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Pulse Counter (PCNT)

Introduction

The PCNT (Pulse Counter) module is designed to count the number of rising and/or falling edges of input signals. The {IDF_TARGET_NAME} contains multiple pulse counter units in the module. [1] Each unit is in effect an independent counter with multiple channels, where each channel can increment/decrement the counter on a rising/falling edge. Furthermore, each channel can be configured separately.

PCNT channels can react to signals of edge type and level type, however for simple applications, detecting the edge signal is usually sufficient. PCNT channels can be configured react to both pulse edges (i.e., rising and falling edge), and can be configured to increase, decrease or do nothing to the unit's counter on each edge. The level signal is the so-called control signal, which is used to control the counting mode of the edge signals that are attached to the same channel. By combining the usage of both edge and level signals, a PCNT unit can act as a quadrature decoder.

Besides that, PCNT unit is equipped with a separate glitch filter, which is helpful to remove noise from the signal.

Typically, a PCNT module can be used in scenarios like:

  • Calculate periodic signal's frequency by counting the pulse numbers within a time slice
  • Decode quadrature signals into speed and direction

Functional Overview

Description of the PCNT functionality is divided into the following sections:

.. list::

    - :ref:`pcnt-resource-allocation` - covers how to allocate PCNT units and channels with properly set of configurations. It also covers how to recycle the resources when they finished working.
    - :ref:`pcnt-setup-channel-actions` - covers how to configure the PCNT channel to behave on different signal edges and levels.
    - :ref:`pcnt-watch-points` - describes how to configure PCNT watch points (i.e., tell PCNT unit to trigger an event when the count reaches a certain value).
    - :ref:`pcnt-register-event-callbacks` - describes how to hook your specific code to the watch point event callback function.
    - :ref:`pcnt-set-glitch-filter` - describes how to enable and set the timing parameters for the internal glitch filter.
    :SOC_PCNT_SUPPORT_CLEAR_SIGNAL: - :ref:`pcnt-set-clear-signal` - describes how to set the parameters for the external clear signal.
    - :ref:`pcnt-enable-disable-unit` - describes how to enable and disable the PCNT unit.
    - :ref:`pcnt-unit-io-control` - describes IO control functions of PCNT unit, like enable glitch filter, start and stop unit, get and clear count value.
    - :ref:`pcnt-power-management` - describes what functionality will prevent the chip from going into low power mode.
    - :ref:`pcnt-iram-safe` - describes tips on how to make the PCNT interrupt and IO control functions work better along with a disabled cache.
    - :ref:`pcnt-thread-safe` - lists which APIs are guaranteed to be thread safe by the driver.
    - :ref:`pcnt-kconfig-options` - lists the supported Kconfig options that can be used to make a different effect on driver behavior.

Resource Allocation

The PCNT unit and channel are represented by :cpp:type:`pcnt_unit_handle_t` and :cpp:type:`pcnt_channel_handle_t` respectively. All available units and channels are maintained by the driver in a resource pool, so you do not need to know the exact underlying instance ID.

Install PCNT Unit

To install a PCNT unit, there is a configuration structure that needs to be given in advance: :cpp:type:`pcnt_unit_config_t`:

Note

Since all PCNT units share the same interrupt source, when installing multiple PCNT units make sure that the interrupt priority :cpp:member:`pcnt_unit_config_t::intr_priority` is the same for each unit.

Unit allocation and initialization is done by calling a function :cpp:func:`pcnt_new_unit` with :cpp:type:`pcnt_unit_config_t` as an input parameter. The function will return a PCNT unit handle only when it runs correctly. Specifically, when there are no more free PCNT units in the pool (i.e., unit resources have been used up), then this function will return :c:macro:`ESP_ERR_NOT_FOUND` error. The total number of available PCNT units is recorded by :c:macro:`SOC_PCNT_UNITS_PER_GROUP` for reference.

If a previously created PCNT unit is no longer needed, it is recommended to recycle the resource by calling :cpp:func:`pcnt_del_unit`. Which in return allows the underlying unit hardware to be used for other purposes. Before deleting a PCNT unit, one should ensure the following prerequisites:

#define EXAMPLE_PCNT_HIGH_LIMIT 100
#define EXAMPLE_PCNT_LOW_LIMIT  -100

pcnt_unit_config_t unit_config = {
    .high_limit = EXAMPLE_PCNT_HIGH_LIMIT,
    .low_limit = EXAMPLE_PCNT_LOW_LIMIT,
};
pcnt_unit_handle_t pcnt_unit = NULL;
ESP_ERROR_CHECK(pcnt_new_unit(&unit_config, &pcnt_unit));

Install PCNT Channel

To install a PCNT channel, you must initialize a :cpp:type:`pcnt_chan_config_t` structure in advance, and then call :cpp:func:`pcnt_new_channel`. The configuration fields of the :cpp:type:`pcnt_chan_config_t` structure are described below:

Channel allocating and initialization is done by calling a function :cpp:func:`pcnt_new_channel` with the above :cpp:type:`pcnt_chan_config_t` as an input parameter plus a PCNT unit handle returned from :cpp:func:`pcnt_new_unit`. This function will return a PCNT channel handle if it runs correctly. Specifically, when there are no more free PCNT channel within the unit (i.e., channel resources have been used up), then this function will return :c:macro:`ESP_ERR_NOT_FOUND` error. The total number of available PCNT channels within the unit is recorded by :c:macro:`SOC_PCNT_CHANNELS_PER_UNIT` for reference. Note that, when install a PCNT channel for a specific unit, one should ensure the unit is in the init state, otherwise this function will return :c:macro:`ESP_ERR_INVALID_STATE` error.

If a previously created PCNT channel is no longer needed, it is recommended to recycle the resources by calling :cpp:func:`pcnt_del_channel`. Which in return allows the underlying channel hardware to be used for other purposes.

#define EXAMPLE_CHAN_GPIO_A 0
#define EXAMPLE_CHAN_GPIO_B 2

pcnt_chan_config_t chan_config = {
    .edge_gpio_num = EXAMPLE_CHAN_GPIO_A,
    .level_gpio_num = EXAMPLE_CHAN_GPIO_B,
};
pcnt_channel_handle_t pcnt_chan = NULL;
ESP_ERROR_CHECK(pcnt_new_channel(pcnt_unit, &chan_config, &pcnt_chan));

Set Up Channel Actions

The PCNT will increase/decrease/hold its internal count value when the input pulse signal toggles. You can set different actions for edge signal and/or level signal.

// decrease the counter on rising edge, increase the counter on falling edge
ESP_ERROR_CHECK(pcnt_channel_set_edge_action(pcnt_chan, PCNT_CHANNEL_EDGE_ACTION_DECREASE, PCNT_CHANNEL_EDGE_ACTION_INCREASE));
// keep the counting mode when the control signal is high level, and reverse the counting mode when the control signal is low level
ESP_ERROR_CHECK(pcnt_channel_set_level_action(pcnt_chan, PCNT_CHANNEL_LEVEL_ACTION_KEEP, PCNT_CHANNEL_LEVEL_ACTION_INVERSE));

Watch Points

Each PCNT unit can be configured to watch several different values that you are interested in. The value to be watched is also called Watch Point. The watch point itself can not exceed the range set in :cpp:type:`pcnt_unit_config_t` by :cpp:member:`pcnt_unit_config_t::low_limit` and :cpp:member:`pcnt_unit_config_t::high_limit`. When the counter reaches either watch point, a watch event will be triggered and notify you by interrupt if any watch event callback has ever registered in :cpp:func:`pcnt_unit_register_event_callbacks`. See :ref:`pcnt-register-event-callbacks` for how to register event callbacks.

The watch point can be added and removed by :cpp:func:`pcnt_unit_add_watch_point` and :cpp:func:`pcnt_unit_remove_watch_point`. The commonly-used watch points are: zero cross, maximum/minimum count and other threshold values. The number of available watch point is limited, :cpp:func:`pcnt_unit_add_watch_point` will return error :c:macro:`ESP_ERR_NOT_FOUND` if it can not find any free hardware resource to save the watch point. You can not add the same watch point for multiple times, otherwise it will return error :c:macro:`ESP_ERR_INVALID_STATE`.

It is recommended to remove the unused watch point by :cpp:func:`pcnt_unit_remove_watch_point` to recycle the watch point resources.

// add zero across watch point
ESP_ERROR_CHECK(pcnt_unit_add_watch_point(pcnt_unit, 0));
// add high limit watch point
ESP_ERROR_CHECK(pcnt_unit_add_watch_point(pcnt_unit, EXAMPLE_PCNT_HIGH_LIMIT));
.. only:: not SOC_PCNT_SUPPORT_RUNTIME_THRES_UPDATE

    .. note::

        Due to the hardware limitation, after adding a watch point, you should call :cpp:func:`pcnt_unit_clear_count` to make it take effect.

Register Event Callbacks

When PCNT unit reaches any enabled watch point, specific event will be generated and notify the CPU by interrupt. If you have some function that want to get executed when event happens, you should hook your function to the interrupt service routine by calling :cpp:func:`pcnt_unit_register_event_callbacks`. All supported event callbacks are listed in the :cpp:type:`pcnt_event_callbacks_t`:

  • :cpp:member:`pcnt_event_callbacks_t::on_reach` sets a callback function for watch point event. As this function is called within the ISR context, you must ensure that the function does not attempt to block (e.g., by making sure that only FreeRTOS APIs with ISR suffix are called from within the function). The function prototype is declared in :cpp:type:`pcnt_watch_cb_t`.

You can save their own context to :cpp:func:`pcnt_unit_register_event_callbacks` as well, via the parameter user_ctx. This user data will be directly passed to the callback functions.

In the callback function, the driver will fill in the event data of specific event. For example, the watch point event data is declared as :cpp:type:`pcnt_watch_event_data_t`:

Registering callback function results in lazy installation of interrupt service, thus this function should only be called before the unit is enabled by :cpp:func:`pcnt_unit_enable`. Otherwise, it can return :c:macro:`ESP_ERR_INVALID_STATE` error.

static bool example_pcnt_on_reach(pcnt_unit_handle_t unit, const pcnt_watch_event_data_t *edata, void *user_ctx)
{
    BaseType_t high_task_wakeup;
    QueueHandle_t queue = (QueueHandle_t)user_ctx;
    // send watch point to queue, from this interrupt callback
    xQueueSendFromISR(queue, &(edata->watch_point_value), &high_task_wakeup);
    // return whether a high priority task has been waken up by this function
    return (high_task_wakeup == pdTRUE);
}

pcnt_event_callbacks_t cbs = {
    .on_reach = example_pcnt_on_reach,
};
QueueHandle_t queue = xQueueCreate(10, sizeof(int));
ESP_ERROR_CHECK(pcnt_unit_register_event_callbacks(pcnt_unit, &cbs, queue));

Set Glitch Filter

The PCNT unit features filters to ignore possible short glitches in the signals. The parameters that can be configured for the glitch filter are listed in :cpp:type:`pcnt_glitch_filter_config_t`:

You can enable the glitch filter for PCNT unit by calling :cpp:func:`pcnt_unit_set_glitch_filter` with the filter configuration provided above. Particularly, you can disable the glitch filter later by calling :cpp:func:`pcnt_unit_set_glitch_filter` with a NULL filter configuration.

This function should be called when the unit is in the init state. Otherwise, it will return :c:macro:`ESP_ERR_INVALID_STATE` error.

Note

The glitch filter is clocked from APB. For the counter not to miss any pulses, the maximum glitch width should be longer than one APB_CLK cycle (usually 12.5 ns if APB equals 80 MHz). As the APB frequency would be changed after DFS (Dynamic Frequency Scaling) enabled, which means the filter does not work as expect in that case. So the driver installs a PM lock for PCNT unit during the first time you enable the glitch filter. For more information related to power management strategy used in PCNT driver, please see :ref:`pcnt-power-management`.

pcnt_glitch_filter_config_t filter_config = {
    .max_glitch_ns = 1000,
};
ESP_ERROR_CHECK(pcnt_unit_set_glitch_filter(pcnt_unit, &filter_config));
.. only:: SOC_PCNT_SUPPORT_CLEAR_SIGNAL

    .. _pcnt-set-clear-signal:

    Use External Clear Signal
    ^^^^^^^^^^^^^^^^^^^^^^^^^

    The PCNT unit can receive a clear signal from the GPIO. The parameters that can be configured for the clear signal are listed in :cpp:type:`pcnt_clear_signal_config_t`:

        -  :cpp:member:`pcnt_clear_signal_config_t::clear_signal_gpio_num` specify the GPIO numbers used by **clear** signal. The default active level is high, and the input mode is pull-down enabled.
        -  :cpp:member:`pcnt_clear_signal_config_t::invert_clear_signal` is used to decide whether to invert the input signal before it going into PCNT hardware. The invert is done by GPIO matrix instead of PCNT hardware. The input mode is pull-up enabled when the input signal is inverted.
        -  :cpp:member:`pcnt_clear_signal_config_t::io_loop_back` is for debug only, which enables both the GPIO's input and output paths. This can help to simulate the clear signal by function :cpp:func:`gpio_set_level` for the same GPIO.

    This signal acts in the same way as calling :cpp:func:`pcnt_unit_clear_count`, but is not subject to software latency, and is suitable for use in situations with low latency requirements. Also please note, the flip frequency of this signal can not be too high.

    .. code:: c

        pcnt_clear_signal_config_t clear_signal_config = {
            .clear_signal_gpio_num = PCNT_CLEAR_SIGNAL_GPIO,
        };
        ESP_ERROR_CHECK(pcnt_unit_set_clear_signal(pcnt_unit, &clear_signal_config));

    .. _pcnt-enable-disable-unit:


.. only:: not SOC_PCNT_SUPPORT_CLEAR_SIGNAL

    .. _pcnt-enable-disable-unit:

Enable and Disable Unit

Before doing IO control to the PCNT unit, you need to enable it first, by calling :cpp:func:`pcnt_unit_enable`. Internally, this function:

On the contrary, calling :cpp:func:`pcnt_unit_disable` will do the opposite, that is, put the PCNT driver back to the init state, disable the interrupts service and release the power management lock.

Unit IO Control

Start/Stop and Clear

Calling :cpp:func:`pcnt_unit_start` makes the PCNT unit start to work, increase or decrease counter according to pulse signals. On the contrary, calling :cpp:func:`pcnt_unit_stop` will stop the PCNT unit but retain current count value. Instead, clearing counter can only be done by calling :cpp:func:`pcnt_unit_clear_count`.

Note, :cpp:func:`pcnt_unit_start` and :cpp:func:`pcnt_unit_stop` should be called when the unit has been enabled by :cpp:func:`pcnt_unit_enable`. Otherwise, it will return :c:macro:`ESP_ERR_INVALID_STATE` error.

Get Count Value

You can read current count value at any time by calling :cpp:func:`pcnt_unit_get_count`. The returned count value is a signed integer, where the sign can be used to reflect the direction.

int pulse_count = 0;
ESP_ERROR_CHECK(pcnt_unit_get_count(pcnt_unit, &pulse_count));

Compensate Overflow Loss

The internal hardware counter will be cleared to zero automatically when it reaches high or low limit. If you want to compensate for that count loss and extend the counter's bit-width, you can:

  1. Enable :cpp:member:`pcnt_unit_config_t::accum_count` when installing the PCNT unit.
  2. Add the high/low limit as the :ref:`pcnt-watch-points`.
  3. Now, the returned count value from the :cpp:func:`pcnt_unit_get_count` function not only reflects the hardware's count value, but also accumulates the high/low overflow loss to it.

Note

:cpp:func:`pcnt_unit_clear_count` resets the accumulated count value as well.

Power Management

When power management is enabled (i.e., :ref:`CONFIG_PM_ENABLE` is on), the system will adjust the APB frequency before going into light sleep, thus potentially changing the behavior of PCNT glitch filter and leading to valid signal being treated as noise.

However, the driver can prevent the system from changing APB frequency by acquiring a power management lock of type :cpp:enumerator:`ESP_PM_APB_FREQ_MAX`. Whenever you enable the glitch filter by :cpp:func:`pcnt_unit_set_glitch_filter`, the driver guarantees that the power management lock is acquired after the PCNT unit is enabled by :cpp:func:`pcnt_unit_enable`. Likewise, the driver releases the lock after :cpp:func:`pcnt_unit_disable` is called.

IRAM Safe

By default, the PCNT interrupt will be deferred when the Cache is disabled for reasons like writing/erasing Flash. Thus the alarm interrupt will not get executed in time, which is not expected in a real-time application.

There is a Kconfig option :ref:`CONFIG_PCNT_ISR_IRAM_SAFE` that:

  1. Enables the interrupt being serviced even when cache is disabled
  2. Places all functions that used by the ISR into IRAM [2]
  3. Places driver object into DRAM (in case it is mapped to PSRAM by accident)

This allows the interrupt to run while the cache is disabled but comes at the cost of increased IRAM consumption.

There is another Kconfig option :ref:`CONFIG_PCNT_CTRL_FUNC_IN_IRAM` that can put commonly used IO control functions into IRAM as well. So that these functions can also be executable when the cache is disabled. These IO control functions are as follows:

Thread Safety

The factory functions :cpp:func:`pcnt_new_unit` and :cpp:func:`pcnt_new_channel` are guaranteed to be thread safe by the driver, which means, you can call them from different RTOS tasks without protection by extra locks.

The following functions are allowed to run under ISR context, the driver uses a critical section to prevent them being called concurrently in both task and ISR.

Other functions that take the :cpp:type:`pcnt_unit_handle_t` and :cpp:type:`pcnt_channel_handle_t` as the first positional parameter, are not treated as thread safe. This means you should avoid calling them from multiple tasks.

Kconfig Options

Application Examples

API Reference

.. include-build-file:: inc/pulse_cnt.inc

.. include-build-file:: inc/pcnt_types.inc
[1]Different ESP chip series might have different number of PCNT units and channels. Please refer to the [TRM] for details. The driver does not forbid you from applying for more PCNT units and channels, but it returns error when all available hardware resources are used up. Please always check the return value when doing resource allocation (e.g., :cpp:func:`pcnt_new_unit`).
[2]:cpp:member:`pcnt_event_callbacks_t::on_reach` callback and the functions invoked by itself should also be placed in IRAM, you need to take care of them by themselves.