Table of Contents

ADC configuration

About this document

This document describes how to configure the ADC and respective GPIO pins for a STM32 target board based in ChibiOS HAL/PAL.

Assumptions and design

The STM32 parts can have up to 19 multiplexed channels (being 16 from external sources). Those can be grouped for special conversion scenarios that we are not going to use. Each ADC channel can be exposed in one or more GPIO pins. Despite this providing more flexibility to a system designer it poses an additional complication at the time of configuring the ADC. Considering that the heavy-lifting on the ADC configuration and initial setup is performed by ChibiOS, we've tried to make the remaining configuration as simple as possible, which is pretty much mapping the GPIO pins.

For the remaining of this document we'll be using ST STM32F769I_DISCOVERY reference target and will configure the ADC to use the ADC channels exposed through the CN14 connector. From the schematics of the board (mb1225 F769I-DISCO schematic.pdf downloadable from ST web site) one can see that the following channels exposed:

pad GPIO pin ADC channel
A0 PA6 ADC1_IN6
A1 PA4 ADC1_IN4
A2 PC2 ADC1_IN12
A3 PF10 ADC1_IN8
A4 PF8 ADC3_IN6
A5 PB8 ADC3_IN7

To fully take advantage of the ADC hardware we are going to enable the internal ADC sources. These ones have to be mapped to ADC1.

pad GPIO pin ADC channel
N.A. N.A. ADC1_TEMP_SENSOR
N.A. N.A. ADC1_VREFINT
N.A. N.A. ADC1_VBAT

Configurations

The configurations are all concentrated in the target_windows_devices_adc_config.cpp file in the reference target folder. This source file is added to the CMake target only if the API_Windows.Devices.Adc option is set to ON. See the target CMakeList.txt.

There is a global NF_PAL_ADC_PORT_PIN_CHANNEL array for the ADC controller. On each entry there are the configurations for the ADC block, the GPIO port and pin along with the ADC internal channel reference. Note that for the internal sources channels the GPIO port and pin are to be set to NULL and those are only available on ADC1. All the naming come from existing ChibiOS defines.

The configuration array will look like:

const NF_PAL_ADC_PORT_PIN_CHANNEL AdcPortPinConfig[] = {

    // ADC1
    {1, GPIOA, 6, ADC_CHANNEL_IN6},
    {1, GPIOA, 4, ADC_CHANNEL_IN4},
    {1, GPIOC, 2, ADC_CHANNEL_IN12},
    {1, GPIOF, 10, ADC_CHANNEL_IN8},

    // ADC3
    {3, GPIOF, 8, ADC_CHANNEL_IN6},
    {3, GPIOB, 8, ADC_CHANNEL_IN7},

    // these are the internal sources, available only at ADC1
    {1, NULL, NULL, ADC_CHANNEL_SENSOR},
    {1, NULL, NULL, ADC_CHANNEL_VREFINT},
    {1, NULL, NULL, ADC_CHANNEL_VBAT},
};

There is also a variable with the channel count, like this: const int AdcChannelCount = ARRAYSIZE(AdcPortPinConfig);

To complete the configuration one has to enable ADC1 and ADC3 for ChibiOS HAL. Remember those were the ADC blocks used in the configuration above. This is done by editing the mcuconf.h file inside the target nanoCLR folder. Search for STM32_ADC_USE_ADC1 and STM32_ADC_USE_ADC3 and set those to TRUE.

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