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 physical GPIO pins to which signals will be routed. For this, call the function :cpp:func`i2s_set_pin` and pass the following arguments to it:
- The structure :cpp:type:`i2s_pin_config_t` defining the GPIO pin numbers to which the driver should route the MCK, BCK, WS, DATA out, and DATA in signals. If you want to keep a currently allocated pin number for a specific signal, or if this signal is unused, then 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 controller.
You can temporarily stop the I2S driver by calling the function :cpp:func:`i2s_stop`, which will disable the I2S Tx/Rx units until the function :cpp:func:`i2s_start` is called. If the function :cpp:func`i2s_driver_install` is used, the driver will start up automatically eliminating the need to call :cpp:func:`i2s_start`.
Deleting the Driver
^^^^^^^^^^^^^^^^^^^
If the established communication is no longer required, the driver can be removed to free allocated resources by calling :cpp:func:`i2s_driver_uninstall`.
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. Also you can 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.
If you are using a high sample rate like more than 48 kHz or see the data being lost, the following information might help.
Considering different applications have different requirements, ``dma_desc_num`` and ``dma_frame_num`` are made public. Here is the detailed explanation to these two fields:
-``dma_desc_num``: The total number of descriptors used by I2S DMA to receive/transmit data. A descriptor includes some information such as buffer address, the address of the next descriptor, and the buffer length. Since one descriptor points to one buffer, therefore, ``dma_desc_num`` can be interpreted as the total number of DMA buffers used to store data from DMA interrupt. Notice that these buffers are internal to :cpp:func:`i2s_read` and descriptors are created automatically inside of the I2S driver. Users only need to set the buffer number while the length is derived from the parameter described below.
-``dma_frame_num``: The number of frames for one-time sampling. The frame here means the total data from all the channels in one 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``), then the total number of bytes of a frame is ``channel_format``* ``bits_per_sample`` = 2 * 32 / 8 = 8 bytes. For example, we assume that the current ``dma_frame_num`` is 100, then 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 by DMA reach the size of internal DMA buffer, a receive interrupt is triggered, and an internal message queue will transport this buffer to :cpp:func:`i2s_read`. The main task of :cpp:func:`i2s_read` is to copy the data in the internal DMA buffer into the user given buffer. Since the internal DMA buffer is usually not equal to the user given buffer size, there are two cases:
- If the size of internal DMA buffers is smaller than the user given buffer size, :cpp:func:`i2s_read` will consume several internal DMA buffers to fill up the user given buffer. If the message queue of DMA buffer is not long enough, :cpp:func:`i2s_read` will be blocked on receive message queue untill getting enough data. You can estimate the time :cpp:func:`i2s_read` function is blocked by the formula block_time (sec) = (given_bufer_size) / (sample_rate * channel_num * channel_bytes). If we place :cpp:func:`i2s_read` in a ``while`` together with other functions we should also consider the time the loop execution is blocked by the other functions before :cpp:func:`i2s_read` is executed again. It is not allowed to exceed the 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 given buffer size, it means :cpp:func:`i2s_read` would need to be called in a loop several times to take all the data, the max_wait_time may be exceeded, and the message queue overflow is likely to happen. Therefore it is quite risky to use a small user buffer for data receiving.
Here are a couple of tips when the sample rate is high:
1. Increasing ``dma_frame_num`` can help to reduce the I2S DMA interrupt frequency;
2. Increasing ``dma_desc_num`` can help to alow a longer max_wait_time to be spent by the other functions in the loop conatining :cpp:func:`i2s_read` ;
..code-block:: c
while (1) {
... // Other operations (e.g. Waiting on a semaphore)
... // Other operations (e.g. Sending the data to another thread. Avoid any data processing here.)
}
3. Increasing the size of the buffer internal to :cpp:func:`i2s_read` can help to increase the max_wait_time. When you process I2S data (like storing the data to an SD card) in another thread, the time spent in processing thread for one loop should not exceed the max_wait_time otherwise some data will be lost. So if the processing of the received data received is rather slow, please allow :cpp:func:`i2s_read` a big internal buffer for storing the read data;
4. Allocating at least two user given buffers can improve the data receipt efficiency. One buffer can continue to receive data while the other one is under processing. But we have to guarantee the buffer that is going to read data is not under processing in the other thread. We can use a semaphore or another lock to avoid overwriting the data in the buffer while it is still under processing;
