[time-nuts] Microchip PIC CTMU

Bob Camp kb8tq at n1k.org
Wed May 11 07:27:14 EDT 2016


Hi

The same process issues that make for a “less than perfect” ADC, also 
add more than a little noise into the circuit. Exactly which component(s)
are the biggest culprits … no idea. About all you can do is take data and
look at the results. 

Bob

> On May 11, 2016, at 4:55 AM, David <davidwhess at gmail.com> wrote:
> 
> Was that a couple of nanoseconds of resolution or minimum time
> interval?
> 
> Based on the simplified schematic which shows the current source being
> controlled by a single XOR gate, it looks to me like the minimum time
> interval is much longer than the achievable single shot resolution.
> 
> On Tue, 10 May 2016 21:37:08 -0400, you wrote:
> 
>> Hi
>> 
>> I have not dug into them for several years. Back when I did, you were pushing 
>> their limits as a couple of nanoseconds without getting into things like lots of 
>> averaging. 
>> 
>> Bob
>> 
>>> On May 10, 2016, at 8:29 PM, David <davidwhess at gmail.com> wrote:
>>> 
>>> I ran a search through the time-nuts archives and found only one
>>> mention of this a couple years ago asking the same question which is,
>>> has anybody explored the capabilities of the Microchip PIC CTMU for
>>> use as a time to digital converter or programmable delay?
>>> 
>>> The Microchip datasheets lack detailed performance specifications but
>>> my conservative estimate is that resolution down to 500ps over 200ns
>>> using the built in 10 bit ADC should be possible without complex
>>> calibration.  (The PIC series ADCs with more than 10 bits are pretty
>>> horrible.)
>>> 
>>> See What You Can Do with the CTMU:
>>> http://ww1.microchip.com/downloads/en/AppNotes/CTMU%2001375a.pdf
>>> 
>>> Overview of Charge Time Measurement Unit (CTMU):
>>> http://www.microchip.com/stellent/groups/SiteComm_sg/documents/DeviceDoc/en542792.pdf
>>> 
>>> PIC18(L)F2X/4XK22 w/10 Bit ADC:
>>> http://ww1.microchip.com/downloads/en/DeviceDoc/39977f.pdf
>>> 
>>> PIC18F66K80 w/12 Bit ADC:
>>> http://ww1.microchip.com/downloads/en/DeviceDoc/39977f.pdf
>>> 
>>> I prefer more discrete implementations but this might be useful for
>>> its higher integration, lower cost, and simplicity in less demanding
>>> applications.
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