I am tracing the operation of the ECU. I want to supply it with power but what is the ECU voltage input that I should supply?
It entirely depends on what your trying to do with it.
To power it on usually 9-14.5 volts will be fine.
However most modern (12years or so or newer) are smart enough to block certain functions unless there is at least 12.6v or in some cases 13.5v.
So as said it depends on what you want to do, if you want to power it up to test it with a diagnostic tool or its basic function just power it with a good fully charged 12v battery or even better a AC-DC power supply.
Put if you are trying to reflash/reprogram the ECU, fully test it or communicate with it then I would power it with a AC-DC power supply or battery support unit with around 13.5v