CLOCK REG INFO PLEASE HELP

Hi, I am trying to set a low clock frequency for the PWM can anyone tell me where the register definitions are for the PWM0 clock, I have looked in the BCM2835 manual but cannot find anything, and from others code I have gleaned that there are some registers as follows


#define CLOCK_BASE(BCM2708_PERI_BASE + 0x101000)
#define PWMCLK_CNTL 40
#define PWMCLK_DIV 41

Thanks
Steve

[Gert van Loo]

The following code snippets are all from the Gertboard example code.
It is too big to put all here. To have a fully working PWM I suggest you get the full
example code and remove the parts you don't need.

#define PWMCLK_CNTL *(clk+40)
#define PWMCLK_DIV *(clk+41)

// Derive PWM clock direct from X-tal
// thus any system auto-slow-down-clock-to-save-power does not effect it
// The values below depends on the X-tal frequency!
PWMCLK_DIV = 0x5A000000 | (32<<12); // set pwm div to 32 (19.2MHz/32 = 600KHz)
PWMCLK_CNTL = 0x5A000011; // Source=osc and enable

So the core is the "(32<<12)". That sets your clock divider.
You can change the '32' to some other number but DON'T PLAY WITH THE OTHER VALUES!

You have to mmap the clk pointer before you can use that code.

Dear Gert

Many thanks, I tried changing it to 512<<12 but this had no effect,so tried 32<<12 and 64<<12, always the same. I guess I am doing something wrong. Is there another register I need to write to to have some effect ?

Regards
Steve

[Gert van Loo]

I am not sure what you are trying to do.
What do you expect to change? The PWM clock is just an arbitrary clock it runs off.
To control the PWM output you set a range and a value. Those two are the main control values of your PWM.
The clock only controls how long it takes for the PWM to go through its range.

Gert

Thanks for the input, I am trying to drive an L298 motor drive chip which has slow rise and fall times ( of the order of 1-2 uS) and as such having a clock frequency of 600Khz means that the chip is spending a lot of time transitioning between high and low, hence I am hoping to slow the clock frequency down to something like say 25khz. I could buy a different faster chip , but it seemed like just changing one value in software would be a lot easier.

Thanks
Steve

Gert

Tried changing the PWMCLK_DIV value but it still made no difference, so instead I have changed range value instead and that seems to have worked.

Thanks
Steve

I would like to set the PWM pin up to 38KHz, which is used for Infra Red transmitting - so I can use the RaspPi as an IR blaster.

What register values would I need to get it to work at 38KHz ?

Regards

Nick

[Gert van Loo]

How about you try a search with "clk PWM"

I wonder if it's the range that I need to mess with?

Regards

Nick

[Gert van Loo]

The clock you specify is the clock which is controlling the PWM, the signal data rate coming out depends greatly on the way you are planning to use the PWM to make your signal. How about you tell a bit more of what you are trying to do? What mode are you using the PWM interface in? How do you plan to make the signal?

.

I'm trying to port some Arduino code over to the Pi. It's an InfraRed transmitter, sending NEC format IR codes.
The PWM needs to be 38KHz.
And, the 1 and 0 are transmitted by varying the length of time that the PWM is turned on ( I think ).

So, first, I guess I need to ensure that 38KHz is actually coming out.
Then, I need to check the timing of on/off .

If I understand he wiringPi example code, it sets a range of 0 to 1023. So, If I set the duty cycle to 512 then that's 50%, which should produce a square wave with equal periods of on and off.

Regards

Nick

[Gert van Loo]

I'm trying to port some Arduino code over to the Pi. It's an InfraRed transmitter, sending NEC format IR codes.
The PWM needs to be 38KHz.
And, the 1 and 0 are transmitted by varying the length of time that the PWM is turned on ( I think ).

So, first, I guess I need to ensure that 38KHz is actually coming out.
Then, I need to check the timing of on/off .

If I understand he wiringPi example code, it sets a range of 0 to 1023. So, If I set the duty cycle to 512 then that's 50%, which should produce a square wave with equal periods of on and off.

Regards

Nick

Thus if set-up like that your PWM output frequency will be 1/1024 th of your PWM clock frequency.

Waheey. I've almost got it to work.

It now produces very intermittent IR codes. The problem I have now is with the timing of the pulses. They are not regular enough, so the receiving end of the IR is only accepting about 1 in 10 of the IR bursts.

It looks like I need something more accurate than nanosleep for my purposes.

Regards

Nick

[Gert van Loo]

You can DMA into the PWM.

I'm not sure that's going to help me.

The IR bits are transmitted by varying the length of time between bursts of 38KHz .
The length of each burst of 38KHz is the same each time, but between each burst the PWM is turned off for a precise length of time. A zero is represented by a shorter time than a one.

So, I don't think DMA can do that for me, can it?

Regards

Nick

[Gert van Loo]

I think it can, but it gets rather convoluted.
Read the DMA suggestion for the panalyzer:
http://www.raspberrypi.org/phpBB3/viewtopic.php?f=37&t=7696#p113732

I notice that there is also a 64bit free running counter available.
I presume this can be mmap'ed to make it available in user space.
That might make a more accurate timer without too many changes to my code.

Do you know what the offset for it would be ?
I see that the hardware address for counters starts at 0x7E003000 . So, would that be 0x20003000 in /dev/mem ?

Regards

Nick

Right,
Ok, so using the free running clock is much more accurate with the timing.
Now, the IR Receiver understands the IR commands 9 times out of 10. Which is good enough for my uses.

Thanks.

Nick

I don't get it completely.

I think:
You divide 19,2 MHz by a prescaler 32, so you have 600 KHz "tick"
If I use a range that is 600000 I have a timebase from one second?
So if I want half second on, half second off I use 300000 as value, but I can't find what's wrong

Hi @nickon314

Are you able to publish your IR code somewhere so we can see how you achieved a 9 out of 10 transmit success rate?

Cheers heaps!
Dave

Hi @nickon314 or @dtbaker.
I too would like to understand better how I can use the PWM on the Gertboard to drive an IR. My IR receiver is 40Khz, how did you manage to setup the clock frequency on the gertboard to get a carrier frequency of 40Khz (I believe in your case it needed to be 38Khz).

Thanks !
Roland

Hi @nickon314 or @dtbaker or @hollmanr

I am trying to drive the Ir bluster by PWM output. Please share your codes or concept.

Thanks
anirban