Clock module

This module drives 12 MHz ceramic resonator connected to GPIO pin 417.17, and distributes clock signal to USB module and Video module.

Description

Clock module is composed of only two nodes. It uses classical Chuck's approach to drive watch crystals and ceramic resonators. A resonator has a lower Q than a crystal, hence it is much easier to start oscillating. It's plenty accurate for video and USB.

When booted up, node 417 drives the resonator high and low at the resonance frequency for some time (50 cycles in this case), and then lets the resonator ring, while the GPIO pin is switched to high impedance input. With WD bit set appropriately we can suspend the node until the signal from the resonator goes high or low, in which instant we drive the resonator shortly in the current direction, switch back to high impedance, toggle WD bit and suspend again, waiting for the signal going in the oposite direcion. This keeps the resonator oscillating.

Node 416 then distributes the clock signal to USB and Video modules.