The RMT (Remote Control) module driver can be used to send and receive infrared remote control signals. Due to flexibility of RMT module, the driver can also be used to generate or receive many other types of signals.
The signal, which consists of a series of pulses, is generated by RMT's transmitter based on a list of values. The values define the pulse duration and a binary level, see below. The transmitter can also provide a carrier and modulate it with provided pulses.
The reverse operation is performed by the receiver, where a series of pulses is decoded into a list of values containing the pulse duration and binary level. A filter may be applied to remove high frequency noise from the input signal.
The RMT has eight channels numbered from zero to seven. Each channel is able to independently transmit or receive data. They are referred to using indexes defined in structure :cpp:type:`rmt_channel_t`.
There are several parameters that define how particular channel operates. Most of these parameters are configured by setting specific members of :cpp:type:`rmt_config_t` structure. Some of the parameters are common to both transmit or receive mode, and some are mode specific. They are all discussed below.
* The RMT **operation mode** - whether this channel is used to transmit or receive data, selected by setting a **rmt_mode** members to one of the values from :cpp:type:`rmt_mode_t`.
* What is the **pin number** to transmit or receive RMT signals, selected by setting **gpio_num**.
* How many **memory blocks** will be used by the channel, set with **mem_block_num**.
* A **clock divider**, that will determine the range of pulse length generated by the RMT transmitter or discriminated by the receiver. Selected by setting **clk_div** to a value within [1 .. 255] range. The RMT source clock is typically APB CLK, 80Mhz by default.
The period of a square wave after the clock divider is called a 'tick'. The length of the pulses generated by the RMT transmitter or discriminated by the receiver is configured in number of 'ticks'.
There are also couple of specific parameters that should be set up depending if selected channel is configured in `Transmit Mode`_ or `Receive Mode`_:
Transmit Mode
^^^^^^^^^^^^^
When configuring channel in transmit mode, set **tx_config** and the following members of :cpp:type:`rmt_tx_config_t`:
* Transmit the currently configured data items in a loop - **loop_en**
* Enable the RMT carrier signal - **carrier_en**
* Frequency of the carrier in Hz - **carrier_freq_hz**
* Duty cycle of the carrier signal in percent (%) - **carrier_duty_percent**
* Level of the RMT output, when the carrier is applied - **carrier_level**
* Enable the RMT output if idle - **idle_output_en**
* Set the signal level on the RMT output if idle - **idle_level**
Receive Mode
^^^^^^^^^^^^
In receive mode, set **rx_config** and the following members of :cpp:type:`rmt_rx_config_t`:
* Enable a filter on the input of the RMT receiver - **filter_en**
* A threshold of the filter, set in the number of ticks - **filter_ticks_thresh**. Pulses shorter than this setting will be filtered out. Note, that the range of entered tick values is [0..255].
* A pulse length threshold that will turn the RMT receiver idle, set in number of ticks - **idle_threshold**. The receiver will ignore pulses longer than this setting.
Finalize Configuration
^^^^^^^^^^^^^^^^^^^^^^
Once the :cpp:type:`rmt_config_t` structure is populated with parameters, it should be then invoked with :cpp:func:`rmt_config` to make the configuration effective.
The last configuration step is installation of the driver in memory by calling :cpp:func:`rmt_driver_install`. If :cpp:type:`rx_buf_size` parameter of this function is > 0, then a ring buffer for incoming data will be allocated. A default ISR handler will be installed, see a note in `Use Interrupts`_.
Now, depending on how the channel is configured, we are ready to either `Transmit Data`_ or `Receive Data`_. This is described in next two sections.
Before being able to transmit some RMT pulses, we need to define the pulse pattern. The minimum pattern recognized by the RMT controller, later called an 'item', is provided in a structure :cpp:type:`rmt_item32_t`, see :component_file:`soc/esp32/include/soc/rmt_caps.h`. Each item consists of two pairs of two values. The first value in a pair describes the signal duration in ticks and is 15 bits long, the second provides the signal level (high or low) and is contained in a single bit. A block of couple of items and the structure of an item is presented below.
The items are provided to the RMT controller by calling function :cpp:func:`rmt_write_items`. This function also automatically triggers start of transmission. It may be called to wait for transmission completion or exit just after transmission start. In such case you can wait for the transmission end by calling :cpp:func:`rmt_wait_tx_done`. This function does not limit the number of data items to transmit. It is using an interrupt to successively copy the new data chunks to RMT's internal memory as previously provided data are sent out.
Another way to provide data for transmission is by calling :cpp:func:`rmt_fill_tx_items`. In this case transmission is not started automatically. To control the transmission process use :cpp:func:`rmt_tx_start` and :cpp:func:`rmt_tx_stop`. The number of items to sent is restricted by the size of memory blocks allocated in the RMT controller's internal memory, see :cpp:func:`rmt_set_mem_block_num`.
Receive Data
------------
Before starting the receiver we need some storage for incoming items. The RMT controller has 512 x 32-bits of internal RAM shared between all eight channels. In typical scenarios it is not enough as an ultimate storage for all incoming (and outgoing) items. Therefore this API supports retrieval of incoming items on the fly to save them in a ring buffer of a size defined by the user. The size is provided when calling :cpp:func:`rmt_driver_install` discussed above. To get a handle to this buffer call :cpp:func:`rmt_get_ringbuf_handle`.
With the above steps complete we can start the receiver by calling :cpp:func:`rmt_rx_start` and then move to checking what's inside the buffer. To do so, you can use common FreeRTOS functions that interact with the ring buffer. Please see an example how to do it in :example:`peripherals/rmt/ir_protocols`.
To stop the receiver, call :cpp:func:`rmt_rx_stop`.
Change Operation Parameters
---------------------------
Previously described function :cpp:func:`rmt_config` provides a convenient way to set several configuration parameters in one shot. This is usually done on application start. Then, when the application is running, the API provides an alternate way to update individual parameters by calling dedicated functions. Each function refers to the specific RMT channel provided as the first input parameter. Most of the functions have `_get_` counterpart to read back the currently configured value.
Parameters Common to Transmit and Receive Mode
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
* Selection of a GPIO pin number on the input or output of the RMT - :cpp:func:`rmt_set_pin`
* Number of memory blocks allocated for the incoming or outgoing data - :cpp:func:`rmt_set_mem_pd`
* Setting of the clock divider - :cpp:func:`rmt_set_clk_div`
* Selection of the clock source, note that currently one clock source is supported, the APB clock which is 80Mhz - :cpp:func:`rmt_set_source_clk`
Transmit Mode Parameters
^^^^^^^^^^^^^^^^^^^^^^^^
* Enable or disable the loop back mode for the transmitter - :cpp:func:`rmt_set_tx_loop_mode`
* Binary level on the output to apply the carrier - :cpp:func:`rmt_set_tx_carrier`, selected from :cpp:type:`rmt_carrier_level_t`
* Determines the binary level on the output when transmitter is idle - :cpp:func:`rmt_set_idle_level()`, selected from :cpp:type:`rmt_idle_level_t`
Receive Mode Parameters
^^^^^^^^^^^^^^^^^^^^^^^
* The filter setting - :cpp:func:`rmt_set_rx_filter`
* The receiver threshold setting - :cpp:func:`rmt_set_rx_idle_thresh`
* Whether the transmitter or receiver is entitled to access RMT's memory - :cpp:func:`rmt_set_memory_owner`, selection is from :cpp:type:`rmt_mem_owner_t`.
Use Interrupts
--------------
Registering of an interrupt handler for the RMT controller is done be calling :cpp:func:`rmt_isr_register`.
When calling :cpp:func:`rmt_driver_install` to use the system RMT driver, a default ISR is being installed. In such a case you cannot register a generic ISR handler with :cpp:func:`rmt_isr_register`.
The RMT controller triggers interrupts on four specific events describes below. To enable interrupts on these events, the following functions are provided:
Setting or clearing an interrupt enable mask for specific channels and events may be also done by calling :cpp:func:`rmt_set_intr_enable_mask` or :cpp:func:`rmt_clr_intr_enable_mask`.
When servicing an interrupt within an ISR, the interrupt need to explicitly cleared. To do so, set specific bits described as ``RMT.int_clr.val.chN_event_name`` and defined as a ``volatile struct`` in :component_file:`soc/esp32/include/soc/rmt_struct.h`, where N is the RMT channel number [0, 7] and the ``event_name`` is one of four events described above.
If you do not need an ISR anymore, you can deregister it by calling a function :cpp:func:`rmt_isr_deregister`.
Uninstall Driver
----------------
If the RMT driver has been installed with :cpp:func:`rmt_driver_install` for some specific period of time and then not required, the driver may be removed to free allocated resources by calling :cpp:func:`rmt_driver_uninstall`.
