[time-nuts] Generating a solid PPS from 10Mhz source

Bob Camp kb8tq at n1k.org
Mon Jan 18 14:34:56 EST 2016


Hi

The same issue come into random logic. The most common chip logic way to do this is with rippling 
divide by 10’s.  Been there / done that.. The output pulse in that case is delayed  by quite a bit. Put
a “sync” flip flop on the output and the (obvious) question is — do you know the delay is << 100 ns?
When it’s not, you start doing full up programmable dividers …. you then have a Fmax question. 

The nice thing about a FPGA (or CPLD) is that they come with a cute timing analyzer. You can indeed
answer questions like this with a quite high level of confidence. That *assumes* that you bother to set
up the timing analyzer :) 

With an MCU, you have pretty good information on the output delay. The ambiguity comes on the clock
input. It’s rare to see that fully documented. You are then back to a “measure it and see” situation. You 
probably could guess that the input delay and output delays relate to the clock delay. 

Regardless of the divider it’s self, you will have the sine to square conversion. You also
may have a sensitivity on a square back to sine conversion. All of it (and the divider) will have both temperature
and voltage sensitivity. Most of that will be in the “measure it and see” category.  

Based on measurements, all of it comes out in the “no big deal” range for normal applications. 

Bob




> On Jan 18, 2016, at 12:28 PM, Vlad <time at patoka.org> wrote:
> 
> 
> Looking to the complex solutions for the frequency divider (CPLD/MCU), I start to think about skews and propagation delays. Its not obvious from the first glance. But I think such things exists.
> It could be interesting to compare the numbers.  Is it worth to consider some correction to avoid phase difference between of input and output ?
> 
> 
> On 2016-01-18 08:59, cfo wrote:
>> On Wed, 13 Jan 2016 09:22:09 +0000, Jerome Blaha wrote:
>>> Is there an easy circuit to build that can consistently deliver a 1 PPS
>>> from a 10MHz source with excellent resolution and repeatability?
>> Ulrich B. has made an AVR 1PPS, for those that uses AVR's instead of PIC's
>> AVR PPSDIV 2008-09-06 - in bottom of page.
>> http://www.ulrich-bangert.de/html/downloads.html
>> The Mega8 version should be easy to port to an Arduino clone
>> CFO
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> 
> -- 
> WBW,
> 
> V.P.
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