CrossRoads:
Use a counter input, count the number of pulses that occur in 1mS.
440 & 540. Both will wrap around on an 8-bit counter - so ~185 & ~285 get returned instead.
Use a counter and pick off the appropriate counter stage to measure. Example: dividing the input by 16 will give a cycle time of 29 or 36 microseconds, depending on the frequency.