UnbufferedSerial

UnbufferedSerial class hierarchy

The UnbufferedSerial class provides UART functionality with an API similar to the BufferedSerial class. The classes also share the same default configurations.

Unlike the BufferedSerial class, the UnbufferedSerial class does not use intermediary buffers to store bytes to transmit to or read from the hardware. The user application is responsible for processing each byte as it is received. The method to read data returns only one byte for every call. Therefore, we recommend you use this class when you need more control and for use in interrupt handlers with the RTOS. You can also use this class to write multiple bytes at once. Because it does not acquire a mutex lock, you must ensure only one instance uses the serial port.

For normal blocking applications that require a serial channel for something other than the console, BufferedSerial performs better than UnbufferedSerial and causes less CPU load and fewer latency issues. Only applications that are short of RAM and cannot afford buffering or that need more control of the serial port and use it from IRQ should use UnbufferedSerial.

You can view more information about the configurable settings and functions in the class reference.

Class reference

UnbufferedSerial Example

/*
 * Copyright (c) 2020 Arm Limited and affiliates.
 * SPDX-License-Identifier: Apache-2.0
 */

#include "mbed.h"

// Create a DigitalOutput object to toggle an LED whenever data is received.
static DigitalOut led(LED1);

// Create a UnbufferedSerial object with a default baud rate.
static UnbufferedSerial serial_port(USBTX, USBRX);

void on_rx_interrupt()
{
    char c;

    // Toggle the LED.
    led = !led;

    // Read the data to clear the receive interrupt.
    if (serial_port.read(&c, 1)) {
        // Echo the input back to the terminal.
        serial_port.write(&c, 1);
    }
}

int main(void)
{
    // Set desired properties (9600-8-N-1).
    serial_port.baud(9600);
    serial_port.format(
        /* bits */ 8,
        /* parity */ SerialBase::None,
        /* stop bit */ 1
    );

    // Register a callback to process a Rx (receive) interrupt.
    serial_port.attach(&on_rx_interrupt, SerialBase::RxIrq);
}

Mbed OS use

Common use cases for UnbufferedSerial are IRQ heavy UART operations, such as BLE Cordio in the transport driver.