LPTIM Linux driver

1. Article purpose

This article introduces the LPTIM Linux^® driver for the LPTIM internal peripheral:

• Which LPTIM features are supported by the driver
• How to configure, use and debug the driver
• What is the driver structure, and where the source code can be found.

2. Short description

The LPTIM Linux driver (kernel space) is based on the PWM, IIO and counter frameworks. It provides several functionalities:

MFD driver:

• handles common resources (registers, clock)

PWM driver:

• handles the PWM output channel (single channel)

IIO hardware trigger driver:

• handles hardware trigger sources (synchronously with PWM) for other internal peripherals such as ADC, DAC, DFSDM

Counter driver:

• handles the quadrature encoder interface as well as the external event counter.

3. Configuration

3.1. Kernel configuration

Activate the LPTIM Linux driver in the kernel configuration using the Linux Menuconfig tool: Menuconfig or how to configure kernel.

Enable the following configurations (as well as their dependencies):

• CONFIG_MFD_STM32_LPTIMER
• CONFIG_PWM_STM32_LP
• CONFIG_IIO_STM32_LPTIMER_TRIGGER
• CONFIG_STM32_LPTIMER_CNT

Device Drivers  --->
   -> Multifunction device drivers  --->
      <*> Support for STM32 Low-Power Timer
   -> Pulse-width modulation (PWM) support  --->
      <*> STMicroelectronics STM32 PWM LP
   -> Industrial I/O support  --->
      -> Triggers - standalone  --->
         <*> STM32 Low-Power Timer Trigger
   -> Counter support  --->
      <*> STM32 LP Timer encoder counter driver

3.2. Device tree

Refer to the LPTIM device tree configuration article when configuring the LPTIM Linux kernel driver.

4. How to use

How to use PWM with sysfs interface

How to set up a TIM or LPTIM trigger using the sysfs interface

How to use the quadrature encoder with the sysfs interface

5. How to trace and debug

The LPTIM Linux driver can access LPTIM registers through REGMAP.

It comes with debugfs entries, which allow dumping registers:

$ cd /sys/kernel/debug/regmap
$ ls
40004000.timer  40009000.timer

$ cd 40009000.timer
$ cat registers
000: 00000003
004: 00000000
008: 00000000
...

It also comes with tracepoints:

$ cd /sys/kernel/debug/tracing
$ cat available_events | grep regmap
...
regmap:regmap_reg_read
regmap:regmap_reg_write

6. Source code location

The LPTIM Linux driver is composed of:

• stm32-lptimer.c driver to handle common resources, such as registers and clock.
• pwm-stm32-lp.c driver to handle PWM channel
• stm32-lptimer-trigger.c driver to handle trigger sources for other internal peripherals
• stm32-lptimer-cnt.c driver to handle quadrature encoder and external event counter
• include/linux/mfd/stm32-lptimer.h and include/linux/iio/timer/stm32-lptim-trigger.h header files