Time measurement circuit

A time measurement circuit (digital timer / period meter) measures the time interval between two events by counting pulses of a known, accurate clock during that interval. If a clock of period $T_c$ gives $N$ counts, the measured time is $t = N \times T_c$.

Block diagram

 Start ──> ┌──────┐
           │ S-R  │ Q = gate
 Stop  ──> │  FF  │─────────┐
           └──────┘         v
 Crystal ─> Time-base ─> ┌─────┐
 oscillator divider      │ AND │
                         └──┬──┘
                            v
 Reset ─────────────> ┌──────────────┐
                      │Decade counter│
                      └──────┬───────┘
                             v
                      ┌──────────────┐
                      │    Latch     │
                      └──────┬───────┘
                             v
                      ┌──────────────┐
                      │BCD-to-7-seg  │
                      │dec. + display│
                      └──────────────┘

Blocks

1. Crystal oscillator – generates a very stable frequency (e.g. 1 MHz).
2. Time-base divider – chain of decade counters (÷10 stages) giving selectable clock periods such as 1 µs, 10 µs, 1 ms. This sets the resolution of the reading.
3. Start/stop control (S-R flip-flop) – the start pulse sets the flip-flop ($Q = 1$); the stop pulse resets it ($Q = 0$). So $Q$ is HIGH exactly for the time interval to be measured.
4. AND gate (main gate) – passes time-base clock pulses to the counter only while $Q = 1$.
5. Decade (BCD) counters – count the pulses passed by the gate.
6. Latch, decoder and display – the latch holds the final count; BCD-to-7-segment decoders show it on the display (usually multiplexed).

Operation

1. Before measurement the counters are reset to zero.
2. The start event sets the flip-flop, opening the AND gate. Clock pulses enter the counter.
3. The stop event resets the flip-flop, closing the gate. Counting stops.
4. The count $N$ is latched and displayed; the time is $t = N \times T_c$, so the display reads directly in µs or ms depending on the selected time base.

Example

With a 1 ms time base, if 2,350 pulses pass the gate, then

$$t = 2350 \times 1\ \text{ms} = 2.35\ \text{s}$$

Period measurement

For measuring the period of a signal, the input is shaped into pulses and applied to a toggle (divide-by-2) flip-flop; its output stays HIGH for exactly one period and acts as the gate. The accuracy is $\pm 1$ count plus the accuracy of the crystal.