TMS320F28335: 28335

Dear Cody,

I also have same issue but is not possible without reducing frequency?

Thanks,

Dear Cody,

I have generated chopped pulse by writing software loop. Suppose I have 100us square wave with 50% duty cycle and I toggled that pulse every 20us using timer.

Here square wave not generated from epwm. Its generated by comparing timer generated triangle and reference. There is synchronization issue if I am using epwm module output for generating chopped pulses. The synchronization between epwm module and chopped gpio pin is not same so I am not getting synchronization between chopped pulse and original pulse.

So what I have done was generated own triangle using timer and compared with reference to generate pwm pulse. Then putiing another loop to generate chopped pulses on another gpio pin.

Can you let me know is it a good solution? (Generating own triangle wave and comparing with reference to generate pulses so that synchronization is there between generated pulse and chopped pulse). Below is the generated chopped pulses

Note: If I am using own generated triangle or ramp using timer with 20us then for 1000hz I am getting only 50 samples i.e discrete samples so even though I set duty cycle some value (52.5%) exact value not reflecting in output square wave because its not continuous triangle its formed by discrete points.

Thanks,

Siva,

I don't know what you mean by a "generated triangle" and "Reference"... If you explain it another way I may be able to comment more.

I think the best solution would be to use two PWMs, one running at the chopping frequency and the other running at twice the desired PWM frequency. The higher frequency(chopper) PWM should be connected to the desired output pin. Then the lower frequency PWM can enable and disable the higher frequency PWM in a CTR=PRD ISR. You could disable it using the trip-zone submodule or at the IO buffer, I can think of a reason one way would be better than the other.

siva swami said:

Note: If I am using own generated triangle or ramp using timer with 20us then for 1000hz I am getting only 50 samples i.e discrete samples so even though I set duty cycle some value (52.5%) exact value not reflecting in output square wave because its not continuous triangle its formed by discrete points.

Regards,
Cody

I think triangle means time base counter

Dear Cody,

Firstly we are using counter in up mode so indirectly we are generating high frequency triangle by using time base counter. Then we are comparing getting desired duty by comparing it with reference  ( by setting CMPA  or CMPB value to reference value ). So by setting CMPA  or CMPB values to reference, we are comparing high frequency triangle with reference signal to get deisred duty at the epwm output follow the pwm version of reference wavform.

Secondly there is limitation on chopping pulse with that each pulse should be at least 20us. So I cannot incraese chipping frequency yo high value.

let me know generating triangle using software code ( time base counter triangle ) and comparing to refrence to generate square wave is it ok? And issue of getting exact duty can be resolved??

I will share code with you if you need better understanding.

Thanks,

Dear Cody,

Even though I didn't convey that much clearly, you have understand quite well. Thanks for the efforts. Whatever you understood it's correct main issue is not using epwm.

Because inorder to eleiminate symcronization issues I have generated my own counter ( triangle) and I am comparing it with reference to get square pulse shown in red waveform above. I wrote software loop whenever it's high I am contiously toggling using software loop where we can control frequency of chopping and pulse width of chopping. Result of chopped pulse using software loop shown in green waveform above.

Only issue is the generated triangle will have discrete points it's depends on carrier frequency and ISR time period. If I put carrier frequency 1000Hz and ISR time 20us, I will get 50 points in which 25 for for counting up and 25 for counting down. So if I go higher time for isr or high frequency number of sample points reduces which will give inaccurate results even though synchronization issue resoleved using this method. ( Because while comparing refrence might not get in-between points since triangle or counter has discrete points that too less)

1. Is it a good way to generate own traingle ( counter) and produce chopped wavforms using software loop? (because by this I can eliminate synchronization isssues)

2. How to improve the accuracy without compromising carrier frequency ( frequency of traingle or counter) and timer ISR? (Counter or traingle accuracy depends on number of sample points generated which inturn depends on above factors)

I hope you understand. Issue of chopping for lower frequnecies has been resolved by the above implemention and same waveforms shown in previous mail with no issue of syncronization. Only concern is mentioned issues.

Thanks,