MCP23017 GPIO Extender - Experimenting with Raspberry Pi - page 29

Value R/W GPIOx Bit Value

0 R Current input pin state is low

W Write 0 to OLATx and output pin

1 R Current input pin state is high

W Write 1 to OLATx and output pin

Main Program
Here are some change notes for the main program:

1. If you have a pre-revision 2.0 Raspberry Pi, change line 36 to

use /dev/i2c-0 .

2. Change line 55 if your MCP23017 chip is using a different I2C

address than 0x20 (A2, A1, and A0 grounded).

3. Change line 56 if you use a different Raspberry Pi GPIO for

your interrupt sense pin.

The main program is fairly straightforward. Here are the basic overall steps:

A signal handler is registered in line 180, so ^C will cause the
program to exit cleanly.

1.

Routine i2c_init() is called to open the I2C driver and
initialize.

2.

Routine mcp23017_init() is called to initialize and configure
the MCP23017 device on the bus (only one is currently
supported).

3.

Routine gpio_open_edge() is called to open /sys/class/
gpio17/value , so changes on the interrupt line can be sensed.
This is discussed in more detail later.

4.

5. Finally, the main program enters a loop in lines 190 to 200,

looping until ^C is entered.

Once inside the main loop, the following events occur:

Execution stalls when gpio_poll() is called. This blocks until
the interrupt on GPIO 17 transitions from a high to a low.

1.

The interrupt flags are read using routine mcp23017_
interrupts() . They're only reported and otherwise not used.

2.

Routine mcp23017_captured() is used to read the INTCAPA
and INTCAPB registers in order to clear the interrupt.

3.

Finally, the routine post_outputs() reads the real-time input
values and sends the bits to the outputs.

4.