Nucleo Boards for Control Systems

GPIO read and write

Read from a GPIO

HAL_GPIO_ReadPin (GPIOA, GPIO_PIN_9);

Write to a GPIO

- High level:

HAL_GPIO_WritePin(GPIOA, GPIO_PIN_8, GPIO_PIN_SET);

- Low level:

HAL_GPIO_WritePin(GPIOA, GPIO_PIN_8, GPIO_PIN_RESET);

PWM

Main registers

Prescaler (PSC)

The prescaler is a register that divides the timer clock frequency to obtain a specific frequency. Specifically, the frequency reduction is applied using the following formula:

f_PWM = f_CLK / (PSC + 1)

Therefore, if the frequency to set is the same as the frequency of the clock, the PSC register must be set at PSC=0. The STM32 timers allow a prescaler value up to 65535, through 16 bits registers.

Auto Reload Register (ARR) and Capture Compare Register (CCR)

The ARR register storages the maximum value that the counter can reach before going back to zero. Moreover, the CCR registers the value of the corresponding timer counter after which the output PWM signal goes from high to low level. Every channel of the timer used to generate the PWM will have a different CCR register (for example, CCR1 and CCR2 for channels 1 and 2). In this way, the value of the CCR together with the value of the ARR define the width of the pulses and, therefore, the duty cycle of the PWM signal:

Duty_Cycle (%) = 100 * (CCR / ARR)

CNT

This is the register that storages the value of the counter used to generate the PWM.

PWM Configuration in STM32CubeIDE

To configure a timer as PWM, the first thing to do is to select one of the timers that could be configured as a PWM generator. Additionally, also the channels that will be used as PWM output must be decided.

Advice: As different PWM outputs are needed, it is recommended to use different channels of the same timer to the extent possible.

The main timers that can be set as PWM are: TIM2, TIM3, TIM4 and TIM5, with 4 different channels each of them.

Advice: It is recommended to use the timer and the corresponding channels that match directly with the pins that feed the H-bridge to modulate the speed of the motor. Otherwise, the PWM output signals must be bridged with the H-bridge input signals.

Therefore, the pins that the channels of the timer are using need to be checked in the "Pinout" section of the STM32CubeIDE.

Once the channels of the timer have been activated, some parameters and the configuration of the timer must be changed.

Main parameters:

• Clock Source: reloj a tomar como referencia por el timer. Debemos seleccionar la opción Internal clock. Además, debemos asegurarnos que la frecuencia del reloj usada es la deseada, en la pestaña de clock configuration.
• Channel x: configuración deseada para el canal x del timer. Debemos seleccionar la opción “PWM Generation CHx”, donde x será el número del canal en cuestión.
• Prescaler (PSC): Parámetro para configurar el registro de PSC.
• Counter Period: Parámetro para configurar el valor del registro ARR.
• Pulse (16 bits): parámetro para modificar el registro de CCR.
• Mode: Debemos seleccionar PWM mode 1.

Además, en nuestro código debemos activar el timer ya configurado e inicializado de la siguiente manera:

HAL_TIM_PWM_Start(&htim3, TIM_CHANNEL_1);

Podemos modificar el valor de CCR (aceder al registro) mediante código de la siguiente manera:

htimx.Instance->CCR1 = new_ccr;

donde CCR1 indica que estamos accediendo al registro del canal 1, new_ccr es el nuevo valor entero que queremos establecer y htimx es la instancia correspondiente al timer x (por ejemplo, htim2 o htim3).

De igual manera, para modificar el registro del canal 2:

htimx.Instance->CCR2 = new_ccr;