1. i have a micrcontroller whose all pins are used only 7 left & have to use 35 led's on it. SO i htought of using Charlieplexing.
2. Attached is the circuit. Its using 35 leds on 7 pins (X1-X7). Charlieplexing allows n(n-1), so 7*6=42 leds. Since I am using only 35 leds so haven't connected rest of them. Is it ok?
3. I have reading on wikipedia for refersh rate. It says tyical rate for 1 led is 50Hz, so for 35 leds it would be 35*50=1750hz i.e around 571us.
So does that mean I have to make a timer interrupt of around 571us. On each interrupt one led will be brought to high or low depending on its state.
So consider if at any point all leds need to be high then in that one led seuence would be:
571us on & 19414us off (517*34=19414)
So this will cycle 50 times in a second.
4. Is there any limit to maximum leds that can be used in Charlieplexing.
Since as led grows, on time decreases (and so off time), but I think there would be minimum on time required for led to glow?
So lets suppose I am using 100 pins (this is extreme example just for theory) & at some point all leds need to be on
Then no of led's = 9900
Refresh rate = 9900*50 = 495000Hz
That for single led: on time-2us & off time 20ms. Is 2us pulse enough to turn on led?