10MHz Clock

Got the ceramic resonator working nicely. Strategy is threefold:

1. Start it with 10 cycles of high, tristate, low, tristate at 10 MHz
2. Pump it every 10th cycle as it goes high, then low
3. Count 9 cycles of going high, then low

You detect going high/low by fetching from the up port after selecting a high/low level. Node will sleep till the transition occurs, then synchronize accurately.

Note on the trace the pump pulse followed by 9 counts (actually 10; pretty close). That is repeated 20 times to obtain 50 KHz timing. The duty cycle of pumping can vary from 5% to 100% and controls the amplitude of the sine wave (1.7 V), which gradually decreases. Smaller duty cycle, less power consumed, less RFI. The starting sequence can be as few a 8 cycles.

The resonator is less precise than a crystal, costs less and is easier to start. This non-linear drive requires only 1 pin and pins are a valuable resource. It does require 1 node, but nodes are cheap and plentiful. And the node can do other tasks, such as provide 50KHz and 60Hz signals.

Code is optimized by pre-filling the stack with numbers:

• Level at which to awake: high 8000, low 0
• Value to drive pin: high 30000, low 20000
• Value to tristate pin: 0
• unext count to complete 50ns pulse: 7
• 2 copies of these 4 numbers

The various modes - start, pump, count - each select from these and drop those that are irrelevant. The 8 numbers appear repeatedly from the 8-deep circular portion of the stack.