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Analog Comparator

{IDF_TARGET_ANA_CMPR_SRC_CHAN0: default="NOT UPDATED", esp32h2="GPIO11", esp32p4="GPIO52"} {IDF_TARGET_ANA_CMPR_EXT_REF_CHAN0: default="NOT UPDATED", esp32h2="GPIO10", esp32p4="GPIO51"} {IDF_TARGET_ANA_CMPR_SRC_CHAN1: default="NOT UPDATED", esp32p4="GPIO54"} {IDF_TARGET_ANA_CMPR_EXT_REF_CHAN1: default="NOT UPDATED", esp32p4="GPIO53"}

Introduction

Analog Comparator is a peripheral that can be used to compare a source signal with the internal reference voltage or an external reference signal.

It is a cost effective way to replace an amplifier comparator in some scenarios. But unlike the continuous comparing of the amplifier comparator, ESP Analog Comparator is driven by a source clock, which decides the sampling frequency.

Analog Comparator on {IDF_TARGET_NAME} has {IDF_TARGET_SOC_ANA_CMPR_NUM} unit(s), the channels in the unit(s) are:

UNIT0

  • Source Channel: {IDF_TARGET_ANA_CMPR_SRC_CHAN0}
  • External Reference Channel: {IDF_TARGET_ANA_CMPR_EXT_REF_CHAN0}
  • Internal Reference Channel: Range 0% ~ 70% of the VDD, the step is 10% of the VDD

esp32p4

UNIT1

  • Source Channel: {IDF_TARGET_ANA_CMPR_SRC_CHAN1}
  • External Reference Channel: {IDF_TARGET_ANA_CMPR_EXT_REF_CHAN1}
  • Internal Reference Channel: Range 0% ~ 70% of the VDD, the step is 10% of the VDD

Functional Overview

The following sections of this document cover the typical steps to install and operate an Analog Comparator unit:

  • Resource Allocation - covers which parameters should be set up to get a unit handle and how to recycle the resources when it finishes working.
  • Further Configurations - covers the other configurations that might need to specific and what they are used for.
  • Enable and Disable Unit - covers how to enable and disable the unit.
  • Power Management - describes how different source clock selections can affect power consumption.
  • IRAM Safe - lists which functions are supposed to work even when the cache is disabled.
  • Thread Safety - lists which APIs are guaranteed to be thread safe by the driver.
  • Kconfig Options - lists the supported Kconfig options that can be used to make a different effect on driver behavior.

SOC_ANA_CMPR_SUPPORT_ETM

Resource Allocation

An Analog Comparator unit channel is represented by :cppana_cmpr_handle_t. Each unit can support either an internal or an external reference.

To allocate the resource of the Analog Comparator unit, :cppana_cmpr_new_unit need to be called to get the handle of the unit. Configurations :cppana_cmpr_config_t need to be specified while allocating the unit:

  • :cppana_cmpr_config_t::unit selects the Analog Comparator unit.
  • :cppana_cmpr_config_t::clk_src selects the source clock for Analog Comparator, it can affect the sampling frequency. Note that the clock source of the Analog Comparator comes from the io mux, it is shared with GPIO extension peripherals like SDM (Sigma-Delta Modulation) and Glitch Filter. The configuration will fail if you specific different clock sources for multiple GPIO extension peripherals. The default clock sources of these peripherals are same, typically, we select :cppsoc_periph_ana_cmpr_clk_src_t::ANA_CMPR_CLK_SRC_DEFAULT as the clock source.
  • :cppana_cmpr_config_t::ref_src selects the reference source from internal voltage or external signal.
  • :cppana_cmpr_config_t::cross_type selects which kind of cross type can trigger the interrupt.

The function :cppana_cmpr_new_unit can fail due to various errors such as insufficient memory, invalid arguments, etc. If a previously created Analog Comparator unit is no longer required, you should recycle it by calling :cppana_cmpr_del_unit. It allows the underlying HW channel to be used for other purposes. Before deleting an Analog Comparator unit handle, you should disable it by :cppana_cmpr_unit_disable in advance, or make sure it has not enabled yet by :cppana_cmpr_unit_enable.

#include "driver/ana_cmpr.h"

ana_cmpr_handle_t cmpr = NULL;
ana_cmpr_config_t config = {
    .unit = 0,
    .clk_src = ANA_CMPR_CLK_SRC_DEFAULT,
    .ref_src = ANA_CMPR_REF_SRC_INTERNAL,
    .cross_type = ANA_CMPR_CROSS_ANY,
};
ESP_ERROR_CHECK(ana_cmpr_new_unit(&config, &cmpr));
// ...
ESP_ERROR_CHECK(ana_cmpr_del_unit(cmpr));

Further Configurations

  • :cppana_cmpr_set_intl_reference - Specify the internal reference voltage when :cppana_cmpr_ref_source_t::ANA_CMPR_REF_SRC_INTERNAL is selected as reference source.

It requires :cppana_cmpr_internal_ref_config_t::ref_volt to specify the voltage. The voltage related to the VDD power supply, which can only support a certain fixed percentage of VDD. Currently on {IDF_TARGET_NAME}, the internal reference voltage can be range to 0 ~ 70% VDD with a step 10%.

#include "driver/ana_cmpr.h"

ana_cmpr_internal_ref_config_t ref_cfg = {
    .ref_volt = ANA_CMPR_REF_VOLT_50_PCT_VDD,
};
ESP_ERROR_CHECK(ana_cmpr_set_internal_reference(cmpr, &ref_cfg));
  • :cppana_cmpr_set_debounce - Set the debounce configuration.

It requires :cppana_cmpr_debounce_config_t::wait_us to set the interrupt waiting time. The interrupt is disabled temporarily for :cppana_cmpr_debounce_config_t::wait_us micro seconds, so that the frequent triggering can be avoid while the source signal crossing the reference signal. That is, the waiting time is supposed to be inverse ratio to the relative frequency between the source and reference. If the waiting time is set too short, it can not bypass the jitter totally, but if too long, the next crossing interrupt might be missed.

#include "driver/ana_cmpr.h"

ana_cmpr_debounce_config_t dbc_cfg = {
    .wait_us = 1,
};
ESP_ERROR_CHECK(ana_cmpr_set_debounce(cmpr, &dbc_cfg));
  • :cppana_cmpr_set_cross_type - Set the source signal cross type.

The initial cross type is set int :cppana_cmpr_new_unit, this function can update the cross type, even in ISR context.

#include "driver/ana_cmpr.h"

ESP_ERROR_CHECK(ana_cmpr_set_cross_type(cmpr, ANA_CMPR_CROSS_POS));
  • :cppana_cmpr_register_event_callbacks - Register the callbacks.

Currently it supports :cppana_cmpr_event_callbacks_t::on_cross, it will be called when the crossing event (specified by :cppana_cmpr_config_t::cross_type) occurs.

#include "driver/ana_cmpr.h"

static bool IRAM_ATTR example_ana_cmpr_on_cross_callback(ana_cmpr_handle_t cmpr,
                                                     const ana_cmpr_cross_event_data_t *edata,
                                                     void *user_ctx)
{
    // ...
    return false;
}
ana_cmpr_event_callbacks_t cbs = {
    .on_cross = example_ana_cmpr_on_cross_callback,
};
ESP_ERROR_CHECK(ana_cmpr_register_event_callbacks(cmpr, &cbs, NULL));

Note

When CONFIG_ANA_CMPR_ISR_IRAM_SAFE is enabled, you should guarantee the callback context and involved data to be in internal RAM by add the attribute IRAM_ATTR. (See more in IRAM Safe)

Enable and Disable Unit

  • :cppana_cmpr_enable - Enable the Analog Comparator unit.
  • :cppana_cmpr_disable - Disable the Analog Comparator unit.

After the Analog Comparator unit is enabled and the crossing event interrupt is enabled, a power management lock will be acquired if the power management is enabled (see Power Management). Under the enable state, only :cppana_cmpr_set_intl_reference and :cppana_cmpr_set_debounce can be called, other functions can only be called after the unit is disabled.

Calling :cppana_cmpr_disable does the opposite.

Power Management

When power management is enabled (i.e., CONFIG_PM_ENABLE is on), the system will adjust the APB frequency before going into light sleep, thus potentially changing the resolution of the Analog Comparator.

However, the driver can prevent the system from changing APB frequency by acquiring a power management lock of type :cppESP_PM_NO_LIGHT_SLEEP. Whenever the driver creates a Analog Comparator unit instance that has selected the clock source like :cppANA_CMPR_CLK_SRC_DEFAULT or :cppANA_CMPR_CLK_SRC_XTAL as its clock source, the driver guarantees that the power management lock is acquired when enable the channel by :cppana_cmpr_enable. Likewise, the driver releases the lock when :cppana_cmpr_disable is called for that channel.

IRAM Safe

By default, the Analog Comparator interrupt will be deferred when the Cache is disabled for reasons like programming/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 CONFIG_ANA_CMPR_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 IRAM1
  3. Places driver object into DRAM (in case it is allocated on PSRAM)

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

There is a Kconfig option CONFIG_ANA_CMPR_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 listed as follows:

  • :cppana_cmpr_set_internal_reference
  • :cppana_cmpr_set_debounce
  • :cppana_cmpr_set_cross_type

Thread Safety

The factory function :cppana_cmpr_new_unit is guaranteed to be thread safe by the driver, which means, user can call it 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.

  • :cppana_cmpr_set_internal_reference
  • :cppana_cmpr_set_debounce
  • :cppana_cmpr_set_cross_type

Other functions that take the :cppana_cmpr_handle_t as the first positional parameter, are not treated as thread safe. Which means the user should avoid calling them from multiple tasks.

Kconfig Options

  • CONFIG_ANA_CMPR_ISR_IRAM_SAFE controls whether the default ISR handler can work when cache is disabled, see IRAM Safe for more information.
  • CONFIG_ANA_CMPR_CTRL_FUNC_IN_IRAM controls where to place the Analog Comparator control functions (IRAM or Flash), see IRAM Safe for more information.
  • CONFIG_ANA_CMPR_ENABLE_DEBUG_LOG is used to enabled the debug log output. Enabling this option increases the firmware binary size.

SOC_ANA_CMPR_SUPPORT_ETM

ETM Events

To create an analog comparator cross event, you need to include driver/ana_cmpr_etm.h additionally, and allocate the event by :cppana_cmpr_new_etm_event. You can refer to ETM </api-reference/peripherals/etm> for how to connect an event to a task.

Application Example

  • peripherals/analog_comparator shows the basic usage of the analog comparator, and other potential usages like hysteresis comparator and SPWM generator.

API Reference

inc/ana_cmpr.inc

inc/ana_cmpr_types.inc

  1. :cppana_cmpr_event_callbacks_t::on_cross callback and the functions invoked by itself should also be placed in IRAM, you need to take care of them by themselves.↩