TMS320F28377D: CMPSS sub-system high and low comparators differences

Joel Holland said:

What is the difference between the upper and lower comparators in each CMPSS system? 

In the PDF https://www.ti.com/lit/ug/spruhm8h/spruhm8h.pdf, page 1613, the upper comparator has an additional Mux which is connected to a ramp generator - why?

I think you have correctly assessed the difference. The upper comparator has access to a Ramp Generator.

Joel Holland said:

In most code I see programmers set an upper and a lower threshold, to sense whether a signal is above a maximum or below a minimum value.

Can you not set the internal DAC of the two comparators to the same high level, and use them to sense two separate signals such as an overvoltage and then an overcurrent?

This way, you could save the amount of CMPSS that you power up, and write less code, since you can sense a maximum value on two external signals using one sub-system.

Windowed trip monitoring is an intended use scenario, hence the use of "windowed comparators" in the datasheet. However, there is the caveat that the two-port reference DAC is a shared resource between both comparators of the CMPSS and is thus subject to cross-disturbance, as described in the datasheet as "CMPSS DAC output disturbance."

Joel Holland said:

But my worry is that this is not allowed, since the hardware for the upper and lower devices is slightly different - would it be better to use two of the upper comparators in two separate sub-systems, say CMPSS1 and CMPSS2? 

The high and low comparators are functionally symmetric for trip monitoring.  Using multiple CMPSS instances might be helpful for mitigating the DAC output disturbance if similar threshold levels are needed, but keep in mind that the CMPSS module can only monitor one CMPINxP signal.