# Analog to Digital Converter (ADC) Continuous Mode Driver

[中文]

## 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.

- Conversion Frame: One conversion frame contains multiple conversion results. Conversion frame size is configured in `adc_continuous_new_handle()` in bytes.

- Conversion Result: One conversion result contains multiple bytes, see `SOC_ADC_DIGI_RESULT_BYTES`. Its structure is `adc_digi_output_data_t`, including ADC unit, ADC channel, and raw data.

## 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: covers which parameters should be set up to initialize the ADC continuous mode driver and how to deinitialize it.

- ADC Configurations: describes how to configure the ADC(s) to make it work under continuous mode.

- ADC Control: describes ADC control functions.

- Register Event Callbacks: describes how to hook user-specific code to an ADC continuous mode event callback function.

- Read Conversion Result: covers how to get ADC conversion result.

- Hardware Limitations: describes the ADC-related hardware limitations.

- Power Management: covers power management-related information.

- IRAM Safe: covers the IRAM safe functions.

- Thread Safety: lists which APIs are guaranteed to be thread-safe by the driver.

### 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`:

- `adc_continuous_handle_cfg_t::max_store_buf_size`: set the maximum size of the pool in bytes, and the driver saves ADC conversion result into the pool. If this pool is full, new conversion results will be lost.

- `adc_continuous_handle_cfg_t::conv_frame_size`: set the size of the ADC conversion frame, in bytes.

- `adc_continuous_handle_cfg_t::flags`: set the flags that can change the driver's behavior.

  - `flush_pool`: auto flush the pool when it's full.

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_monitor_register_callbacks()`: register user callbacks to take action when the ADC value exceeds of the threshold.

- `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.

#### 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:

- `adc_continuous_config_t::pattern_num`: number of ADC channels that will be used.

- `adc_continuous_config_t::adc_pattern`: list of configs for each ADC channel that will be used, see the description below.

- `adc_continuous_config_t::sample_freq_hz`: expected ADC sampling frequency in Hz.

- `adc_continuous_config_t::conv_mode`: continuous conversion mode.

- `adc_continuous_config_t::format`: conversion output format.

Set `adc_digi_pattern_config_t` with the following process:

- `adc_digi_pattern_config_t::atten`: ADC attenuation. Refer to the On-Chip Sensor and Analog Signal Processing chapter in TRM.

- `adc_digi_pattern_config_t::channel`: the IO corresponding ADC channel number. See the note below.

- `adc_digi_pattern_config_t::unit`: the ADC that the IO is subordinate to.

- `adc_digi_pattern_config_t::bit_width`: the bitwidth of the raw conversion result.

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`.

- `adc_continuous_evt_cbs_t::on_conv_done`: this is invoked when one conversion frame finishes.

- `adc_continuous_evt_cbs_t::on_pool_ovf`: this is invoked when the internal pool is full. Newer conversion results will be discarded.

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 Datasheet.
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 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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