Chronometer (Time Units)

Introduction link

This block is a stopwatch that speaks real time. You tell it the frequency of its clock (Clock Frequency, in Hz) and the unit you want to read (Time Unit: h, m, s, ms or us); TIME then counts elapsed hours, minutes, seconds, milliseconds or microseconds directly, with no conversion needed downstream.

Three options choose how it behaves:

• EVENT pulses for one clock every time TIME advances by one unit.
• PPS (pulse per second) pulses for one clock on every elapsed second of measured time, whatever the selected unit.

The time base is exact for any integer clock frequency: when the number of clocks per unit is not an integer (for example 62.5 MHz on the us scale = 62.5 clocks per microsecond) a fractional accumulator spaces the ticks 62 and 63 clocks apart so that the average is exact and the error never accumulates.

$$ \mathrm{TIME} = \left\lfloor \frac{N_{\mathrm{clk}}}{f_{\mathrm{clk}} \cdot T_{\mathrm{unit}}} \right\rfloor $$

where $N_{\mathrm{clk}}$ is the number of clocks counted while running and $T_{\mathrm{unit}}$ is the selected unit in seconds.

Pin Description link

Properties link

Functional description link

Time base link

Every counted clock adds $R$ to an accumulator, where $R$ is the number of base ticks per second (1 000 000 for us, 1 000 for ms, 1 for s, m and h). When the accumulator reaches the clock frequency $f$ a base tick is produced and $f$ is subtracted. Over exactly $f$ clocks this gives exactly $R$ base ticks, so the rate is exact for every integer $f \ge R$.

• us, ms, s: every base tick is a unit tick.
• m: one unit tick every 60 base ticks (seconds).
• h: one unit tick every 3600 base ticks (seconds).

On a unit tick TIME increments and EVENT pulses. PPS pulses on every second of measured time: with s, m and h that is every base tick, with ms every 1000 ticks, with us every 1 000 000 ticks. With unit s, EVENT and PPS coincide.

PPS is measured time, not wall-clock time. It pulses only while the chronometer is RUNNING and it keeps its phase across STOP/START. For a free-running 1 Hz reference tie START to 1 (or pulse it once) and STOP to 0.

Control Mode: start and stop link

• Start/Stop: the symbol has START and STOP. A rising edge on START starts, a rising edge on STOP stops. Any pulse width works. If both edges act on the same clock, STOP wins. A START while already running is ignored.
• Enable: the symbol has ENABLE. The chronometer runs while ENABLE is high: its rising edge starts, its falling edge stops.
• In both modes the pins pass a two-register synchroniser and the action lands one clock after the pin edge. Counting begins on the clock after the start; the clock on which the stop acts is still counted. So a start pin edge at clock $p$ and a stop pin edge at clock $q$ measure exactly $q-p$ clocks (in Enable mode: exactly the number of clocks ENABLE was high).
• An unconnected control pin reads ‘0’ (never starts).

After Stop: continue or restart link

• Continue: the next start resumes. TIME and the fraction of the unit already elapsed are both kept, so successive intervals add up exactly, down to the clock. TIME is the total running time.
• Restart: the next start restarts from zero: TIME, the sub-unit fraction and the PPS phase are cleared at that start. Until then the output keeps showing the last value. TIME is the time of the current (or last) interval.

Output Update: real time or on stop link

• Real time: TIME follows the count while running and holds while stopped.
• On stop: TIME changes only when a running measurement stops, in the same sample in which RUNNING falls, and holds that value until the next stop. While running it keeps showing the previous result (0 before the first stop). Combined with Restart this gives a lap timer: TIME is always the length of the last completed interval. EVENT, PPS and OVERFLOW keep following the internal count in real time.

Overflow link

TIME wraps from $2^N-1$ to 0; OVERFLOW pulses for one clock together with the EVENT of that wrap.

RESET and CE link

• RESET clears everything and stops the chronometer: a new START is needed afterwards.
• CE=‘0’ freezes the block completely (counting and synchronisers); the pulse outputs read ‘0’ while CE is low. CE=‘0’ therefore stretches time: only CE-qualified clocks are counted.

Timing diagram link

Unit s with a 10 Hz clock (one second = 10 clocks), TIME shown in units:

RUNNING rises one clock after the START pin; TIME becomes 1 after exactly 10 counted clocks, with EVENT and PPS. The 4 clocks counted after that (0.4 s) are kept when STOP arrives and are credited at the next START.

Enable + Restart + On stop (lap timer), same clock: a 2.9 s ENABLE window then a 1.9 s one. TIME shows 2, then 1, each from the end of its window:

The Test Timing window (right click on the block) shows four simulated demonstrations: seconds with stop/resume/reset (Start/Stop, Continue, Real time), the lap timer (Enable, Restart, On stop), minutes with PPS, and the fractional 2.5-clocks-per-tick case.

Choosing the parameters link

Clock Frequency link

Enter the frequency of the clock actually connected to CLK, in Hz (for example 125000000 for 125 MHz, 62500000 for 62.5 MHz). Range 1 Hz to 1 GHz. The selected unit must not be faster than the clock: us needs at least 1 MHz, ms at least 1 kHz. The property window refuses invalid combinations.

Bit Number link

Maximum TIME before wrapping:

Accuracy link

• Resolution of TIME: one unit. Resolution of the measured interval: one clock.
• With a non-integer number of clocks per unit, each tick may be up to one clock late against the ideal grid; the error does not accumulate.
• The absolute accuracy is the accuracy of the clock itself.

Typical use cases link

• Run-time / live-time counters in seconds or milliseconds readable by software without conversions.
• Experiment duration and dead-time accounting.
• Periodic actions every second/millisecond of an active acquisition (EVENT or PPS used as an enable).
• Uptime counter in hours.
• Lap timer: Enable + Restart + On stop gives the duration of every gate or busy window, in the unit you need, stable for software to read.

Comparison with Chrono (Start/Stop) link

Resources & Timing link

• Latency: START/STOP/ENABLE act 1 clock after the pin edge; TIME, EVENT, PPS registered

• Throughput: 1 clock per sample

One N-bit TIME counter, one accumulator of about log2(Clock Frequency) bits, a PPS prescaler (up to 20 bits for us) and a unit prescaler (12 bits for h). No DSP, no BRAM.