TMS570LC4357: ECAP Duty Cycle for differing pre-scales

The ECAP is to measure the period and duty cycle of the input signal. It is not able to the change the input signal. 

How do you generate the input signal for ECAP? 

You can use PWM to output a PWM signal with different period and duty cycle, and use ECAP to capture those value.

Hi Wang,

I am giving a frequency and duty cycle to ECAP 1 pins from a function generator and trying to capture those values using the ECAP with CAP reg's.

I am able to capture the frequency but not the duty cycle. I am using the CAP registers to calculate duty cycle.

duty cycle = (CAP3 - CAP1)/(CAP 2 - CAP 1)

What are your CAP settings?

If CAP1 is rising edge, CAP2 the falling edge, CAP3 the 2nd rising edge,

DutyCycle(ON) = (CAP2 - CAP1) / (CAP3 - CAP1)

DutyCycle(OFF) = (CAP3 - CAP1) / (CAP3 - CAP1)

Below are my CAP settings

CAP 1 - Rising Edge

CAP 2 - Falling Edge

CAP 3 - Rising Edge

The duty cycle is always captured to 50%, when I am using with pre-scale.

The formula below in your post is not correct:

duty cycle = (CAP3 - CAP1)/(CAP 2 - CAP 1)

I am using the correct one, I typed the wrong formula

i.e., 

duty cycle =(CAP 2 - CAP 1) / (CAP 3 - CAP 1)

I am able to capture the correct duty cycle with no pre scale, but when I add prescale the duty cycle is wrong.

Thanks for clarification. Your observation is correct. If the prescale is enabled, the dutycycle is always 50%. Prescale is used to measure the period of high frequency signal, but not the duty cycle.