Timers: exact periods and measuring time

Basic operation

Timer input clock → divided by PSC (prescaler) → CNT increases by 1 → when it reaches ARR it returns to 0 and raises an update event

Period = (PSC + 1) × (ARR + 1) / timer clock

A 1 kHz interrupt from a 100 MHz timer: PSC = 99 (1 MHz), ARR = 999 → 1 ms

Uses

Mode	Purpose
Update interrupt	Exact control period (1 kHz speed loop)
Output compare / PWM	Motors, LED brightness
Input capture	Pulse width and frequency measurement (servo signals, ultrasonic sensors)
Encoder mode	Counts position automatically from A/B signals
Trigger output (TRGO)	Sets the ADC sampling instant

Measuring time with input capture

At the instant a signal edge arrives, the CNT value is stored in CCR automatically. The difference between the CCR values at the rising and falling edges is the pulse width. Unlike measuring in software, there is no jitter.

How this differs from HAL_Delay

HAL_Delay waits on SysTick and ties up the CPU. Periodic work should run from a timer interrupt or an RTOS task, so that other things can happen at the same time.

A timer is a counter

Every clock tick raises the counter by 1. When it reaches the set value (ARR), it returns to 0 and raises an update event. The prescaler (PSC) divides the clock first, so the counter counts more slowly.

counter clock = timer clock / (PSC + 1)
event period  = (PSC + 1) × (ARR + 1) / timer clock

3D labRun what you have read so far→

Example: a 1 ms period

With a 72 MHz timer clock

PSC = 71    →  72 MHz / 72 = 1 MHz   (1 count = 1 μs)
ARR = 999   →  an event every 1000 counts = 1 ms

TIM2->PSC  = 71;
TIM2->ARR  = 999;
TIM2->DIER |= TIM_DIER_UIE;      // enable the update interrupt
TIM2->CR1  |= TIM_CR1_CEN;       // start
NVIC_EnableIRQ(TIM2_IRQn);

If you forget the + 1 and write 72 to PSC and 1000 to ARR, the period becomes about 1.5% longer. For communication timing or a control period, that error cannot be ignored.

In a picture: the counter and PWM

The same timer does four different jobs depending on its settings.

Mode	What it does	Use
Basic timer	Interrupt at a fixed period	System tick, control loop
Output compare / PWM	Compares with the counter and drives a pin	Motors, LED brightness, servos
Input capture	Stores the counter value at the instant a pin changes	Frequency and pulse-width measurement, Hall sensors
Encoder mode	Counts up and down from A/B signals	Rotational position

The limit of 16 bits

A 16-bit timer only counts to 65535. Counting in 1 μs units, it overflows at 65.5 ms. To measure longer times, increase PSC to make the unit larger, or count the upper digits separately in the overflow interrupt and join them. When taking the difference between two input-capture times, use unsigned subtraction such as (uint16_t)(now - prev); the result is correct even if one overflow happened in between.

Problems that come up in practice

• Timer clock is twice what you expected: on many MCUs only the timer clock doubles when the bus divider is not 1. If the period comes out at half, check the clock tree.
• Changing ARR takes effect immediately and breaks a pulse: with preload enabled, the change is applied all at once at the next update.
• Interrupt flag not cleared: the handler is called again the moment it returns, and the main loop stops.