A simulator will allow you to verify the precise behavior of your firmware within the task, but will tell you nothing about the interaction with the hardware.

In a case wherein you are ABSOLUTELY CERTAIN that the hardware worksin a particular and well-proven way, the simulator will assist you in ensuring that your firmware deals with the inputs in the way in which you wish. It will also allow you to verify that it produces the outputs that you wish.

However, it will tell you nothing about whether the hardware will or even should work properly. If you want to simulate the interaction of the remaining circuitry with the MCU, you must have a simulator that can do that, e.g. SPICE, and an appropriate model for each and every component in the circuit. You must also have a precise specification for the inputs, and a means for stimulating the simulation with in exact accordance with those specifications. Once you have those elements in place, you can verify that the circuitry SHOULD behave in a prescribed way. However, there's no guarantee that your external circuitry or signal sources behave as specified unless you've verified that beforehand.

RE