[time-nuts] Precisione GPS based led clock

[Tom Van Baak]

I also have had need of a high resolution display clock for
photographic time stamping.

For those of you working on this make sure not to use a
LCD or VFD display. The response time is too slow. Also
you can rule out any sort of TV display.

If using LED make sure not to multiplex the digits. This is
a common trick, especially when using microprocessors,
and works well for human eyes, but fails completely with
high resolution photos.

So having ruled out everything but direct drive LED the
only other concern is to make sure the decade counters
which drive the display are synchronous or at least that
all the digits are latched at the same time. Otherwise you
get false readouts due to ripple carry or sequential scan.

Be careful using a microprocessor for this. For millisecond
displays you need a total of 21 pins; for microseconds you
need 42 pins. An external serial-parallel (e.g., shift register
with latch) chip might be safer since neither a PIC nor an
Arduino can update that many pins in one instruction.

/tvb