Raspberry Pi Pinout

GPCLK - General Purpose CLock

General Purpose Clock pins can be set up to output a fixed frequency without any ongoing software control.

The following clock sources are available:

0     0 Hz     Ground
1     19.2 MHz oscillator
2     0 Hz     testdebug0
3     0 Hz     testdebug1
4     0 Hz     PLLA
5     1000 MHz PLLC (changes with overclock settings)
6     500 MHz  PLLD
7     216 MHz  HDMI auxiliary
8-15  0 Hz     Ground

Other frequencies can be achieved by setting a clock-divider in the form of SOURCE/(DIV_I + DIV_F/4096). Note, that the BCM2835 ARM Peripherals document contains an error and states that the denominator of the divider is 1024 instead of 4096.