Analog to Digital Converter (ADC) Continuous Mode Driver

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Introduction

The Analog to Digital Converter is integrated on the chip and is capable of measuring analog signals from specific analog IO pads. Additionally, the Direct Memory Access (DMA) functionality is utilized to efficiently retrieve ADC conversion results.

ESP32-S2 has two ADC unit(s), which can be used in scenarios like:

  • Generate one-shot ADC conversion result

  • Generate continuous ADC conversion results

This guide introduces ADC continuous mode conversion.

Driver Concepts

ADC continuous mode conversion is made up of multiple conversion frames.

../../_images/adc_conversion_frame.png

Functional Overview

The following sections of this document cover the typical steps to install the ADC continuous mode driver, and read ADC conversion results from a group of ADC channels continuously:

Resource Allocation

The ADC continuous mode driver is implemented based on ESP32-S2 SAR ADC module. Different ESP targets might have different numbers of independent ADCs.

To create an ADC continuous mode driver handle, set up the required configuration structure adc_continuous_handle_cfg_t:

After setting up the above configurations for the ADC, call adc_continuous_new_handle() with the prepared adc_continuous_handle_cfg_t. This function may fail due to various errors such as invalid arguments, insufficient memory, etc.

Especially, when this function returns ESP_ERR_NOT_FOUND, this means the SPI3 peripheral is in use. See Hardware Limitations for more information.

If the ADC continuous mode driver is no longer used, you should deinitialize the driver by calling adc_continuous_deinit().

IIR filter

Two IIR filters are available when ADC is working in continuous mode. To create an ADC IIR filter, you should set up adc_continuous_iir_filter_config_t and call adc_new_continuous_iir_filter().

  • adc_digi_filter_config_t::unit: ADC unit.

  • adc_digi_filter_config_t::channel: ADC channel to be filtered.

  • adc_digi_filter_config_t::coeff: Filter coefficient.

On ESP32-S2, the filter is per ADC unit. Once a filter is enabled, all the enabled ADC channels in this ADC unit will be filtered. However, we suggest only enabling one ADC channel per unit, when using the filter feature. Because the filtered results depend on the previous filtered result. So you should not enable multiple ADC channels, to avoid mixing the filtered results.

To recycle a filter, you should call adc_del_continuous_iir_filter().

Monitor

2 monitors are available when ADC is working under continuous mode, you can set one or two threshold(s) of a monitor on a working ADC channel, then the monitor will invoke interrupts every sample loop if conversion result outranges of the threshold. To create an ADC monitor, you need to set up the adc_monitor_config_t and call adc_new_continuous_monitor().

  • adc_monitor_config_t::adc_unit: Configures which ADC unit the channel you want to monitor belongs to.

  • adc_monitor_config_t::channel: The channel you want to monitor.

  • adc_monitor_config_t::h_threshold: The high threshold, conversion result larger than this value invokes interrupt, set to -1 if do not use.

  • adc_monitor_config_t::l_threshold: The low threshold, conversion result less than this value invokes interrupt, set to -1 if do not use.

Once a monitor is created, you can operate it by following APIs to construct your apps.

  • adc_continuous_monitor_enable(): Enable a monitor.

  • adc_continuous_monitor_disable(): Disable a monitor.

  • adc_continuous_monitor_register_event_callbacks(): register user callbacks to take action when the ADC value exceeds of the thresholds.

  • adc_del_continuous_monitor(): Delete a created monitor and free resources.

Note

There are some hardware limitations on ESP32-S2: 1. Only one threshold is supported for one monitor. 2. Only one monitor is supported for one ADC unit. 3. All enabled channel(s) of a certain ADC unit in ADC continuous mode driver will be monitored. The adc_monitor_config_t::channel parameter will not be used.

Specifically, the monitor function can be used to implement zero-crossing detection. As ADC cannot directly process negative input signals, an extra DC bias should be applied to the original signal before measurement.

First, add a DC bias to the input signal through a circuit to "shift" the negative signal into the ADC's measurement range. For the measurement range, please refer to the On-Chip Sensor and Analog Signal Processing chapter in TRM. For example, adding a 1 V bias would transform a signal from -1 V to +1 V into 0 V to 2 V range. Then by setting the appropriate high and low thresholds, the ADC can detect if the input signal approaches zero, allowing for the identification of phase changes in the signal. Refer to the example code below for details.

// Initialize the ADC monitor handle
adc_monitor_handle_t adc_monitor_handle = NULL;

// Configure the ADC monitor
adc_monitor_config_t zero_crossing_config = {
    .adc_unit = EXAMPLE_ADC_UNIT_1,      // Specify the ADC unit to monitor
    .channel = EXAMPLE_ADC_CHANNEL_0,    // Specify the ADC channel to monitor
    .h_threshold = 1100,                 // Set the high threshold close to the DC bias and adjust it as needed
    .l_threshold = 900,                 // Set the low threshold close to the DC bias and adjust it as needed
};

// Create the ADC monitor
ESP_ERROR_CHECK(adc_new_continuous_monitor(&zero_crossing_config, &adc_monitor_handle));

// Register the callback function
adc_monitor_evt_cbs_t zero_crossing_cbs = {
    .on_over_high_thresh = example_on_exceed_high_thresh,
    .on_below_low_thresh = example_on_below_low_thresh,
};

ESP_ERROR_CHECK(adc_continuous_monitor_register_event_callbacks(adc_monitor_handle, &zero_crossing_cbs, NULL));

// Enable the ADC monitor
ESP_ERROR_CHECK(adc_continuous_monitor_enable(adc_monitor_handle));

// Disable and delete the ADC monitor
ESP_ERROR_CHECK(adc_continuous_monitor_disable(adc_monitor_handle));
ESP_ERROR_CHECK(adc_del_continuous_monitor(adc_monitor_handle));

Initialize the ADC Continuous Mode Driver

adc_continuous_handle_t handle = NULL;
adc_continuous_handle_cfg_t adc_config = {
    .max_store_buf_size = 1024,
    .conv_frame_size = 256,
};
ESP_ERROR_CHECK(adc_continuous_new_handle(&adc_config, &handle));

Recycle the ADC Unit

ESP_ERROR_CHECK(adc_continuous_deinit(handle));

ADC Configurations

After the ADC continuous mode driver is initialized, set up the adc_continuous_config_t to configure ADC IOs to measure analog signal:

Set adc_digi_pattern_config_t with the following process:

Note

For the IO corresponding ADC channel number, check TRM to acquire the ADC IOs. Besides, adc_continuous_io_to_channel() and adc_continuous_channel_to_io() can be used to acquire the ADC channels and ADC IOs.

To make these settings take effect, call adc_continuous_config() with the configuration structure above. This API may fail due to reasons like ESP_ERR_INVALID_ARG. When it returns ESP_ERR_INVALID_STATE, this means the ADC continuous mode driver is started, you should not call this API at this moment.

See ADC continuous mode example peripherals/adc/continuous_read to see configuration codes.

To enable/disable the ADC IIR filter, you should call adc_continuous_iir_filter_enable() / adc_continuous_iir_filter_disable().

To enable/disable the ADC monitor, you should call adc_continuous_monitor_enable() / adc_continuous_monitor_disable().

ADC Control

Start and Stop

Calling adc_continuous_start() makes the ADC start to measure analog signals from the configured ADC channels, and generate the conversion results.

On the contrary, calling adc_continuous_stop() stops the ADC conversion.

ESP_ERROR_CHECK(adc_continuous_stop(handle));

Register Event Callbacks

By calling adc_continuous_register_event_callbacks(), you can hook your own function to the driver ISR. Supported event callbacks are listed in adc_continuous_evt_cbs_t.

As the above callbacks are called in an ISR context, you should always ensure the callback function is suitable for an ISR context. Blocking logic should not appear in these callbacks. The callback function prototype is declared in adc_continuous_callback_t.

You can also register your own context when calling adc_continuous_register_event_callbacks() by the parameter user_data. This user data will be passed to the callback functions directly.

This function may fail due to reasons like ESP_ERR_INVALID_ARG. Especially, when CONFIG_ADC_CONTINUOUS_ISR_IRAM_SAFE is enabled, this error may indicate that the callback functions are not in the internal RAM. Check the error log for more details. Besides, when it fails due to ESP_ERR_INVALID_STATE, it indicates that the ADC continuous mode driver is started, and you should not add a callback at this moment.

Conversion Done Event

When the driver completes a conversion, it triggers the adc_continuous_evt_cbs_t::on_conv_done event and fills the event data. Event data contains a buffer pointer to a conversion frame buffer, together with the size. Refer to adc_continuous_evt_data_t to know the event data structure.

Note

It is worth noting that, the data buffer adc_continuous_evt_data_t::conv_frame_buffer is maintained by the driver itself. Therefore, never free this piece of memory.

Note

When the Kconfig option CONFIG_ADC_CONTINUOUS_ISR_IRAM_SAFE is enabled, the registered callbacks and the functions called by the callbacks should be placed in IRAM. The involved variables should be placed in internal RAM as well.

Pool Overflow Event

The ADC continuous mode driver has an internal pool to save the conversion results. When the pool is full, a pool overflow event will emerge. Under this condition, the driver will not fill in the event data. This usually happens because the speed to read data from the pool by calling adc_continuous_read() is much slower than the ADC conversion speed.

Read Conversion Result

After calling adc_continuous_start(), the ADC continuous conversion starts. Call adc_continuous_read() to get the conversion results of the ADC channels. You need to provide a buffer to get the raw results.

Function adc_continuous_read() tries to read the expected length of conversion results each time.

  • When calling adc_continuous_read(), you can request to read a conversion result of the specified length. Sometimes, however, the actual available conversion results may be less than the requested length, in which case the function still moves the data from the internal pool into the buffer you provided. Therefore, to learn the number of conversion results actually moved into the buffer, please check the value of out_length.

  • If there is no conversion result generated in the internal pool, the function will block for timeout_ms until the conversion results are generated. If there are still no generated results, the function will return ESP_ERR_TIMEOUT.

  • If the generated results fill up the internal pool, newly generated results will be lost. Next time when adc_continuous_read() is called, this function will return ESP_ERR_INVALID_STATE to indicate this situation.

This API aims to give you a chance to read all the ADC continuous conversion results.

The ADC conversion results read from the above function are raw data. To calculate the voltage based on the ADC raw results, this formula can be used:

Vout = Dout * Vmax / Dmax       (1)

where:

Vout

Digital output result, standing for the voltage.

Dout

ADC raw digital reading result.

Vmax

Maximum measurable input analog voltage, this is related to the ADC attenuation, please refer to the On-Chip Sensor and Analog Signal Processing chapter in TRM.

Dmax

Maximum of the output ADC raw digital reading result, which is 2^bitwidth, where the bitwidth is the adc_digi_pattern_config_t::bit_width configured before.

To do further calibration to convert the ADC raw result to voltage in mV, please refer to Analog to Digital Converter (ADC) Calibration Driver.

Hardware Limitations

  • A specific ADC unit can only work under one operating mode at any one time, either continuous mode or one-shot mode. adc_continuous_start() has provided the protection.

  • Random Number Generator (RNG) uses ADC as an input source. When ADC continuous mode driver works, the random number generated from RNG will be less random.

  • ADC2 is also used by Wi-Fi. adc_continuous_start() has provided the protection between Wi-Fi driver and ADC continuous mode driver.

  • ADC continuous mode driver uses SPI3 peripheral as hardware DMA FIFO. Therefore, if SPI3 is in use already, the adc_continuous_new_handle() will return ESP_ERR_NOT_FOUND.

Power Management

When power management is enabled, i.e., CONFIG_PM_ENABLE is on, the APB clock frequency may be adjusted when the system is in an idle state, thus potentially changing the behavior of ADC continuous conversion.

However, the continuous mode driver can prevent this change by acquiring a power management lock of type ESP_PM_APB_FREQ_MAX. The lock is acquired after the continuous conversion is started by adc_continuous_start(). Similarly, the lock will be released after adc_continuous_stop(). Therefore, adc_continuous_start() and adc_continuous_stop() should appear in pairs, otherwise, the power management will be out of action.

IRAM Safe

All the ADC continuous mode driver APIs are not IRAM-safe. They are not supposed to be run when the Cache is disabled. By enabling the Kconfig option CONFIG_ADC_CONTINUOUS_ISR_IRAM_SAFE, the driver's internal ISR handler is IRAM-safe, which means even when the Cache is disabled, the driver will still save the conversion results into its internal pool.

Thread Safety

ADC continuous mode driver APIs are not guaranteed to be thread-safe. However, the share hardware mutual exclusion is provided by the driver. See Hardware Limitations for more details.

Application Examples

  • peripherals/adc/continuous_read demonstrates how to use the ADC Continuous Read Mode (DMA Mode) on ESP32-S2 development boards to read from GPIO pins via on-chip ADC modules.

API Reference

Header File

  • components/esp_adc/include/esp_adc/adc_continuous.h

  • This header file can be included with:

    #include "esp_adc/adc_continuous.h"

  • This header file is a part of the API provided by the esp_adc component. To declare that your component depends on esp_adc, add the following to your CMakeLists.txt:

    REQUIRES esp_adc

    or

    PRIV_REQUIRES esp_adc

Functions

esp_err_t adc_continuous_new_handle(const adc_continuous_handle_cfg_t *hdl_config, adc_continuous_handle_t *ret_handle)

Initialize ADC continuous driver and get a handle to it.

Parameters

  • hdl_config -- [in] Pointer to ADC initialization config. Refer to adc_continuous_handle_cfg_t.

  • ret_handle -- [out] ADC continuous mode driver handle

Returns

  • ESP_ERR_INVALID_ARG If the combination of arguments is invalid.

  • ESP_ERR_NOT_FOUND No free interrupt found with the specified flags

  • ESP_ERR_NO_MEM If out of memory

  • ESP_OK On success

esp_err_t adc_continuous_config(adc_continuous_handle_t handle, const adc_continuous_config_t *config)

Set ADC continuous mode required configurations.

Parameters

  • handle -- [in] ADC continuous mode driver handle

  • config -- [in] Refer to adc_digi_config_t.

Returns

  • ESP_ERR_INVALID_STATE: Driver state is invalid, you shouldn't call this API at this moment

  • ESP_ERR_INVALID_ARG: If the combination of arguments is invalid.

  • ESP_OK: On success

esp_err_t adc_continuous_register_event_callbacks(adc_continuous_handle_t handle, const adc_continuous_evt_cbs_t *cbs, void *user_data)

Register callbacks.

Note

User can deregister a previously registered callback by calling this function and setting the to-be-deregistered callback member in the cbs structure to NULL.

Note

When CONFIG_ADC_CONTINUOUS_ISR_IRAM_SAFE is enabled, the callback itself and functions called by it should be placed in IRAM. Involved variables (including user_data) should be in internal RAM as well.

Note

You should only call this API when the ADC continuous mode driver isn't started. Check return value to know this.

Parameters

  • handle -- [in] ADC continuous mode driver handle

  • cbs -- [in] Group of callback functions

  • user_data -- [in] User data, which will be delivered to the callback functions directly

Returns

  • ESP_OK: On success

  • ESP_ERR_INVALID_ARG: Invalid arguments

  • ESP_ERR_INVALID_STATE: Driver state is invalid, you shouldn't call this API at this moment

esp_err_t adc_continuous_start(adc_continuous_handle_t handle)

Start the ADC under continuous mode. After this, the hardware starts working.

Parameters

handle -- [in] ADC continuous mode driver handle

Returns

  • ESP_ERR_INVALID_STATE Driver state is invalid.

  • ESP_OK On success

esp_err_t adc_continuous_read(adc_continuous_handle_t handle, uint8_t *buf, uint32_t length_max, uint32_t *out_length, uint32_t timeout_ms)

Read bytes from ADC under continuous mode.

Parameters

  • handle -- [in] ADC continuous mode driver handle

  • buf -- [out] Conversion result buffer to read from ADC. Suggest convert to adc_digi_output_data_t for ADC Conversion Results. See the subsection Driver Backgrounds in this header file to learn about this concept.

  • length_max -- [in] Expected length of the Conversion Results read from the ADC, in bytes.

  • out_length -- [out] Real length of the Conversion Results read from the ADC via this API, in bytes.

  • timeout_ms -- [in] Time to wait for data via this API, in millisecond.

Returns

  • ESP_ERR_INVALID_STATE Driver state is invalid. Usually it means the ADC sampling rate is faster than the task processing rate.

  • ESP_ERR_TIMEOUT Operation timed out

  • ESP_OK On success

esp_err_t adc_continuous_stop(adc_continuous_handle_t handle)

Stop the ADC. After this, the hardware stops working.

Parameters

handle -- [in] ADC continuous mode driver handle

Returns

  • ESP_ERR_INVALID_STATE Driver state is invalid.

  • ESP_OK On success

esp_err_t adc_continuous_deinit(adc_continuous_handle_t handle)

Deinitialize the ADC continuous driver.

Parameters

handle -- [in] ADC continuous mode driver handle

Returns

  • ESP_ERR_INVALID_STATE Driver state is invalid.

  • ESP_OK On success

esp_err_t adc_continuous_flush_pool(adc_continuous_handle_t handle)

Flush the driver internal pool.

Note

This API is not supposed to be called in an ISR context

Parameters

handle -- [in] ADC continuous mode driver handle

Returns

  • ESP_ERR_INVALID_STATE Driver state is invalid, you should call this API when it's in init state

  • ESP_ERR_INVALID_ARG: Invalid arguments

  • ESP_OK On success

esp_err_t adc_continuous_io_to_channel(int io_num, adc_unit_t *const unit_id, adc_channel_t *const channel)

Get ADC channel from the given GPIO number.

Parameters

  • io_num -- [in] GPIO number

  • unit_id -- [out] ADC unit

  • channel -- [out] ADC channel

Returns

  • ESP_OK: On success

  • ESP_ERR_INVALID_ARG: Invalid argument

  • ESP_ERR_NOT_FOUND: The IO is not a valid ADC pad

esp_err_t adc_continuous_channel_to_io(adc_unit_t unit_id, adc_channel_t channel, int *const io_num)

Get GPIO number from the given ADC channel.

Parameters

  • unit_id -- [in] ADC unit

  • channel -- [in] ADC channel

  • io_num -- [out] GPIO number

  • - -- ESP_OK: On success

    • ESP_ERR_INVALID_ARG: Invalid argument

Structures

struct adc_continuous_handle_cfg_t

ADC continuous mode driver initial configurations.

Public Members

uint32_t max_store_buf_size

Max length of the conversion results that driver can store, in bytes.

uint32_t conv_frame_size

Conversion frame size, in bytes. This should be in multiples of SOC_ADC_DIGI_DATA_BYTES_PER_CONV.

uint32_t flush_pool

Flush the internal pool when the pool is full.

struct adc_continuous_handle_cfg_t::[anonymous] flags

Driver flags.

struct adc_continuous_config_t

ADC continuous mode driver configurations.

Public Members

uint32_t pattern_num

Number of ADC channels that will be used.

adc_digi_pattern_config_t *adc_pattern

List of configs for each ADC channel that will be used.

uint32_t sample_freq_hz

The expected ADC sampling frequency in Hz. Please refer to soc/soc_caps.h to know available sampling frequency range

adc_digi_convert_mode_t conv_mode

ADC DMA conversion mode, see adc_digi_convert_mode_t.

adc_digi_output_format_t format

ADC DMA conversion output format, see adc_digi_output_format_t.

struct adc_continuous_evt_data_t

Event data structure.

Note

The conv_frame_buffer is maintained by the driver itself, so never free this piece of memory.

Public Members

uint8_t *conv_frame_buffer

Pointer to conversion result buffer for one conversion frame.

uint32_t size

Conversion frame size.

struct adc_continuous_evt_cbs_t

Group of ADC continuous mode callbacks.

Note

These callbacks are all running in an ISR environment.

Note

When CONFIG_ADC_CONTINUOUS_ISR_IRAM_SAFE is enabled, the callback itself and functions called by it should be placed in IRAM. Involved variables should be in internal RAM as well.

Public Members

adc_continuous_callback_t on_conv_done

Event callback, invoked when one conversion frame is done. See the subsection Driver Backgrounds in this header file to learn about the conversion frame concept.

adc_continuous_callback_t on_pool_ovf

Event callback, invoked when the internal pool is full.

Macros

ADC_MAX_DELAY

ADC read max timeout value, it may make the adc_continuous_read block forever if the OS supports.

Type Definitions

typedef struct adc_continuous_ctx_t *adc_continuous_handle_t

Type of adc continuous mode driver handle.

typedef bool (*adc_continuous_callback_t)(adc_continuous_handle_t handle, const adc_continuous_evt_data_t *edata, void *user_data)

Prototype of ADC continuous mode event callback.

Param handle

[in] ADC continuous mode driver handle

Param edata

[in] Pointer to ADC continuous mode event data

Param user_data

[in] User registered context, registered when in adc_continuous_register_event_callbacks()

Return

Whether a high priority task is woken up by this function

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