# RaspberryPI GPIO library

This egg provides an interface to the RaspberryPi GPIO pin for CHICKEN Scheme.

You'll need to install [Wiring Pi](http://wiringpi.com) before using this
library. The [Wiring Pi documentation](http://wiringpi.com/reference) may
help you understand some of these functions in more detail.

## Example

```scheme
(use raspberry-pi-gpio)

(setup-gpio)

;; digital read / write

(pin-mode 1 'output)

(digital-write 1 HIGH)
(digital-write 1 LOW)

(printf "Pin 1 is: ~S~n" (digital-read 1))

;; using interrupts to respond to events

(set-edge 7 'falling)
(set-edge 8 'both)

(define (loop)
  (let ((pin (receive-gpio-event)))
    (printf "Interrup on pin ~S, value: ~S~n" pin (digital-read pin))
    (loop)))

(loop)
```

## API

One of the setup functions must be called at the start of your program, or
your program will fail to work correctly.

#### (setup-gpio)

Use the Broadcom GPIO pin numbers when referencing pins.

#### (setup-virtual)

Use the wiringPi pin numbering scheme. This is a simplified numbering
scheme which provides a mapping from virtual pin numbers 0 through 16 to
the real underlying Broadcom GPIO pin numbers. See the [Wiring Pi
documentation](http://wiringpi.com/reference/setup/) for more info.

#### (setup-physical)

Use the physical pin numbers *on the P1 connector only*.

#### (setup-system)

Use the /sys/class/gpio interface rather than accessing the hardware
directly. This can be called as a non-root user provided the GPIO pins have
been exported before-hand using the gpio program.

#### (board-rev)

Returns the board revision (1 or 2).

#### (pin-mode pin mode)

Sets the pin to the given mode. Mode is one of the following symbols:
`input`, `output`, `pwm-output`, `gpio-clock`.

#### (pull-up-dn-control pin pud)

This sets the pull-up or pull-down resistor mode on the given pin, which
should be set as an input. Pud should be one of the following symbols:
`off`, `up`, `down`.

#### (pwm-write pin value)

Writes the value to the PWM register for the given pin. The Raspberry Pi
has one on-board PWM pin, pin 1 (BMC\_GPIO 18, Phys 12) and the range is
0-1024. Other PWM devices may have other PWM ranges.

This function is not able to control the Pi’s on-board PWM when in Sys
mode.

#### (digital-write pin value)

Writes the value HIGH or LOW (1 or 0) to the given pin which must have been
previously set as an output.

#### (digital-read pin)

This function returns the value read at the given pin. It will be HIGH or
LOW (1 or 0) depending on the logic level at the pin.

#### (analog-write pin value)

This writes the given value to the supplied analog pin.

#### (analog-read pin)

This returns the value read on the supplied analog input pin.

#### (set-edge pin edge)

Sets up an interrupt handler for the pin and given edge using wiringPiISR.
Use this to hook up events for use with `recieve-gpio-event`.

Type should be one of the following symbols: `falling`, `rising`, `both`,
`setup`. If `setup` is used then no initialisation of the pin will happen –
it’s assumed that you have already setup the pin elsewhere (e.g. with the
gpio program).

#### (receive-gpio-event)

Blocks until a GPIO interrupt occurs on a pin that had set-edge called on it.
Returns the pin number that caused the interrupt.

#### HIGH

The HIGH value (1) exported by wiringPi.

#### LOW

The LOW value (0) exported by wiringPi.