n> - set input mux (bits 4,5 of config) to n (default 0, four independent inputs; other values for various differential modes, see datasheet)
• DAC
• -o <n> - output n as value to the DAC (lower 8 bits of n, 0..255)
• -O - force DAC output without specifying DAC value; keep value, prevent switching off of the output
• I2C
• -a <n> - select the n-th chip on the bus (0..7), default n=0; adds to base addr 0x48
• -d /dev/i2c-something - specify the I2C device to access
• general
• -v - verbose mode, report every read and device open; suitable for debugging
• -h - help
• --help - help; seriously, why every software has one or the other and never both?!?

output

If all is okay, result code is 0. Otherwise, if something crashes, result is 1.

If not verbose or help, only the measured value of the selected channel, or all four separated by space, is spat to stdout.

examples

read all four channels (0..3)

Compiling

The code doesn't need any special compilation. A simple call will do:

• cc -o pcf8591 pcf8591.c

• cc -o pcf8591 -static pcf8591.c

Files

• pcf8591.c - source code
• pcf8591-raspi - binary for raspberry pi
• pcf8591-raspi-static - binary for raspberry pi, statically linked

TODO

• config of the ADC inputs setting (default: all singles, can be differentials)