28X short delays

tim griff

Member
I'm building 3no NIXIE tube clocks. I've worked out most of the code including how to decipher the DCF77 signal! ands set the clocks

However the PCB's I bought multiplex the tubes in pair ( 3 pairs - 6 tubes in total)
The board takes a pre programmed PIC (inc 4MHZ res). This is bit I'm replacing with a 28X. The instructions refers to the duty cyle of the tubes; these are
full brightness on- 600 microsecs/ off 233microsec
low brightness on - 119microsec / off 680microsecs

I know the new 28x1 chips can have microsec delays, but I've bought the standard 28x chips.
The programme will be small and I can build some of the code into the off cycles
Any thoughts on how to implement these short delays.

Regards
Tim
 

hippy

Senior Member
The duty cycle is the ratio of on time to off time, the higher the ratio the greater the apparent brightness will be. Unless there's something special about Nixies, it's the ratio which sets the brightness, not the actual times, so you can approach this in a different way.

If you know how long it takes to cycle through your multiplexed Nixies to have dealt with them all, that will give a refresh time. That specifies the off period, so from that you can determine what the on period must be to give the required duty ratio. Simply stay with a particular Nixie for that time to give the required on period.

Obviously the change to the on-time alters the refresh period and it all goes round in circles to get the numbers right. There's probably some equation to calculate this but should be easy enough to work out approximately.

The only issue then is what your final refresh time is. If that is too slow the Nixies may appear to flicker. Run the 28X faster and you can do more work in a time period, go through all your Nixies quicker, reduce the on/off periods and improve the overall refresh rate.

The PICmicro program has a 1mS refresh rate, and that might be hard to match on a 28X, especially if it's doing other things. Hopefully a slower refresh rate won't cause too many problems.

If you want to dim your Nixies, simply extend the refresh time while keeping the on periods constant. You're probably going to have to do some experimenting to find what does and doesn't work, and I'm not familiar with driving Nixies.
 

tim griff

Member
Hi Hippy,
Thanks.. I'll experiment. The times seem short to me as the Nixie's are in effect discharge tubes and can accomodate some "flicker".

I had thought about puting in a small "for next" loop instead of pause.

Thanks again
Tim
 
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