PWM Frequency Problem

Suraj143 · May 26, 2013

Guys I have used the PWM module & I have some basic knowledge.

I'm working according to below formulas.

PWM period = [(PR2) + 1] • 4 • TOSC • (TMR2 prescale value)

PWM duty cycle = (DCxB9 : DCxB0 bits value) • Tosc • (TMR2 prescale value)

My question is datasheet says when you go for higher PWM frequency you must decrease the resolution so I cannot use full 10 bits resolution.

Why is that?

Pommie · May 27, 2013

An easier way to think about the PWM resolution is that the duty cycle can go from zero to (PR2+1)*4 so for the best resolution use as large a value as possible in PR2 and use the prescaler to adjust the frequency.

The reason the resolution decreases is because the duty cycle is compared to timer2 and timer2 resets when it matches PR2 therefore any value greater than PR2 will result in 100% duty cycle.

Mike.

Suraj143 · May 27, 2013

Pommie said:
An easier way to think about the PWM resolution is that the duty cycle can go from zero to (PR2+1)*4 so for the best resolution use as large a value as possible in PR2 and use the prescaler to adjust the frequency.

The reason the resolution decreases is because the duty cycle is compared to timer2 and timer2 resets when it matches PR2 therefore any value greater than PR2 will result in 100% duty cycle.

Mike.

Very nice easier to understood.

Thanks mike

dr pepper · May 28, 2013

It took me a while to suss out the pwm module, perosnally I found the datasheet very difficult to understand.
Theres a couple of websites that explain its operation well.
Some pics you can define the o/p of the pwm to a choice of pins.

PWM Frequency Problem

Suraj143

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PWM Frequency Problem

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