Cycle-Based Period Meter

The block starts a measurement on request, waits for a first rising edge, then counts system cycles until the next rising edge.

Behaviour

• i_signal first passes through a two-flip-flop synchronizer and edge detector.
• An i_start pulse is accepted while o_ready='1'.
• The first synchronized edge arms the counter without producing a result.
• The second edge completes the measurement. o_period_cycles contains the exact distance between the two synchronized edges.
• o_done pulses for one cycle with the result.
• If the period exceeds g_max_period_cycles, the result saturates to that value and o_overflow pulses with o_done.
• The active-high asynchronous reset cancels the measurement and clears the outputs.

The measured signal must remain low and high long enough to be observed by the system clock after synchronization. The circuit measures sampled edges, not a frequency beyond what the clock can observe.

Frequency can then be calculated with f_signal = f_clk / o_period_cycles. That division is outside this exercise.