TMS320F28377D: Questions about Cycle-by-Cycle in the datasheet

Hi Julia,

N here refers to threshold setting/value of the CMPSS filter assuming you get a continuous stream of 1s or 0s. If the 1s or 0s are not continuous, then N here refers to the sample window setting/value. The CMPSS filter changes state when the number of 1s or 0s in the sample window exceeds the threshold setting. Please read the CMPSS section for this.

Essentially what the TRM is saying is that you need to design your application such that the CMPSS filter will resolve it's output before the next cycle as the CMPSS filter essentially introduces a delay in the trip or untrip.

For example let's assume your PWM period is 20 clocks long, sampwin=10, thresh=7 and comparator output is initially low at the beginning of the first pwm cycle.

Let's assume at the start of cycle 1 i.e clock = 0, the comparator trips. the pwm does not see this trip until thresh clocks later i.e 7 clocks later. Same argument applies to the untrip situation. Let's assume you are 15 clocks into cycle1 and the comparator untrips as it gets ready for cycle 2. The pwm will see this untrip 2 clocks into cycle 2 (5 + 2). At the beginning of cycle 2, the pwm still sees a trip condition even though the comparator untripped 5 clocks ago. The TRM is saying that you need to design your application such that cycle 2 doesn't begin until the untrip condition has been registered by the pwm

If the description above is not clear, let me know and i can sketch something up.