Speed of PWM generated from 32khz clock

anil23 · ‎2006-04-06

Posted on April 06, 2006 at 12:06

anil23 · ‎2006-04-02

Posted on April 02, 2006 at 14:21

Hi:

I'm trying to generate 1800Hz out of the timer1 PWM output with 32khz internal clock(RTC clock). I learn that the max output that i could generate was close to 1100Hz. Is there any theoretical explanation for this limit?

If there is something I'm doing wrong, please direct me with the right direction (code)...

Regards,

edb · ‎2006-04-05

Posted on April 05, 2006 at 18:17

In order to get an 1800 Hz signal from a 32 kHz clock, you need a full output cycle every 17.7778 timer ticks. The closest integer is 18 which will give you a frequency of 1777.78 Hz. This means for a 50% duty cycle wave you need a transition every 9 ticks.

Assuming that you are using the 710 set up the timer this way:

Because the clock resets to FFFC, you need to subtract 4 from the timer tick value for the total length

Set the prescaler to 0 (divide by 1)

Set OCBR to 14

Set OCAR to 5 (or other value between 0 and 14 to set the pulse width)

Set up the OLVLA and OLVLB bits to select which part of the wave form has logic high level.

In the TIM1_CR1 register ...

Set OCAE to enable output on the pin

Set PWM to enable PWM output

Set EN to start the clock

As an alternitave, if you want a fixed 50% duty cycle wave, set both OCAR and OCBR to 5.

anil23 · ‎2006-04-06

Posted on April 06, 2006 at 12:06

Hi:

Thanks for the information. I used the setting recommended by you and was getting close to 1800hz.

I would use the same math to generate the other frequencies for DTMF(1633,1477,1336,1209,941,852,770,697Hz).I understand that 5 is being subtracted in the library(tim.c) if we use the TIM_PWMOModeConfig() routine to set the parameters for the timer. So, 4 shouldnâ€™t be subtracted for calculating the OCAR and OCBR.

Again, thanks for the help.