Hello all,

I am trying to learn how to do PWM with timers on 8051 architecture. My goal is this:

Generate a pulse with a width of 1.4 ms with frequency of 55 Hz on pin P1.0.


So after reading a number of tutorials, my logic went like this:

For my chip and crystal 1 machine cycle is 0.001085 ms.

I want the frequency to be 55 times/second so the period is:

1 / 55 = 0.018181818 or 18.181818182 ms.

The peak or high state should be 1.4 ms long. So the low state should be:

18.181818182 - 1.4 = 16.781818182 ms

So in terms of machine cycles, for 1.4 ms. I need:

1.4 / .001085 =~ 1290 cycles

and for 16.781818182 ms I need:

16.781818182 / .001085 =~ 15467 cycles


So my assumption is that the timer in Mode 1 (16-bit) will always overflow at 65535.

So for a 1.4 ms time I need to start the timer at:

65535 - 1290 = 64245 = FAF5 hex

And for the 16.781818182 ms time I need to start the timer at:

65535 - 15467 = 50068 = C394 hex



So I think the flow should be like this:

1. Reset everything.
2. Enable interrupts on Timer 0.
3. Set TH0 = 0xC3 and TL0 = 0x94 to initiate a low period.
4. Set a flag to 0.
5. Start the Timer.

When the timer overflows and interrupts do the following:

1. Set the Flag to 1.
2. Set TH0 = 0xFA and TL0 = 0xF5 to initiate a high period.
3. Clear the overflow flag.
4. Turn on P1.0.
5. Return.

When it overflows again then:

1. Set the Flag to 0.
2. Set TH0 = 0xC3 and TL0 = 0x94 to initiate a low period.
3. Clear the overflow flag.
4. Turn off P1.0.
5. Return.

So each time the timer overflows, the interrupt code checks the flag value and decides to initiate a high period or a low period.

To my mind this should generate the goal I am trying to achieve, but I am having no luck. Is my logic flawed? Am I making some bad assumptions? If anyone is willing to read all of this and check my logic I would be most appreciative. I also have the SDCC code that I am trying to use if it would help.