On 3/2/13 2:52 PM, Tom Van Baak wrote:
Hi Jim,
I had a similar challenge a while ago. I ended up capturing a
4-digit, 7-segment display with a USB/LAN webcam, converting the JPG
to BMP, analyzing pixel gradients, matching the image with heuristic
masks, and appending an ascii log file with the 4 decimal digit
result once a minute. It worked amazingly well. I can send you the C
source code (contact me off-line). See also these legacy links:
http://leapsecond.com/webcam/
http://tycho.usno.navy.mil/ptti/ptti2003/paper35.pdf
More recently I used the same webcam to capture AC mains wall clock
time vs. a cesium clock:
http://leapsecond.com/pages/tec/mains-clock-ani.gif
Now, it may seem very old-fashion and crude to use a web cam to
capture data.
I've used that scheme more than once.. as well as similar approaches of
spinning wheels or counters driving LEDs to measure camera shutter timing.
But, if I understand correctly, in your case, you are more interested
using the external LCD display as a *proxy* for the internal crystal
oscillator.
exactly
Thus you are not concerned with the actual numeric value
that the LCD segments are displaying; you are simply interested in
measuring the time at which the LCD digits and segments change --
because that corresponds to the phase/frequency of the internal
oscillator.
yes
Measuring clock drift and tempco would be very easy and would produce
interesting results. The higher the rate of web cam capture the finer
the resolution of your TIC detection. Note that multiplexed displays
are not a problem; in fact, if you're clever it can actually improve
your resolution by observing the segment transition times. That is,
instead of *polling* the display, you use *changes* in the display as
your timing trigger. Edge detection. You move from a world of
periodic/gated frequency counters to a world of a reciprocal period
counters, or even time-stamping counters.
In other words, since you are interested in the underlying clock
performance rather than the RSA algorithm itself, just focus a few
photo-transistor(s) on the LCD segment(s).
That's what I was thinking.. the segments are pretty small, though, so
some optics and fixturing would need to be fabricated. The idea of
"point webcam at RSA fob and run software" was pretty appealing.
The transition from
light/dark/light/dark will exactly correspond to some internal
crystal clock phase/frequency, from which you can gather precise
long-term phase, frequency, and stability information. Edges are
always better than polling.
Yes indeed..
I can't think of a good reason why they would decouple the segment mux
from the underlying clock (unless there's some built in RC oscillator in
the chip doing the mux, separate from the timekeeping).
Actually, there's probably some information around on the internal
design of the fob.. After all, the security is in the algorithm, not the
hardware.
The idea is that segment *transition* instants rather than periodic
observation of segment *state* provides much greater accuracy. What
you want to do is measure the time of transition of individual
segments. I would use time-stamping rather than polling or time
interval or frequency measurement. That is, use a CNT-91, or picPET
(www.leapsecond.com/pic).
If you look at a 7-segment digit encoding table
(http://en.wikipedia.org/wiki/Seven-segment_display) you can see that
focusing on segment 'e' gives you 8 transitions out of every 10, and
using two opto-detectors on segments 'a' and 'e' will allow you to
generate a timing event for every change in LCD digit. This will be
several orders of magnitude more precise than using a web cam. You
don't need photo-transistors on all segments.
Even better, there's a little segment that blinks on and off every
second, as well as a stack of 6 that go away, one every 10 seconds, and
then, finally, the 6 digits. If I rig up an optical system, the 1 pps
segment is the logical one to look at.
Time to look for some suitable optics.
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