harvey/sys/man/3/gpio

.TH GPIO 3
.SH NAME
gpio \- access to Raspberry Pi GPIO pins
.SH SYNOPSIS
.B bind -a #G /dev
.PP
.B /dev/gpio
.fi
.SH DESCRIPTION
.I Gpio
serves a single file that provides access to the GPIO pins on the
Raspberry Pi.
Reads from the file receive a 16-character hexadecimal string
representing a
.B vlong
giving the values of up to 64 GPIO lines.
(The processor on both the first and second generation Pis provides
54 actual GPIO lines.)
The proper method for sampling GPIO 27 is as follows:
.IP
.B read(gfd, buf, 16);
.br
.B buf[16] = 0;
.br
.B gvals = strtoull(buf, nil, 16);
.br
.B "pin27 = gvals & (1 << 27);"
.PP
Writes to
.B gpio
control the characteristics and output values of GPIO lines.
The exact operation is specified by one of the following commands:
.TP
.BI function " pin f"
Set the function of GPIO
.I pin
to
.I f.
The valid values for
.I f
are:
.BR in ,
.BR out ,
.BR alt0 ,
.BR alt1 ,
.BR alt2 ,
.BR alt3 ,
.BR alt4 ,
.BR atl5 ,
and
.BR pulse .
The functions
.B in
and
.B out
set the specified GPIO line to be a general purpose input
and output, respectively.
The various
.BI alt n
functions are as specified in the Broadcom documentation and
differ on a per-pin basis.
The
.B pulse
function is somewhat specialized.
It causes the pin to be set to an output for
2μS and then to be set to a input.
With the value of the line set to 0 and a pullup resistor on the
line, this operation provides a short low pulse suitable for bit-banging
a 1-wire interface.
.TP
.BI pullup " pin"
Enables the internal pullup resistor for the specified GPIO pin.
.TP
.BI pulldown " pin"
Enables the internal pulldown resistor for the specified GPIO pin.
.TP
.BI float " pin"
Disables both internal pullup and pulldown resistors for the specified
GPIO pin.
.TP
.BI set " pin value"
For GPIO pins set to the output function, this command sets the output
value for the pin.
The
.I value
should be either 0 or 1.
.SH NOTES
All pin number references are according to the SoC documentation.
These GPIO signal numbers do
.I not
match the pin numbers on the header on the Pi board.
Reads sample the external signal values.
As a result, the values read for output pins might not match
the value written to them if externally they are driven harder
than the SoC drives them.
.SH SOURCE
.B /sys/src/9/bcm/devgpio.c
.br
.B /sys/src/9/bcm/gpio.c