Learn it by examples – (Timers)

By David Kebo Houngninou

On this page, you will find some interfacing experiments using the ARM Cortex M3.
– The evaluation board we target is the MCBSTM32C running on the STM32F107VC microcontroller.
– The tutorial focuses on configuring the MCBSTM32C and interfacing.
– Datasheets and documentation for STMicroelectronics STM32F107VC core is available at: http://www.keil.com/dd/chip/4889.htm.
– The reference used for this tutorial is the RM0008 Reference manual for STM32F107xx advanced ARM®-based 32-bit MCUs.

Tutorial 4: Timers

Timers are clock sources used as the ‘heartbeats’ for operations.

e.g. of applications using timers:

Counting pulses
Measuring time periods of waveforms
Generating pulse width modulation (PWM) signals
Triggering external devices
Timing special events

03 groups of timers:

Basic Timers
General Purpose Timers
Advanced Timers (TIM1&TIM8)

1- Basic Timers

TIM6, TIM7

Applications:

no I/O channels for input capture
no PWM generation
Only used for time-base generation

2- General Purpose Timers

TIM2, TIM3, TIM4, TIM5

Applications:

PWM generation
Input capture
Time-base generation
Output compare

3- Advanced Timers

TIM1, TIM8

Applications:

Advanced PWM generation
Input capture
Time-base generation
Output compare

Timers pin

Each timer is associated with an I/O pin
Configure the alternate function to use the timer pin

What is an alternate function (A.F)?

In addition to general-purpose input and output the ARM subsystem can implement other specialized input output functions.
e.g. TIM4_Ch4 is an alternate function for pin PB9

TIMx functional description

The main block of the timer is a 16-bit counter TIMx_CNT with an auto-reload register TIMx_ARR
The counter can count up, down, or both up and down.
The counter clock can be divided by a pre-scaler TIMx_PSC

TIMx functional description

Counter timing diagram with pre-scaler division change from 1 to 2

Timer configuration

RCC->APB1ENR //Enable clock to the timer

RCC->APB2ENR //Enable the Alternate function and the GPIO clock

(TIMx_SMCR)TIMx slave mode control register //Set the internal clock as system clock

(TIMx_PSC)TIMx pre-scaler register // Set the pre-scaler

(TIMx_CR1)TIMx control register // Enable the timer

Steps:

1.Enable clock to the timer
2.Enable the Alternate function and the GPIO clock
3.Set the internal clock as system clock
4.Set the pre-scaler
5.Enable the timer

Notes about the clock usage:

The HSI clock signal is generated from an internal 8 MHz RC oscillator and can be used directly as a system clock.
(HSI) high-speed internal oscillator

To set the pre-scaler register:
Use the counter clock frequency (CK_CNT), CK_CNT = fCK_PSC / (PSC[15:0] + 1)

Timer example

A developer is writing a function for a counter user a timer.
The internal HIS clock signal runs at 8 MHz
What is the value to set the pre-scaler to count every millisecond?

CK_CNT = fCK_PSC / (PSC[15:0] + 1)
PSC[15:0] + 1 = fCK_PSC / 1000
PSC[15:0] = (fCK_PSC / 1000) - 1
PSC[15:0] = 7999

Timer clock

Timer clock

Advanced Peripheral Bus 1 Enable Register

1	RCC->APB1ENR \|= (1<<2); //enable clock to TIM4.

Timer clock

Advanced Peripheral Bus 2 Enable Register

1 2	// e.g: To use TIM4 RCC->APB2ENR \|= (1)\|(1<<3); //Enable the A.F clock and the GPIO clock

Timer pre-scaler register

TIM1&TIM8 pre-scaler (TIMx_PSC)
PSC[15:0]: Pre-scaler value

Timer slave mode register

TIM1&TIM8 slave mode control register (TIMx_SMCR)
SMS = 000, the pre-scaler is clocked directly by the internal clock.

Timer control register

TIM1&TIM8 control register 1 (TIMx_CR1)
CEN: Counter enable
0: Counter disabled
1: Counter enabled