[time-nuts] Thunderbolt? (re simple gpsdo.)

Tom Harris celephicus at gmail.com
Sun Jan 1 21:52:07 UTC 2012


Kasper,

I like your style! Small ROM = no inessential features! I shall be
studying this code, getting capture working with no capture hardware
is a real pain.

On 2 January 2012 00:53, Kasper Pedersen <time-nuts at kasperkp.dk> wrote:
> On 01/01/2012 12:23 PM, Tom Harris wrote:
>
>> I too have been mulling over a minimal GPSDO.
>>
>> Has anyone mentioned a PWM DAC? Even a "toy" microcontroller like the
>> Atmel AVRs can generate 16 bit resolution PWM, which sounds like it
>
> ..
>
>> So a minimal GPSDO looks like a simple microcontroller (I would use
>> Arduino since we use these at work whenever we need to build something
>> in an afternoon) with a filter for a PWM DAC, so one opamp and a few
>> discretes. Of course as I write software for such beasts as a trade I
>> am giving the software a zero cost.
>>
>
>
> A 'bike light' sized microcontroller can do the job. Today I would have
> used something larger, and saved development time. On the other hand,
> having only room for 512 instructions prevented spending time on
> non-essential features.
>
> http://n1.taur.dk/simplexdo/
>
> I used 8 bit PWM into a two-stage RC filter, making it easier to filter.
> To get 16 bit resolution I added delta-sigma modulation on top of the PWM.
>
> The ATTiny13 is not the right part for the job; It has no input capture.
> So space- and time consuming tricks are necessary to get cycle-accurate
> capture.
>
> Recently I discovered that some of the microcontrollers in the drawer
> would do nanosecond capture all on their own, but with a working tbolt,
> need has been lacking.
>
>
> /Kasper Pedersen
>
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-- 

Tom Harris <celephicus at gmail.com>




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