An Universal Asynchronous Receiver/Transmitter (UART) is a component known to handle the timing requirements for a variety of widely-adapted protocols (RS232, RS485, RS422, ...). An UART provides a widely adopted and cheap method to realize full-duplex data exchange among different devices.
There are three UART controllers available on the ESP32 chip. They are compatible with UART-enabled devices from various manufacturers. All UART controllers integrated in the ESP32 feature an identical set of registers for ease of programming and flexibility. In this documentation, these controllers are referred to as UART0, UART1, and UART2.
The following overview describes functions and data types used to establish communication between ESP32 and some other UART device. The overview reflects a typical workflow when programming ESP32's UART driver and is broken down into the following sections:
uart-api-setting-communication-parameters- baud rate, data bits, stop bits, etc,uart-api-setting-communication-pins- pins the other UART is connected touart-api-driver-installation- allocate ESP32's resources for the UART driveruart-api-running-uart-communication- send / receive the datauart-api-using-interrupts- trigger interrupts on specific communication eventsuart-api-deleting-driver- release ESP32's resources, if UART communication is not required anymore
The minimum to make the UART working is to complete the first four steps, the last two steps are optional.
The driver is identified by :cppuart_port_t, that corresponds to one of the tree UART controllers. Such identification is present in all the following function calls.
There are two ways to set the communications parameters for UART. One is to do it in one shot by calling :cppuart_param_config provided with configuration parameters in :cppuart_config_t structure.
The alternate way is to configure specific parameters individually by calling dedicated functions:
- Baud rate - :cpp
uart_set_baudrate- Number of transmitted bits - :cpp
uart_set_word_lengthselected out of :cppuart_word_length_t- Parity control - :cpp
uart_set_parityselected out of :cppuart_parity_t- Number of stop bits - :cpp
uart_set_stop_bitsselected out of :cppuart_stop_bits_t- Hardware flow control mode - :cpp
uart_set_hw_flow_ctrlselected out of uart_hw_flowcontrol_t
All the above functions have a _get_ equivalent to retrieve the current setting, e.g. :cppuart_get_baudrate.
In next step, after configuring communication parameters, we are setting physical GPIO pin numbers the other UART will be connected to. This is done in a single step by calling function :cppuart_set_pin and providing it with GPIO numbers, that driver should use for the Tx, Rx, RTS and CTS signals.
Instead of GPIO pin number we can enter a macro :cppUART_PIN_NO_CHANGE and the currently allocated pin will not be changed. The same macro should be entered if certain pin will not be used.
Once configuration of driver is complete, we can install it by calling :cppuart_driver_install. As result several resources required by the UART will be allocated. The type / size of resources are specified as function call parameters and concern:
- size of the send buffer
- size of the receive buffer
- the event queue handle and size
- flags to allocate an interrupt
If all above steps have been complete, we are ready to connect the other UART device and check the communication.
The processes of serial communication are under control of UART's hardware FSM. The data to be sent should be put into Tx FIFO buffer, FSM will serialize them and sent out. A similar process, but in reverse order, is done to receive the data. Incoming serial stream is processed by FSM and moved to the Rx FIFO buffer. Therefore the task of API's communication functions is limited to writing and reading the data to / from the respective buffer. This is reflected in some function names, e.g.: :cppuart_write_bytes to transmit the data out, or :cppuart_read_bytes to read the incoming data.
The basic API function to write the data to Tx FIFO buffer is :cppuart_tx_chars. If the buffer contains not sent characters, this function will write what fits into the empty space and exit reporting the number of bytes actually written.
There is a 'companion' function :cppuart_wait_tx_done that waits until all the data are transmitted out and the Tx FIFO is empty.
An easier to work with function is :cppuart_write_bytes. It sets up an intermediate ring buffer and exits after copying the data to this buffer. When there is an empty space in the FIFO, the data are moved from the ring buffer to the FIFO in the background by an ISR.
There is a similar function as above that adds a serial break signal after sending the data - :cppuart_write_bytes_with_break. The 'serial break signal' means holding TX line low for period longer than one data frame.
To retrieve the data received by UART and saved in Rx FIFO, use function :cppuart_read_bytes. You can check in advance what is the number of bytes available in Rx FIFO by calling :cppuart_get_buffered_data_len.
If the data in Rx FIFO is not required and should be discarded, call :cppuart_flush.
When the hardware flow control is disabled, then use :cppuart_set_rts and :cppuart_set_dtr to manually set the levels of the RTS and DTR signals.
There are nineteen interrupts reported on specific states of UART or on detected errors. The full list of available interrupts is described in ESP32 Technical Reference Manual (PDF). To enable specific interrupts call :cppuart_enable_intr_mask, to disable call :cppuart_disable_intr_mask. The mask of all interrupts is available as :cppUART_INTR_MASK. Registration of an handler to service interrupts is done with :cppuart_isr_register, freeing the handler with :cppuart_isr_free. To clear the interrupt status bits once the handler is called use :cppuart_clear_intr_status.
The API provides a convenient way to handle specific interrupts discussed above by wrapping them into dedicated functions:
- Event detection - there are several events defined in :cpp
uart_event_type_tthat may be reported to user application using FreeRTOS queue functionality. You can enable this functionality when calling :cppuart_driver_installdescribed inuart-api-driver-installation. Example how to use it is covered inperipherals/uart_events. - FIFO space threshold or transmission timeout reached - the interrupts on TX or Rx FIFO buffer being filled with specific number of characters or on a timeout of sending or receiving data. To use these interrupts, first configure respective threshold values of the buffer length and the timeout by entering them in :cpp
uart_intr_config_tstructure and calling :cppuart_intr_config. Then enable interrupts with functions :cppuart_enable_rx_intrand :cppuart_enable_tx_intr. To disable these interrupts there are corresponding functions :cppuart_disable_rx_intror :cppuart_disable_tx_intr. - Pattern detection - an interrupt triggered on detecting a 'pattern' of the same character being sent number of times. The functions that allow to configure, enable and disable this interrupt are :cpp
uart_enable_pattern_det_intrand cppuart_disable_pattern_det_intr.
The API provides several macros to define configuration parameters, e.g. :cppUART_FIFO_LEN to define the length of the hardware FIFO buffers, :cppUART_BITRATE_MAX that gives the maximum baud rate supported by UART, etc.
If communication is established with :cppuart_driver_install for some specific period of time and then not required, the driver may be removed to free allocated resources by calling :cppuart_driver_delete.
Configure UART settings and install UART driver to read/write using UART1 interface: peripherals/uart_echo.
Demonstration of how to report various communication events and how to use patern detection interrupts: peripherals/uart_events.
Transmitting and receiveing with the same UART in two separate FreeRTOS tasks: peripherals/uart_async_rxtxtasks.
Using synchronous I/O multiplexing for UART file descriptors: peripherals/uart_select.
You can use macros to specify the direct GPIO (UART module connected to pads through direct IO mux without the GPIO mux) number of a UART channel, or vice versa. The pin name can be omitted if the channel of a GPIO number is specified, e.g.:
UART_NUM_2_TXD_DIRECT_GPIO_NUMis the GPIO number of UART channel 2 TXD pin (17);UART_GPIO19_DIRECT_CHANNELis the UART channel number of GPIO 19 (channel 0);UART_CTS_GPIO19_DIRECT_CHANNELis the UART channel number of GPIO 19, and GPIO 19 must be a CTS pin (channel 0).