topic Re: PWM Cycle in 8-bit Microcontrollers

PWM Cycle

rapa — Sun, 04 Jan 2009 22:57:56 GMT

Hi All! I am working with MC9S08SH8 micro. In processor expert, i set period to 15 ms.Is it cycle time? I am asking,because i have measured PWM cycle by scope and i saw, that cycle is 7.5 ms Starting pulse width i set to be 100 Hz. Am i write?

Re: PWM Cycle

peg — Mon, 05 Jan 2009 05:56:11 GMT

Hi rapa,

There seems to be some mangling of terms here.
Period and cycle in this context should mean the same thing, The time to complete one cycle or until it repeats itself. e.g. the upward curved AND the downward curved parts of a sine wave.
For PWM it is the on time and the off time added together.
With PWM, the cycle time remains constant while the duty cycle changes. It can usually go from 0 to 100%. At 0 and 100 it is no longer a waveform but simply DC.
I don't know why you are talking of the pulse width in Hertz. Pulse width is a time or, in PWM as the frequency is fixed, a percentage.

Many people, both on these forums and elsewhere, seem to refer to any square wave as PWM which is wrong.
PWM stands for Pulse Width Modulation. That is, the only thing you are changing (or modulating) is the pulse width. The frequency (or cycle time or period) remains fixed.

Re: PWM Cycle

ProcessorExpert — Mon, 12 Jan 2009 20:10:20 GMT

Hello rapa,

I have not reproduced such behaviour. Could you please send us more detailed information how to reproduce this problem, including information about CPU and PWM beans setting. What hardware are you using (EVB board, value of external oscillator/crystal)? What version of CodeWarrior and Processor Expert are you using? The project reproducing the problem will be the best.

best regards
Vojtech Filip
Processor Expert Support Team
UNIS