A touch sensor system is built on a substrate which carries electrodes and relevant connections under a protective flat surface. When the surface is touched, the capacitance variation is used to evaluate if the touch was valid.
The sensing pads can be arranged in different combinations (e.g., matrix, slider), so that a larger area or more points can be detected. The touch pad sensing process is under the control of a hardware-implemented finite-state machine (FSM) which is initiated by software or a dedicated hardware timer.
For design, operation, and control registers of a touch sensor, see **{IDF_TARGET_NAME} Technical Reference Manual** > **On-Chip Sensors and Analog Signal Processing** [`PDF <{IDF_TARGET_TRM_EN_URL}#sensor>`__].
In-depth design details of touch sensors and firmware development guidelines for {IDF_TARGET_NAME} are available in `Touch Sensor Application Note <https://github.com/espressif/esp-iot-solution/blob/release/v1.0/documents/touch_pad_solution/touch_sensor_design_en.md>`_.
For more information about testing touch sensors in various configurations, please check the `Guide for ESP32-Sense-Kit <https://docs.espressif.com/projects/espressif-esp-dev-kits/en/latest/esp32/esp32-sense-kit/user_guide.html>`_.
For detailed description of a particular function, please go to Section :ref:`touch_pad-api-reference`. Practical implementation of this API is covered in Section :ref:`Application Examples <touch_pad-api-examples>`.
Before using a touch pad, you need to initialize the touch pad driver by calling the function :cpp:func:`touch_pad_init`. This function sets several ``.._DEFAULT`` driver parameters listed in :ref:`touch_pad-api-reference` under **Macros**. It also removes the information about which pads have been touched before, if any, and disables interrupts.
Enabling the touch sensor functionality for a particular GPIO is done with :cpp:func:`touch_pad_config()`. The following 10 capacitive touch pads are supported for {IDF_TARGET_NAME}.
..list-table::
:align:center
:widths:50 50
:header-rows:1
* - Touch Pad
- GPIO Pin
* - T0
- GPIO4
* - T1
- GPIO0
* - T2
- GPIO2
* - T3
- MTDO
* - T4
- MTCK
* - T5
- MTDI
* - T6
- MTMS
* - T7
- GPIO27
* - T8
- 32K_XN
* - T9
- 32K_XP
..only:: esp32s2 or esp32s3
Enabling the touch sensor functionality for a particular GPIO is done with :cpp:func:`touch_pad_config()`. The following 14 capacitive touch pads are supported for {IDF_TARGET_NAME}.
Use the function :cpp:func:`touch_pad_set_fsm_mode` to select if touch pad measurement (operated by FSM) should be started automatically by a hardware timer, or by software. If software mode is selected, use :cpp:func:`touch_pad_sw_start` to start the FSM.
The following two functions come in handy to read raw or filtered measurements from the sensor:
*:cpp:func:`touch_pad_read_raw_data`
*:cpp:func:`touch_pad_read_filtered`
They can also be used, for example, to evaluate a particular touch pad design by checking the range of sensor readings when a pad is touched or released. This information can be then used to establish a touch threshold.
Before using :cpp:func:`touch_pad_read_filtered`, you need to initialize and configure the filter by calling specific filter functions described in Section `Filtering of Measurements`_.
It can also be used, for example, to evaluate a particular touch pad design by checking the range of sensor readings when a pad is touched or released. This information can be then used to establish a touch threshold.
For the demonstration of how to read the touch pad data, check the application example :example:`peripherals/touch_sensor/touch_sensor_{IDF_TARGET_TOUCH_SENSOR_VERSION}/touch_pad_read`.
The touch sensor counts the number of charge/discharge cycles over a fixed period of time (specified by :cpp:func:`touch_pad_set_measurement_clock_cycles`). The count result is the raw data that read from :cpp:func:`touch_pad_read_raw_data`. After finishing one measurement, the touch sensor sleeps until the next measurement start, this interval between two measurements can be set by :cpp:func:`touch_pad_set_measurement_interval`.
If the specified clock cycles for measurement is too small, the result may be inaccurate, but increasing clock cycles will increase the power consumption as well. Additionally, the response of the touch sensor will slow down if the total time of the interval and measurement is too long.
The touch sensor records the period of time (i.e., the number of clock cycles) over a fixed charge/discharge cycles (specified by :cpp:func:`touch_pad_set_charge_discharge_times`). The count result is the raw data that read from :cpp:func:`touch_pad_read_raw_data`. After finishing one measurement, the touch sensor sleeps until the next measurement start, this interval between two measurements can be set by :cpp:func:`touch_pad_set_measurement_interval`.
If the specified charge and discharge cycles for measurement is too small, the result may be inaccurate, but increasing charge and discharge cycles will increase the power consumption as well. Additionally, the response of the touch sensor will slow down if the total time of the interval and measurement is too long.
A touch sensor has several configurable parameters to match the characteristics of a particular touch pad design. For instance, to sense smaller capacity changes, it is possible to narrow down the reference voltage range within which the touch pads are charged/discharged. The high and low reference voltages are set using the function :cpp:func:`touch_pad_set_voltage`.
Besides the ability to discern smaller capacity changes, a positive side effect is reduction of power consumption for low power applications. A likely negative effect is an increase in measurement noise. If the dynamic range of obtained readings is still satisfactory, then further reduction of power consumption might be done by reducing the measurement time with :cpp:func:`touch_pad_set_measurement_clock_cycles`.
Besides the ability to discern smaller capacity changes, a positive side effect is reduction of power consumption for low power applications. A likely negative effect is an increase in measurement noise. If the dynamic range of obtained readings is still satisfactory, then further reduction of power consumption might be done by reducing the measurement time with :cpp:func:`touch_pad_set_charge_discharge_times`.
All functions are provided in pairs to **set** a specific parameter and to **get** the current parameter's value, e.g., :cpp:func:`touch_pad_set_voltage` and :cpp:func:`touch_pad_get_voltage`.
If measurements are noisy, you can filter them with provided API functions. Before using the filter, please start it by calling :cpp:func:`touch_pad_filter_start`.
The filter type is IIR (infinite impulse response), and it has a configurable period that can be set with the function :cpp:func:`touch_pad_set_filter_period`.
You can stop the filter with :cpp:func:`touch_pad_filter_stop`. If not required anymore, the filter can be deleted by invoking :cpp:func:`touch_pad_filter_delete`.
If measurements are noisy, you can filter them with provided API functions. The {IDF_TARGET_NAME}'s touch functionality provide two sets of APIs for doing this.
There is an internal touch channel that is not connected to any external GPIO. The measurements from this denoise pad can be used to filters out interference introduced on all channels, such as noise introduced by the power supply and external EMI.
There is also a configurable hardware implemented IIR-filter (infinite impulse response). This IIR-filter is configured with the function :cpp:func:`touch_pad_filter_set_config` and enabled by calling :cpp:func:`touch_pad_filter_enable`
Touch detection is implemented in ESP32's hardware based on the user-configured threshold and raw measurements executed by FSM. Use the functions :cpp:func:`touch_pad_get_status` to check which pads have been touched and :cpp:func:`touch_pad_clear_status` to clear the touch status information.
If measurements are noisy and capacity changes are small, hardware touch detection might be unreliable. To resolve this issue, instead of using hardware detection/provided interrupts, implement measurement filtering and perform touch detection in your own application. For sample implementation of both methods of touch detection, see :example:`peripherals/touch_sensor/touch_sensor_{IDF_TARGET_TOUCH_SENSOR_VERSION}/touch_pad_interrupt`.
Before enabling an interrupt on a touch detection, you should establish a touch detection threshold. Use the functions described in `Touch State Measurements`_ to read and display sensor measurements when a pad is touched and released. Apply a filter if measurements are noisy and relative capacity changes are small. Depending on your application and environment conditions, test the influence of temperature and power supply voltage changes on measured values.
Once a detection threshold is established, it can be set during initialization with :cpp:func:`touch_pad_config` or at the runtime with :cpp:func:`touch_pad_set_thresh`.
In the next step, configure how interrupts are triggered. They can be triggered below or above the threshold, which is set with the function :cpp:func:`touch_pad_set_trigger_mode`.
When interrupts are operational, you can obtain the information from which particular pad an interrupt came by invoking :cpp:func:`touch_pad_get_status` and clear the pad status with :cpp:func:`touch_pad_clear_status`.
Interrupts on touch detection operate on raw/unfiltered measurements checked against user established threshold and are implemented in hardware. Enabling the software filtering API (see :ref:`touch_pad-api-filtering-of-measurements`) does not affect this process.
If touch pad interrupts are used to wake up the chip from a sleep mode, you can select a certain configuration of pads (SET1 or both SET1 and SET2) that should be touched to trigger the interrupt and cause the subsequent wakeup. To do so, use the function :cpp:func:`touch_pad_set_trigger_source`.
Configuration of required bit patterns of pads may be managed for each 'SET' with:
-:example:`peripherals/touch_sensor/touch_sensor_v1/touch_pad_read` demonstrates how to read and print raw or IIR filtered values from capacitive touch sensors on {IDF_TARGET_NAME}, and how to calibrate the sensors.
-:example:`peripherals/touch_sensor/touch_sensor_v1/touch_pad_interrupt` demonstrates how to set up the {IDF_TARGET_NAME}'s capacitive touch pad to trigger an interrupt when touched, and how to use software detection for greater sensitivity, comparing the performance of hardware interrupt mode and software polling mode.
..only:: esp32s2 or esp32s3
-:example:`peripherals/touch_sensor/touch_sensor_v2/touch_pad_read` demonstrates how to read and display raw values from capacitive touch pad sensors on {IDF_TARGET_NAME}, including how to calibrate the sensors and detect touch actions.
-:example:`peripherals/touch_sensor/touch_sensor_v2/touch_pad_interrupt` demonstrates how to set up {IDF_TARGET_NAME}'s capacitive touch pad peripheral to trigger an interrupt when a pad is touched, and how to detect the touch event by the software for sensor designs when greater touch detection sensitivity is required.
