- Last edited 3 days ago ago
ADC OpenSTLinux drivers
- 1 Article purpose
- 2 Short description
- 3 Configuration
- 4 How to use
- 5 How to trace and debug
- 6 Source code location
- 7 References
1 Article purpose
This article introduces the Linux® and U-Boot drivers for the ADC internal peripheral. It answers the following questions:
- Which ADC 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 ADC internal peripheral is supported in Linux and U-boot software components through dedicated drivers. Refer to ADC internal peripheral to know the actual support of an ADC particular instance, depending on STM32 MPU device and execution context (here U-Boot or Linux context).
2.1 U-Boot driver
The U-Boot driver is based on the adc-uclass framework. This framework offers an API  which can be used to implement ADC client applications.
The U-Boot driver supports one shot conversions in single and scan modes. It uses poll mechanism to monitor the end of ADC conversions, interrupts, or DMA being not supported.
2.2 Linux kernel driver
The ADC Linux® driver (kernel space) is based on the IIO framework.
It supports two modes:
- IIO direct mode: single capture on a channel (using interrupts).
- IIO triggered buffer mode: capture on one or more channels (preferably using DMA).
It uses the hardware triggers available in IIO. See TIM Linux driver and LPTIM Linux driver for more details.
2.3 Calibration handover between U-Boot and Linux kernel drivers
The U-boot and Linux kernel drivers implement calibration support to improve the ADC accuracy. The recommendations to enhance ADC accuracy can be found in the application note AN2834. The ADC internal peripheral offers two kind of calibration:
- Offset calibration
This calibration is fast and may vary over time depending on voltage and temperature variations. The offset single-ended and differential calibration is performed on each boot in U-boot driver, and each time, new conversions are launched in the Linux kernel driver.
The linear calibration is SoC dependent. This calibration is time consuming and does not change over time because it is not voltage or temperature dependent. It can be then done only once, on first boot.
3.1 U-Boot configuration
The ADC U-Boot driver can be activated in the U-Boot configuration using the U-Boot Menuconfig tool (enable CONFIG_STM32_ADC).
Device Drivers ---> [*] Enable STMicroelectronics STM32 ADC driver
The ADC command line interface can be activated by enabling CMD_ADC flag.
Command line interface ---> Device access commands ---> [*] adc - Access Analog to Digital Converters info and data
3.2 Linux kernel configuration
The ADC Linux driver can be activated in the kernel configuration using the Linux Menuconfig tool (enable both CONFIG_STM32_ADC_CORE and CONFIG_STM32_ADC).
Device Drivers ---> <*> Industrial I/O support ---> Analog to digital converters ---> <*> STMicroelectronics STM32 adc core <*> STMicroelectronics STM32 adc
3.3 Device tree
Refer to the ADC device tree configuration article where the configuration of ADC Linux kernel or U-boot drivers is explained.
4 How to use
4.1 U-Boot driver
The analog-to-digital converters information and data can be accessed through the ADC command line tool. For that, the command line interface must be enabled in U-Boot configuration. This interface makes use of U-Boot adc API .
adc Usage: adc list - list ADC devices adc info <name> - Get ADC device info adc single <name> <channel> - Get Single data of ADC device channel adc scan <name> [channel mask] - Scan all [or masked] ADC channels
4.2 Linux kernel driver
In "IIO direct mode", the conversion result can be read directly from sysfs (refer to How to do a simple ADC conversion using the sysfs interface).
In "IIO triggered buffer mode", the configuration must be performed using sysfs first. Then, character device (/dev/iio:deviceX) is used to read data (refer to Convert one or more channels using triggered buffer mode).
5 How to trace and debug
5.1 In U-Boot
Refer to U-Boot - How to debug for details about debug means in U-Boots.
The ADC linear calibration factors are saved as persistent environment variables named "adc<addr>_lincalfact<index>". These variables can be displayed through the printenv command (STM32MP15x lines only):
printenv adc48003000_lincalfact1=1fe80602 adc48003000_lincalfact2=1da7c9ff adc48003000_lincalfact3=1ff7e9b8 ...
5.2 In Linux kernel
Refer to How to trace with dynamic debug for how to enable the debug logs in the driver and in the framework.
Refer to How to debug with debugfs for how to access information on ADC driver.
The ADC has system wide dependencies towards other key resources:
- runtime power management can be disabled, for example it may be forced on via power/control sysfs entry:
cd /sys/devices/platform/soc/48003000.adc/48003000.adc:adc@0 cat power/autosuspend_delay_ms 2000 cat power/control auto # kernel is allowed to automatically suspend the ADC device after autosuspend_delay_ms echo on > power/control # force the kernel to resume the ADC device (e.g. keep clocks and regulators enabled)
- clock usage can be verified by reading clk_summary:
cat /sys/kernel/debug/clk/clk_summary | grep adc adc12_k 1 1 0 24000000 0 0 adc12 1 1 0 196607910 0 0
- regulator tree and usage can be verified (for example, use count, open count or regulator reference voltage) as follows:
cat /sys/kernel/debug/regulator/regulator_summary regulator use open bypass voltage current min max ------------------------------------------------------------------------------- v3v3 4 5 0 3300mV 0mA 3300mV 3300mV vdda 1 2 0 2900mV 0mA 2900mV 2900mV 40017000.dac 0mV 0mV 48003000.adc 0mV 0mV
- pinctrl usage can be verified by reading pinmux-pins:
cd /sys/kernel/debug/pinctrl/soc\:pin*@50002000/ cat pinmux-pins | grep adc pin 92 (PF12): device 48003000.adc function analog group PF12 # check pin is assigned to ADC and is configured as "analog"
- interrupts can be verified by reading "interrupts":
cat /proc/interrupts CPU0 CPU1 56: 2 0 dummy 0 Edge 48003000.adc:adc@0
cat /sys/kernel/debug/iio/iio:device*/lincalfact* 527955453 521648626 ...
6 Source code location
The ADC source code is composed of:
- stm32-adc-core driver to handle common resources such as clock (selection, prescaler), regulator used as reference voltage, interrupt and common registers.
- stm32-adc driver to handle the resources available for each ADC such as channel configuration.
6.2 Linux kernel
The ADC source code is composed of:
- stm32-adc-core driver to handle common resources such as clock (selection, prescaler), regulator used as reference voltage, interrupt, and common registers.
- stm32-adc driver to handle the resources available for each ADC such as channel configuration and buffer handling.