Re: [rtl] help for a newbie

[Ron <rgray@intermind.net>]

Loren Frank wrote:

> The frame grabber can indeed service the interrupts without a problem, but I
> need to know, with an error of at most 1 or 2 ms, exactly when the frame was
> taken. I know that I would be likely to be within that margin 99% of the
> time, but I really need consistent precision....

I'm assuming the video is coming from a continuous source, i.e. video
camera, which would mean that it takes 16ms to acquire a field. Thus
your frame "clock" only has 16ms resolution anyway.

If the video source is not continuous, you still have a 16ms window
in which to mark that field with a time code before the next possible
time code interval could arrive. If your trying for "strobe" type
image acquisition, i.e. an external event triggers the camera, then you
really should add a little piece of hardware, like a counter
triggered by the strobe. I've had success using a 6522 (wired to an ISA 
bus no less) that is triggered by the "event" signal, then when
the unix box gets around to processing the interrupt, you read your
time base then subtract the time accumulated in the counter, a very
simple modification to any kernel interrupt handler. I've used something
similar for some traffic cameras controlled from a BSD box.