This is io23017, an utility and a daemon for MCP23017 i2c-connected GPIO chips.
Copyright (c)2015 Kadir A. Mueller (kadir.mueller@theflatnet.de) and distributed
under the terms of the GNU General Public License version 2.

 io23017 lets you easily read and write MCP23017 i2c to GPIO chips.  It contains
a little utility for writing to and  a  daemon for reading from these chips. The
utility will let  you write  to the GPIO ports while  the  daemon reads from one
port and executes something with the port's value and timing information.
 Please note  that the daemon currently always needs all eight bits of one port,
since it is operating on complete octets.  Also note  that it's polling the chip
just ten times per second by default, which  is  pretty enough for  connecting a
button or reading a door contact, but not much more.  You can change this in the
Makefile (DELAY), if you want to. But if you change that, keep your hardware and
ressources in mind. The default just fit's my own needs, which may or may not be
close to yours.
 io23017d reads from port a or b every DELAY microseconds and compares the port
inputs between reads, executing /usr/local/libexec/io23017 if there was a change
on the GPIO port. It's a very simplicistic approach, not using any interrupt and
running completely in userspace, but despite that, still runs without even using
1% of an Atheros AR9132 CPU.  And not needing things like an interrupt, much CPU
or huge amounts of memory makes it easy to use on platforms like Atheros - based
routers.
 BTW: The TP-Link TL-WR1043ND rev.1 (!) has two unused GPIOs that are wired from
the SOC with open ends on the PCB.  And two GPIOs are exactly what one needs for
opening an I2C-bus.  But there's absolutely no reason why you shouldn't use this
also with your PC or Raspberry Pi. io23017 should work at least on any linux, as
long as there's i2c connectivity.

Please note that this stuff is just quickly hacked together and not well tested.


Usage:

io23017d <dev> <address> <port>

 Executes "/usr/local/libexec/io23017 <bit> <state> <time>"  for every change on
port <port>,  where <dev> means the i2c-busdevice,  <address> the i2c-address of
the 23017 on the  bus  and  <port>  identifies which port of the 23017 should be
read, 'a' or 'b'.
The arguments for /usr/local/libexec/io23017:  <bit> will tell you which bit has
been changed, <state> gives you an "high" or "low" for the corresponding bit and
<time> gives you the time in tenths of seconds since the last change of the bit.
 io23017d will execute "/usr/local/libexec/io23017" for every single bit change,
regardless of how many bits may have changed since the read before.

Example for invoking io23017d: "/usr/sbin/io27013d /dev/i2c-0 0x20 a"


io23017 <dev> <address> <port> (<octet>)

 <dev> and <address> have the same meanings as with io23017d above. <port> also,
but there's a difference  if  the port is written in lower or upper case: if you
use lower case,  you're just accessing the GPIO- port,  while an upper case port
letter means to initialize the data direction register (IODIR) with <octed>. You
have to do this explicitely, even if you tell io23017 to write to the port. I'll
tell the reason in the examples later.
 Using lower port letters for <port>,  you can tell io23017 with issuing <octed>
which value to write to the port.  If you don't issue <octed>, io23017 will read
from the port and write the resulting octed to stdout.

Examples for using io23017:

io23017 /dev/i2c-0 0x20 A 0x00	# Tell our MCP23017 to set port a to output
io23017 /dev/i2c-0 0x20 a 0x00	# Tell out MCP23017 to set port a to zero
io23017 /dev/i2c-0 0x20 a	# Read from port a and echo the result

 If you have configured a port to output and read from it, io23017 will give you
what has been written to it:
io23017 /dev/i2c-0 0x20 A 0x00	# Tell our MCP23017 to set port a to output
io23017 /dev/i2c-0 0x20 a 0xaa	# Write 0xaa to port a
io23017 /dev/i2c-0 0x20 a	# Read port a
170				# Result 170 (0xaa)

 This is a difference to switching the port to input and read from it:
io23017 /dev/i2c-0 0x20 A 0xff	# Tell our MCP23017 to set port a to input
io23017 /dev/i2c-0 0x20 a	# Reading now will give the levels on the GPIOs

 Of course, you can use the GPIOs of one port with input and output mixed:
io23017 /dev/i2c-0 0x20 A 0x0f	# Tell our  MCP23017 to set the upper four GPIOs
				# to output and the lower four to input

And here is an example for /usr/local/libexec/io23017:

========= SNIP =================================================================
#!/bin/sh

echo Button on GPIO-pin $1 has changed after $3 10/seconds to $2.
========= SNAP =================================================================


Corrections and additions might be available in future at:
http://www.theflatnet.de/pub/io23017/