TMS320F2800156-Q1: VREGENZ

The VREGENZ pin is the active low enable to the internal 1.2V LDO.  If you only supply 3.3V to the VDDIO/VDDA pins, you can keep this low so that the internal LDO is "enabled" and 1.2V will come from inside the device.  There is a requirement of local bulk capacitance to keep the output stable across the operating conditions of the device.

If this pins is pulled high(tied to VDDIO), then the internal LDO is disabled and the expectation is that the 1.2V is supplied off chip, via an external regulator.

Your observation of VREGENZ = 3.3V and the MCU doesn't run is expected if you have no external 1.2V supply.

The second observation, the ADC interrupt doesn't occur, likely has nothing to do with the VREGENZ pin/state, but some configuration of the ADC.

Keep in mind that VDDA(analog supply) also requires 3.3V for ADC to function.

Best,

Matthew