gpio - General Purpose IO Macros

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This include file provides symbolic access to GPIO pins through a few
generic macros and a configuration table. The configuration table holds
information about the GPIO layout and mapping to the hardware ports of
the AVR chip. Using a table to describe the GPIOs in a symbolic way has
several advantages:
1. All bit mappings can be made at a central project place, in one file.
2. Changes to the layout of the GPIOs do not enforce any changes to other
source code places - simply change the config table.
3. In the program itself there is no need to worry about bit positions
or bit masks - bits are handled in a symbolic way.
4. No more worry about anding/oring bits and bitfields.
5. ...

The configuration table is made up from preprocessor #defines. The syntax
is as below:

#define SYMBOL PORT, MSB, LSB

// SYMBOL : A unique symbolic name for the GPIO (or GPIO group).
// PORT : AVR Hardware Port (A,B,C,D,...).
// MSB : Bit# of the (M)ost (S)ignificant (B)it in this GPIO group.
/ /LSB : Bit# of the (L)east (S)ignificant (B)it in this GPIO group.

Once the table is created, the GPIOs can be accessed by several macros
(please find the exact calling conventions in the source code below):

gpio_conf(cfg,io,p_da) - Configures the GPIOs (set PORT/DDR reg.)
gpio_wr(px,cfg,da) - Writes to GPIO registers
gpio_rd(px,cfg) - Reads GPIO registers (right aligned)
gpio_rd8(px,cfg) - Reads GPIO registers (unaligned)
gpio_toggle(cfg) - Toggles GPIOs (write 1's to PIN reg.)
gpio_rd_inv(cfg) - Reads GPIO regs. right aligned, and inversed
gpio_set(cfg) - Sets GPIO registers (write 1's to PORT reg.)
gpio_clr(cfg) - Clears GPIO registers (write 0's to PORT reg.)

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