PWM assembly code for generation of 5 PWM signals in 8051 mc

I exactly want what Mr.Graham told. A software method. I don't want a very fast system. The period can be even upto 2 second. But the duration of PWM about 20-100ms. Is it possible to have such a system? Is it possible using only one timer? Can you explain some logic for that. Actually, I want that both period and duration of all the 5 PWMs to be different. The values for duration and period for 5 signals(5*2=10 values) should be taken from some memory location where we are already stored the corresponding values for period and duration.

One way or another, you need five timers - one for each PWM. You don't have five hardware timers, so it is necessary to "simulate" these timers in software. Each timer is simply a variable in memory.

If it helps, think of each timer as being like the second hand on a clock. The second hand moves with every tick and after 60 ticks it is back where it started. To generate a PWM from the second hand of a clock, the output is simply a matter of how far round the clock the second hand has travelled. To put out 25% PWM, the output will be on for the first 15 seconds and then off for 45 seconds, and so on.

The hardware timer you do have can provide a "tick" that triggers an interrupt. It is slightly more efficient to count down than up, therefore I suggest a simulated timer that runs backwards. You will need something like this for each PWM timer:

    MOV    A,pwm_timer_n
    DJNZ   A,xxx
    MOV    A,#pwm_timer_n_period
xxx:
    MOV    pwm_timer_n,A

Now, the only remaining question is whether the coresponding PWM output should be on or off. This is simply a question of whether the value of the timer is greater or less than a threshold value. You will need something like this for each PWM timer:

    MOV    A,pwm_timer_n
    CJNE   A,pwm_on_time_n,$+3
    MOV    pwm_output_bit,C

Note that here the CJNE instruction is just being used to set or clear the carry as a result of the comparison.

That should be enough to get you started although there are plenty of details to to fill in.

I exactly want what Mr.Graham told. A software method. I don't want a very fast system. The period can be even upto 2 second. But the duration of PWM about 20-100ms. Is it possible to have such a system? Is it possible using only one timer? Can you explain some logic for that. Actually, I want that both period and duration of all the 5 PWMs to be different. The values for duration and period for 5 signals(5*2=10 values) should be taken from some memory location where we are already stored the corresponding values for period and duration.

One way or another, you need five timers - one for each PWM. You don't have five hardware timers, so it is necessary to "simulate" these timers in software. Each timer is simply a variable in memory.

If it helps, think of each timer as being like the second hand on a clock. The second hand moves with every tick and after 60 ticks it is back where it started. To generate a PWM from the second hand of a clock, the output is simply a matter of how far round the clock the second hand has travelled. To put out 25% PWM, the output will be on for the first 15 seconds and then off for 45 seconds, and so on.

The hardware timer you do have can provide a "tick" that triggers an interrupt. It is slightly more efficient to count down than up, therefore I suggest a simulated timer that runs backwards. You will need something like this for each PWM timer:

    MOV    A,pwm_timer_n
    DJNZ   A,xxx
    MOV    A,#pwm_timer_n_period
xxx:
    MOV    pwm_timer_n,A

Now, the only remaining question is whether the coresponding PWM output should be on or off. This is simply a question of whether the value of the timer is greater or less than a threshold value. You will need something like this for each PWM timer:

    MOV    A,pwm_timer_n
    CJNE   A,pwm_on_time_n,$+3
    MOV    pwm_output_bit,C

Note that here the CJNE instruction is just being used to set or clear the carry as a result of the comparison.

That should be enough to get you started although there are plenty of details to to fill in.