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Timers

2025-07-14 · embedded · basic · timers

🟠 Timers are hardware modules of the microcontroller for actions with time counting and event generation without the participation of the main code.
The accuracy of the timer and its maximum time range depends on its bit depth, the clock frequency to which it is connected and the value of the divider.

🔍 Timers Use Cases

• Time Measurement — Signal Duration, Interval Between Events
• Event Generation — Periodic Interrupts, PWM, ADC Triggers
• Hang Control — Watchdog Timer (Restart MCU when hung)
• System Intervals — SysTick, used in RTOS and Basic Delays (System Timer, built into Cortex-M core)

🕒 Timers and Clock

• Timers operate from a clock signal coming from the clock system.
• The timer frequency depends on:
  • Input signal frequency
  • prescaler value
• The lower the frequency, the longer the interval between ticks, and vice versa.

⚡ Timer and Interrupts

Timers can generate different types of interrupts:

• Update Event — triggered when the end of the count is reached (ARR)

• Compare Match — triggered when the counter matches the specified value

• Overflow — counter overflow

• Used for:

• Precise periodic sampling (e.g. every 10 ms)

• PWM generation

• Time delays (sleep/delay)

⚙️ Timer Settings

1. Counting Direction

• Up, Down, or Both

2. Resolution (Timer Bit Capacity)

• The number of bits determines the maximum duration of the timer.
• At a frequency of 1 mHz - the clock ticks 1 million times per second. (1 time per 1 microsecond µs)
  • 8-bit timer: max interval = 255 µs
  • 16-bit timer: ≈ 65,535 µs (0.065536 seconds)
  • 32-bit: ≈ 2^32 / 1,000,000 Hz = 4.294967296 seconds

3. Prescaler Factor

• Timers are often connected to a high frequency system clock (e.g. 12.5 MHz).
• Without a prescaler, a 16-bit timer at 12.5 MHz will “overflow” in ~5 ms.
• Divides the input frequency, increasing the interval between ticks
• By decreasing the frequency, you can increase the total amount of time that the timers can count, since each value represents a longer period of time.
• formula Maximum time = (2^n - 1) / (clock_freq / prescaler)

Frequency: 1 MHz = 1,000,000 Hz, which means 1,000,000 pulses per second.
Pulse period: 1 / 1,000,000 = 0.000001 seconds (1 microsecond).
The maximum value of a 16-bit timer is 65535.
Overflow time: 65535 * 0.000001 = 0.065535 seconds.

⚙️ Timer programming

What we want to do is manipulate the pre-scaling. To do this, we need to configure the clock configuration of the operating mode.

• Manipulate the RCC register to set the system clock divider.
• (Context: microcontroller in QEMU, PLL frequency is 400 MHz, divided by 2 → SYSCLK = 200 MHz).

Steps:

1. Define the RCC base address

const RCC: u32 = 0x400FE060;  					// just paste this address, but remember that it has offset 060

2. Определим нужное значение делителя

const SYSCTL_SYSDIV_16: u32 = 0xF; 				// Divisor = 16 (frequency 12.5 megahertz)

3. Установим новое значение в RCC

unsafe {
	let sysdiv = SYSCTL_SYSDIV_16 << 23; 		// we are going to shift it 23 bits to the left so that it occupies bits 23 through 26
	let orig = *(RCC as *const u32); 			// to avoid overwriting the entire RCC register, we need to read the value that is in RCC.
	let mask = !(0b1111 << 23); 				// create a mask of 4 bits and invert them. The mask will occupy bits 23 through 26
	let rcc = (orig & mask) | sysdiv; 			// replace the bit values in the original with the mask.
	*(RCC as *mut u32) = rcc; 				// set the preliminary scaling value.
}

4. Change the divider to increase the frequency

const SYSCTL_SYSDIV_12: u32 = 0xB; 		// Divisor = 12 → higher frequency (new frequency 16.67 MHz)

With a change in the divider, the timer frequency changes, and therefore the interval between interrupts. A smaller divider means that the timers work faster, a larger one means that the timers work slower, but the counting takes longer. A higher frequency means that the interruption occurs more often.