I'm designing a device that will have an IR photodiode connected to a low power microcontroller's ADC pin. At times, another device will be transmitting a 48KHz square wave, and I'd like to be able to detect when that is the case entirely in software, as well as being able to detect individual high and low transitions.
My understanding is that a PLL is the correct solution to this problem. However, I have extremely limited DSP knowledge, so I have a fuzzy idea at best on how to actually implement this in software, and much of the documentation available goes well over my head with a lot of domain specific terminology. This is further complicated by the low power nature of the microcontroller - I have limited processor cycles available, and thus I'd like to sample as infrequently as possible while still reliably locking on to the signal. If it helps, the sampling timer can be adjusted to match the observed frequency of the input signal.
Can anyone suggest an approach to this that is as straightforward and understandable as possible, while also minimizing CPU resources consumed?