beale wrote:
I see Microchip application note AN1375 at
http://ww1.microchip.com/downloads/en/AppNotes/01375a.pdf suggests that the
CTMU module included in some recent PIC parts, for example PIC18F46J11 can be
used as a TIC building block for interpolating time intervals with sub-ns
resolution. I gather this module was originally designed for touch sensing
applications; they also mention measurement of absolute and relative
capacitance.
The application note merely asserts the possibility, but neglects to present a
specific design. Has anyone here attempted to use a PIC CTMU in that way?
I've been studying Richard McCorkle's PICTIC-II design (1) with an eye to
implementing the digital parts in a CPLD, and so far have a start/stop counter
working (2). I haven't yet started on the analog part of the circuit, but now
I'm wondering if Microchip has already done the work for me, in a $4 part ? (3)
(1) http://www.ko4bb.com/dokuwiki/doku.php?id=precision_timing:pictic
(2) http://bealecorner.org/best/measure/time/CPLD-TIC/
(3)
http://parts.digikey.com/1/parts/1671850-ic-pic-mcu-flash-64kb-44-tqfp-pic18f46j11-i-pt.html
Spotted that feature several months ago.
It should work well to within the limits of the CTMU module design.
IIRC the external capacitance needs to be kept low for sub ns resolution.
Crosstalk from within the PIC chip as well as ground and Vcc bounce may
be an issue.
A statistical fill the buckets linearity test will allow evaluation the
integral and differential nonlinearities in such a TDC.
In this case a relatively unstable source is required to enable such
testing.
One of the resistance tuned silicon oscillators available from LTC and
elsewhere should work well.
Bruce
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