I2S (Inter-IC Sound) is a serial, synchronous communication protocol that is usually used for transmitting audio data between two digital audio devices.
{IDF_TARGET_NAME} contains {IDF_TARGET_I2S_NUM} I2S peripheral(s). These peripherals can be configured to input and output sample data via the I2S driver.
I2S0 output can be routed directly to the digital-to-analog converter's (DAC) output channels (GPIO 25 & GPIO 26) to produce direct analog output without involving any external I2S codecs. I2S0 can also be used for transmitting PDM (Pulse-density modulation) signals.
The I2S peripherals also support LCD mode for communicating data over a parallel bus, as used by some LCD displays and camera modules. LCD mode has the following operational modes:
For high accuracy clock applications, use the APLL_CLK clock source, which has the frequency range of 16 ~ 128 MHz. You can enable the APLL_CLK clock source by setting :cpp:member:`i2s_config_t::use_apll` to ``TRUE``.
If :cpp:member:`i2s_config_t::use_apll` = ``TRUE`` and :cpp:member:`i2s_config_t::fixed_mclk` > ``0``, then the master clock output frequency for I2S will be equal to the value of :cpp:member:`i2s_config_t::fixed_mclk`, which means that the mclk frequency is provided by the user, instead of being calculated by the driver.
The clock rate of the word select line, which is called audio left-right clock rate (LRCK) here, is always the divisor of the master clock output frequency and for which the following is always true: 0 < MCLK/LRCK/channels/bits_per_sample < 64.
Once the driver is installed, configure the physical GPIO pins to which the I2S signals will be routed. This is accomplished by calling the function :cpp:func:`i2s_set_pin` with the following arguments:
- The structure :cpp:type:`i2s_pin_config_t` which defines the GPIO pin numbers for the MCK, BCK, WS, DATA out, and DATA in signals. To keep a current pin allocatopm pin for a specific signal, or to indicate an unused signal, pass the macro :c:macro:`I2S_PIN_NO_CHANGE`. See the example below.
To retrieve received data, use the function :cpp:func:`i2s_read`. It will retrieve the data from the DMA Rx buffer once the data is received by the I2S peripheral.
You can temporarily halt the I2S driver by calling :cpp:func:`i2s_stop`, which disables the I2S Tx/Rx units until :cpp:func:`i2s_start` is called. The driver automatically starts the I2S peripheral after :cpp:func:`i2s_driver_install` is called, eliminating the need to call :cpp:func:`i2s_start`.
I2S on {IDF_TARGET_NAME} support TDM mode, up to 16 channels are available in TDM mode. If you want to use TDM mode, set field ``channel_format`` of :cpp:type:`i2s_config_t` to ``I2S_CHANNEL_FMT_MULTIPLE``. Then enable the channels by setting ``chan_mask`` using masks in :cpp:type:`i2s_channel_t`, the number of active channels and total channels will be calculate automatically. You can also set a particular total channel number for it, but it shouldn't be smaller than the largest channel you use.
If active channels are discrete, the inactive channels within total channels will be filled by a constant automatically. But if ``skip_msk`` is enabled, these inactive channels will be skiped.
In order to accomodate different application requirements, some aspects of DMA are made public via the ``dma_desc_num`` and ``dma_frame_num`` parameters. The following section provides a description of these two fields:
-``dma_desc_num``: The total number of descriptors used by I2S DMA to receive/transmit data. A descriptor includes information such as buffer address, the address of the next descriptor, and the buffer length. As each descriptor points to one buffer, ``dma_desc_num`` can be interpreted as the total number of DMA buffers used to store data in the DMA interrupt. Note that these buffers are internal to :cpp:func:`i2s_read` and descriptors are created automatically inside the I2S driver. Users only need to configure the number of buffers while their size is derived from the parameter described below.
-``dma_frame_num``: The number of frames for one-time sampling. In this context, "frame" refers to the data from all channels in a single WS cycle. For example, if two channels in stereo mode (i.e., ``channel_format`` is set to ``I2S_CHANNEL_FMT_RIGHT_LEFT``) are active, and each channel transfers 32 bits (i.e., ``bits_per_sample`` is set to ``I2S_BITS_PER_CHAN_32BIT``), the total number of bytes in a frame is ``channel_format``* ``bits_per_sample`` = 2 * 32 / 8 = 8 bytes. Assuming the current value of ``dma_frame_num`` is 100, the length of the DMA buffer is 8 * 100 = 800 bytes. Note that the length of an internal DMA buffer shouldn't be greater than 4092.
When the data received fills the DMA buffer, a receive interrupt is triggered, and an internal message queue transports this buffer to :cpp:func:`i2s_read`. The main task of :cpp:func:`i2s_read` is to copy the data from the DMA buffer into the buffer provided by the user. Since the internal DMA buffer is not necessarily equal to the size of the user buffer, there are two cases:
- If the size of internal DMA buffers is smaller than the user buffer, :cpp:func:`i2s_read` will consume several DMA buffers to fill up the user provided buffer. If the message queue of DMA buffers is not long enough, :cpp:func:`i2s_read` will block on the receive message queue until it receives enough data. Estimate the time :cpp:func:`i2s_read` blocks by the formula block_time (sec) = (user_buffer_size) / (sample_rate * channel_num * channel_bytes). If we place :cpp:func:`i2s_read` in a ``while``-loop with other functions we also need to consider the time the loop execution is blocked by the other functions before :cpp:func:`i2s_read` is executed again. Do not exceed max_wait_time (sec) = ((``dma_desc_num`` - 1) * dma_buffer_size) / (sample_rate), otherwise the internal message queue will overflow and in consequence some data will be lost.
- If the DMA buffer size is greater than the user buffer, :cpp:func:`i2s_read` needs to be called repeatedly in order to consume all available data. ``max_wait_time`` may be exceeded, and the message queue is likely to overflow. This makes it quite risky to use a small user buffer to receive data.
3. Increasing the size of :cpp:func:`i2s_read`'s internal buffer may help increase max_wait_time. When processing I2S data (like storing the data to an SD card) in another thread, the time spent in the processing thread for one loop should not exceed max_wait_time, otherwise data will be lost. The longer the processing of the samples take, the larger :cpp:func:`i2s_read`'s internal buffers need to be.
4. Allocating at least two user buffers can improve the efficiency of reads. One buffer can continue to receive data while the other is being processed. This requires us to ensure the buffer reading data is not being processed in another thread. We can use a semaphore or another lock to avoid overwriting the data during processing;
