gpio - Read and set GPIO pins (of the Samsung S3C2442 MCU)
==========================================================

Copyright (C) 2008 by OpenMoko, Inc.
Written by Werner Almesberger <werner@openmoko.org>
All Rights Reserved

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.


Displaying all GPIOs
--------------------

# gpio

If invoked without arguments, "gpio" displays the state of all GPIO
pins of the MCU. The state is represented as follows:

   0  Input, reads 0
   1  Input, reads 1
  >0  Output, reads 0 (1)
  >1  Output, reads 1 (1)
  F0  Function, reads 0 (2)
  F1  Function, reads 1 (2)
  X0  Secondary function or reserved, reads 0 (2)
  X1  Idem, reads 1 (2)

(1) The value read back from a port pin reflects the voltage level on
    that pin. For an output, this is normally the value that we try to
    output. However, if the pin is externally forced high or low, the
    value read back may be different from the value we try to output.

(2) According to Samsung documentation, the value read back from a
    function pin is undefined. However, it usually represents the real
    state of that pin.

The pin state can be followed by "R", which indicates that the
corresponding pull-down resistor is active.

Changes in the GPIO settings can be monitored by running "gpio" under
"watch", e.g., with

# watch -n 1 -d ./gpio


Displaying individual GPIOs
---------------------------

Individual GPIOs can be displayed by specifying their names on the command
line, e.g.,

# gpio b0 b1 b2

Names are case-insensitive. The GPIO's state is displayed as described
above.


Setting GPIOs
-------------

A GPIO can be set by specifying an assignment on the command line, as
follows:

# gpio gpio=value

The GPIO is specified in the same way as when displaying individual
GPIOs. The following values can be set:

  0  Output 0
  1  Output 1
  Z  Input
  F  Function
  X  Secondary function

In addition to this, the value can be followed by "R" to set the
pull-down resistor. "ZR" can be abbreviates to "R".

Example:
# gpio b0=1 b1=r b2=z

Assignments can be mixed with reads, e.g.,
# gpio b0=1 b0