4 docs tagged with "timers"

PWM points the timer outward: the counter drives a pin. Input capture points it inward. The counter free-runs, an edge on a pin tells the hardware "now", and the value of CNT at that instant is copied into a capture register before software has had a chance to be late. That last clause is the entire value of the peripheral. A GPIO interrupt can also tell you an edge happened, but by the time your handler reads a counter it has been anywhere from 12 to several hundred cycles — jittering with whatever else the NVIC was doing — and the measurement carries that jitter. The capture unit's latch has no jitter at all.

A microcontroller pin has two output voltages and nothing in between. Pulse-width modulation is the trick that gets the third: switch fast enough and whatever is downstream — an LED and your eye, a motor and its inductance, an RC filter and a slow ADC — averages the square wave into a level. The pin is still only ever fully on or fully off, which is why it dissipates almost no power doing it. That is the whole reason PWM won over analogue drive for everything from a status LED to a 10 kW inverter.

There are three clocks in a system running an RTOS and it is worth separating them before writing a single delay. There is the hardware timer, counting a crystal-derived clock in silicon, which does not know or care what software is doing. There is the tick, a periodic interrupt that increments a counter and is the only time the kernel can see. And there is the task's own experience of time, which is the tick as filtered through whether that task was actually running when something expired.

A timer is the least interesting peripheral to describe and the one you will use most. It is a counter that increments on a clock edge, and a comparator that notices when the count reaches a number you chose. Everything else in the chapter — PWM, input capture, encoder decoding, one-pulse output, triggering the ADC — is that counter with different plumbing bolted onto the comparator.