I don't think the ECU should control the power to the other PCB devices, the key should do that. The ECU should just control whether it is asleep or awake based on an interrupt from a trigger input from the key supply. You could potentially use that knowledge to keep, say, a fuel pump, or cooling fan, or electric water pump on after shutdown until things cool off, or for fixed delays. The main point, though, is to let it shut itself down such that information about time of shutdown, the last run time, the shutdown conditions can be remembered, and hot start better catered for by way of inference from previous conditions.
Now, as for how to divide it up you have three things :
- Constant battery power to supply the CPU and what else?
- Key switched battery power to tell the CPU that it should wake up, and power what else?
- One input pin on the CPU to receive the key switched signal.
- It doesn't matter much how much power is used when the key is on, but it does matter very much how much is used when the key is off, it should be absolutely minimal.
- The vref signal should match the analogue sensor supply, but should also match the CPU supply pretty closely.
- It doesn't matter much, if at all, if the analogue sensor supply and vref come off different regulators, it will be close enough by definition anyway
- Anything else?
Fred.