[time-nuts] ANFSCD - Synchronizing time in home video recorders

Tom Van Baak tvb at LeapSecond.com
Thu Feb 2 15:35:50 UTC 2012

Hi Roberto,

The motivation for this, I assume most list members know, is to
drive cheap quartz stepper motor clocks with precise 32 kHz
frequency, one derived from an atomic or GPS 10 MHz.

The 10 MHz to 32 kHz PIC divider I wrote uses a sort of binary
"leap year" algorithm to adjust the digital output phase to be as
close as possible to the ideal 32.768 kHz phase on each cycle
and also to have zero long-term error.

I'm not sure how well a multi-level leap year algorithm relates
Breseham's algorithm. I tracked down his 1965 plotter article.
There might be common ground there.

With non-integral ratios like this case, or without external analog
components (e.g., PLL), it seems some level of jitter is always
unavoidable. So the goal was to make it as mathematically small
as possible, and furthermore, to be able to do the math within a
half cycle, which is only 15 microseconds.

I'll send you an early draft of the PIC code; the version that was
most clear before I had to pinch too many cycles and added too
many features. Let me know what you think.

I also simulated the algorithm on a PC and measured the ADEV
and phase noise. That simulation code is file 10m32k.c under:



----- Original Message ----- 
From: "Roberto Barrios" <rbarrioss at msn.com>
To: "Tom Van Baak" <tvb at leapsecond.com>; "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Thursday, February 02, 2012 5:09 AM
Subject: Re: [time-nuts] ANFSCD - Synchronizing time in home video recorders

> Hi Tom,
> I'm interested in that divider. Actually, insterested in knowing how it 
> works, not in the .HEX file.
> Breseham's algorith works but has inherent jitter and I've found no other 
> solutions for situations like that.
> I'd live to know how it is done.
> Thank you,
> Roberto EB4EQA
> http://www.rbarrios.com
> -----Mensaje original----- 
> From: Tom Van Baak
> Sent: Thursday, February 02, 2012 10:34 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] ANFSCD - Synchronizing time in home video recorders
>> I think I've seen comments about making 32 KHz from 10 MHz in a PIC or 
>> AVR.
>> tvb has this web page, but I don't see a 32 KHz option:
>>  http://www.leapsecond.com/pic/picdiv.htm
> Hal,
> Yes, I have a PIC divider that takes 5 or 10 MHz input and
> outputs a 32.768 kHz square wave with minimal jitter and
> no long-term phase offset. Contact me off-line if interested.
> /tvb

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