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Accurate timing ( 0-500Hz +/- 5ppm)

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I need accurate timing for my project. I'm going to use PIC16F88 (20MHz).


What is the best solution for external oscillator/clock, is there accurate crystals?


can i use SPG8651A  (selectable output crystal oscillator) for pics oscillator...

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The best way would be using LP configuration to draw a minimal current from the PIC's internal osc driver (although an external clock wiring will let the OSC2 pin unconnected, and no current will flow through it anyway).


Then, the simpliest and most accurate solution should be a crytal oscillator, with a frequency (let's call it Fq) equal to a power of 2 if u wanna use interrupts.

Therefore, you can use this subcircuit to drive a 74HC161, which will provide you with 4 selectable clock rates : Fq/2 , Fq/4, Fq/8, Fq/16.


Then, with a 4MHz oscillator, you can get a 500kHz frequency from pin12 (Q2) of the 74HC161.


This is a sure way to get many different clock sources from a cheap circuit (u can get a 74HC161 for less than half a buck !).


Another solution would be to use a 1MHz ext clock combined with a "divide by 2" subcirtcuit. But the number of device will reach the first I described, without as many rates (ie : five) as you could desire !


Don't think about ACCURATE clock sources neither with RC pairs, nor resonators.

I almost forgot : SPG8651A offers a 100ppm stability. Far away from your 5ppm needs !!!!


Nota Bene : I was talking n'bout CRYSTAL clock sources, cuz, as you said, you need 5ppm accuracy.

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