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

[Nick Sayer]

> On Jan 14, 2016, at 4:11 AM, Bob Camp <kb8tq at n1k.org> wrote:
> 
> Hi
> 
> The way pretty much all of these work is to take the “10 MHz” in on the clock input port. 
> The critical spec is the upper frequency for an external clock input. With some chips this
> is in the vicinity of 50 MHz. On others it tops out at 4 MHz.

For the AVRs I generally use (including the ATTinyx5s the subject here), it’s 10 MHz if you’re running at 3.3 volts and 20 MHz at 5 volts.

> 
> The next step after wiring it up is to check the jitter on the resulting output. It would be 
> nice if all input circuits were designed equally well. There is evidence out there that this
> is not the case…..In some cases the performance can be improved by feeding the MCU
> input with a high slew rate signal rather than a sine wave. About all that takes is a single
> gate. 

My testing last night (I described the methodology elsewhere) went to the limits of my TIA. The output from my GPSDO is a square wave, so I didn’t condition the input. In the past what I’ve used for that is a DC blocking cap, followed by a Thevenin termination (100Ω to ground and Vcc) feeding a 74LVC1G17 Schmitt trigger buffer. I don’t know if that’s up to Time Nuts standard or not. The one downside I’ve seen with it is that it kind of requires a larger input amplitude than is sometimes convenient.

> 
> Bob
> 
> 
>> On Jan 13, 2016, at 11:11 PM, Bryan _ <bpl521 at outlook.com> wrote:
>> 
>> Nick, can the Attiny25 divide down a 20Mhz input. I understand the 12fxxx pics max out at 20Mhz on a input so not sure if they would be suitable for my purpose. Can't seem to find anything in the datasheet for the tiny that explains the maximum frequency on a input pin.
>> 
>> -=Bryan=-
>> 
>>> Date: Wed, 13 Jan 2016 12:12:39 -0800
>>> To: nsayer at kfu.com; time-nuts at febo.com
>>> Subject: Re: [time-nuts] Generating a solid PPS from 10Mhz source
>>> From: time-nuts at febo.com
>>> 
>>> Just shy of a half dozen folks have asked, so I'll post here as soon as I finish cleaning it up. I'll put it on Github when it's ready. I just need a day or two. 
>>> 
>>> Sent from my iPhone
>>> 
>>>> On Jan 13, 2016, at 6:43 AM, Nick Sayer via time-nuts <time-nuts at febo.com> wrote:
>>>> 
>>>> If anyone is interested in the equivalent functionality using an ATTiny25 (for instance, if you’re already heavily invested in AVR instead of PIC, like I am), ping me. I’ve privately written code to solve almost the same problem and it could easily be adapted into doing the same job.
>>>> 
>>>>> On Jan 13, 2016, at 5:23 AM, Edesio Costa e Silva <time-nuts at tardis.net.br> wrote:
>>>>> 
>>>>> Hi!
>>>>> 
>>>>> Try TVB's picDiv at http://www.leapsecond.com/pic/picdiv.htm
>>>>> 
>>>>> Edésio
>>>>> 
>>>>>> On Wed, Jan 13, 2016 at 09:22:09AM +0000, Jerome Blaha wrote:
>>>>>> Hey Guys,
>>>>>> 
>>>>>> Is there an easy circuit to build that can consistently deliver a 1 PPS from a 10MHz source with excellent resolution and repeatability?  My first application is to test different 10MHz oscillators without a TIC always attached and then compare the PPS output change over time against a master GPSDO PPS with an HP53132A.
>>>>>> 
>>>>>> The circuit used for PPS generation would have to deliver consistent PPS output with preferably not more than 100ps noise or jitter, assuming a perfect source.  I'm totally guessing that for this resolution, the PPS would have to be generated and accurate to within 0.001Hz every second.  If this is too difficult, maybe the integration time can be increased to generate one pulse every 10second or every 100,000,000.00 cycles?
>>>>>> 
>>>>>> Finally, is a square 10Mhz reference any better in this case than a sinusoidal input for generating the PPS?
>>>>>> 
>>>>>> Thanks,
>>>>>> Jerome
>>>>>> 
>>>>>> _______________________________________________
>>>>>> time-nuts mailing list -- time-nuts at febo.com
>>>>>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>>>>> and follow the instructions there.
>>>>> _______________________________________________
>>>>> time-nuts mailing list -- time-nuts at febo.com
>>>>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>>>> and follow the instructions there.
>>>> 
>>>> _______________________________________________
>>>> time-nuts mailing list -- time-nuts at febo.com
>>>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>>> and follow the instructions there.
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at febo.com
>>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>> and follow the instructions there.
>> 		 	   		  
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
> 
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.