How do I start a timer after power-on, or when the input signal goes away?

How do I start a timer when the SigmaDSP powers on, and then output a GPIO flag when the timer is complete? This could be used, for example, in power sequencing for a system that requires staggered startup times between functional blocks.

Also, how would I start a timer when the audio input signal goes away, so that I could mute other parts of the system via GPIO after a set period of time?