Lpc84x PWM max source clock frequency and duty_cycle

[mjx1995mjx]

    Im using lpc845-BRK to generate a PWM pluse, which need 250KHz frequency with high dutycycle accuracy. But 845 max clock is 30MHz, after approximate calculation I find that the dutycycle accuracy is 0.83%(250K/30M * 100%) and affirmed by oscilloscope. I have no find any PWM clock frequency multiplica.

    According to Lpc84x user manual(UM11029, page 122),' The loop filter filters these control signals and drives the current controlled oscillator (CCO), which generates the CCO clock. The CCO frequency range is 156 MHz to 320 MHz. This clock is divided by 2xP by theprogrammable post divider to create the output clock. '

Does user manual means that  I can use a clock which is the output clock of the CCO as PWM clock source? Or the clock source max frequency only is 30M, the CCO is not availble?    

If CCO can't be the source, is there any way to get a higher PWM clock source to increase the dutycycle accuracy?

Thanks for your help

[jeremyzhou]

Hi Peter Ma 

Thank you for your interest in NXP Semiconductor products and
for the opportunity to serve you.
Firstly, I have no idea with the duty cycle accuracy is 0.83%(250K/30M * 100%) you mention, please explains how to get it.
Next, I'd highly recommend you to use the SCTimer module to implement it, I think is definitely capable of doing it. In further, the AN11538 describes SCTimer/PWM applications and code examples, of course, it includes the PWM generation.
Have a great day,
TIC

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[mjx1995mjx]

Thanks for your help.

The '0.83%' is calculated by follow and I don't know if it's right.

          targat frequence/clock frequence * 100%, because the clock is 30MHz, per PWM period needs 120 clock period, the duty cycle's percent what I can change is 1/120(0.83%). I used  oscilloscope to observe that, 50% in program with 50.0% on oscilloscope, 51% in program with 50.8% on oscilloscope, 52% in program with 51.6% and so on. What I need is 0.01% or 0.1%, and now is just 0.8%.

AN11538 mention that some family(LPC15xx) has SCTPLL so could generate 72MHz, but I can't find this PLL in LPC84x.

Does LPC84x has SCTPLL or same PLL to make Sctimer clocks break through 30MHz?

Appreciate for your patience

[jeremyzhou · Accepted solution]

Hi Peter Ma ,

Thanks for your explanation.
1. I'm afraid that duty cycle accuracy can't achieve the 0.01% or 0.1% when the frequency is 250 KHz, as LPC845's maximum system clock is 30 MHz.
2. Does LPC84x have SCTPLL or same PLL to make Sctimer clocks break through 30MHz?
-- No, it doesn't have any similar PLL to make Sctimer clocks break through 30MHz.

Have a great day,
TIC

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