want to display 1,2,3,4 on four seven segemnt display connected to 8051 in such way that the display reads as "1234", but I am not able to write delay routine which refreshes the displays fast enough that our eyes is not able to catch it, kindly guide. using 12Mhz crystal.
I've done something along those lines.
I illuminated a digit for 750µs, followed by 250µs of dark time. Then I'd switch to the next digit. 8 digits overall, meaning every digit was updated every 8ms, that's a 125Hz refresh rate. No flickering.
I used plain old timer 0 to generate a 250µs heartbeat. Nothing fancy, really.
Never use a delay for a realtime application. The very concept is flawed. You want to control the interval at which your code is executed, not the time it is _not_ executed.
Some people are more sensitive to flicker than others.
125Hz sounds a bit low to me, considering that many people complain of flicker from standard fluorescent lamps - which is 100Hz over here.
Erik has much experience in this area...
Personally, I find that I see the flicker in lots of the LED exterior lamps used on cars mowadays. It's not so much when I look directly at them, but when they cross my field of view...
Without the dark time, you might get "ghosting" from one digit to the next.
A 250µs gap with 750µs illumination sounds a bit large - but I guess that was down to your timer granularity?
No, I started with 1ms and no dark time. As you say there was ghosting. So I switched to 750/250, there was no ghosting and no recognizable loss of brightness. So I stuck with it. It was a demo application on a tight schedule, only to be used once. No reason to do it perfect. The pain threshold for usable heartbeats would have been somewhere between 5 and 10µs (I use 5µs for flash writing). So switching to something like 950/50 wouldn't have been an issue.
There's a picture: sourceforge.net/.../rollout.jpg
125Hz sounds a bit low to me, considering that many people complain of flicker from standard fluorescent lamps - which is 100Hz over here. I once had a bunch of people looking at a 24*132 LED sign. all saw flicker at 75Hz some saw flicker at 100HZ none saw flicker at 120Hz
Erik