CCS/TMS320F28027: Difference in cmpa

Hello, Madhuri

At first, I think you meant

Epwm1Regs.CMPA.all = 600;
Epwm1Regs.CMPA.half.CMPA = 600;

There is a difference. CMPA is divided into two parts: CMPA for regular ePWM counter-compare and CMPAHR( where HR means High Resolution) for High-Resolution compare. There is an ability to divide each TBCLK into more clocks (up to 255, i guess), so you can achieve incredible accuracy in PWM signal generation. You can refer to documentation on ePWM and HRPWM modules.

So if you have PRD = 10000, and want to genereate 50% duty cycle, you must write 

Epwm1Regs.CMPA.half.CMPA = 5000;

If you want to achieve something like 50.00054% duty cycle, you will have to write into CMPA.half.CMPAHR register. Of course, you need to set up HR submodule in first place.

I also think, that the first way won't even build. 

Epwm1Regs.CMPA = 600;

You can write any value to EPwmXRegs.CMPA.half.CMPA. But this value must not be larger, than your period.

Just to make sure:

Let's assume we have TBPRD = 200 and counter is counting in UP-COUNT mode. In this case, what is the accuracy of your PWM? It's (1/200)*100% = 0.5%

That means, that you can set CORRECTLY any duty cycle with step 0.5%: you can set 0.0% (CMPA = 0), 10.0% (CMPA = 20), 13.5%(CMPA = 27), 30.0% (CMPA = 60)... 99.5%(CMPA = 199) 100.0% (CMPA = 200).

All of this CMPA must be written in EPwmXRegs.CMPA.half.CMPA register.

But if you want to set more accurate duty cycle, for example 10.2% - you are not able to do it. Because in this case you need CMPA = (10.2 / 100%) * TBPRD = 0.102*200 = 20.4. But CMPA can only be integer value, so you can set only CMPA = 20 or CMPA = 21 (10.0% and 10.5% respectively). 

So in this case High Resolution Module can help: you can set CMPA = 20 (10.0%) and than set CMPAHR to add this missing 0.2% percent.