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pmoliveira
Posts: 4
Joined: Fri Dec 18, 2015 12:33 pm

PWM 19,2 MHz Max?

Fri Dec 18, 2015 12:40 pm

Hi.

Could somebody explain to me where did the 19,2 Mhz PWM Max freq. come from?

I can't find any literature regarding this.

Is it a default value? Is possible to change it?

The datasheet says: "Both modes clocked by clk_pwm which is nominally 100MHz, but can be varied by
the clock manager".

Many thanks.

Pedro
Lisbon - PORTUGAL

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joan
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Joined: Thu Jul 05, 2012 5:09 pm
Location: UK

Re: PWM 19,2 MHz Max?

Fri Dec 18, 2015 1:32 pm

I expect the 19,2MHz figure is from the physical crystal.

As far as I can tell the PWM signal is derived from the 500MHz PLLD (derived I presume from the crystal).

For reasons I have forgotten the maximum frequency appears to be 125 MHz (although it's unlikely that anything over 30 MHz will work on the Pi).

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pmoliveira
Posts: 4
Joined: Fri Dec 18, 2015 12:33 pm

Re: PWM 19,2 MHz Max?

Fri Dec 18, 2015 2:00 pm

Hi joan. Thanks for your reply.

After a while using bcm2835 and WiringPi libs, I'm starting programming in a more low-level way, and I got stuck here.

It's hard to find the right answers. This should be better documented.

Thanks,

Pedro

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joan
Posts: 15383
Joined: Thu Jul 05, 2012 5:09 pm
Location: UK

Re: PWM 19,2 MHz Max?

Fri Dec 18, 2015 2:10 pm

pmoliveira wrote:Hi joan. Thanks for your reply.

After a while using bcm2835 and WiringPi libs, I'm starting programming in a more low-level way, and I got stuck here.

It's hard to find the right answers. This should be better documented.

Thanks,

Pedro
It could be better documented but I doubt there is anyone left at Broadcom who has that responsibility. Some stuff you will need to discover by experiment.

gregeric
Posts: 1509
Joined: Mon Nov 28, 2011 10:08 am

Re: PWM 19,2 MHz Max?

Fri Dec 18, 2015 2:17 pm

I read an old post by Gert yesterday, where he said even he didn't understand how all the clocks work (or maybe the software that controls them), and he was the guy that designed the chip!

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