Introduction to Using UART in MaixPy

2024-05-14

Introduction to UART

UART (Universal Asynchronous Receiver/Transmitter) is a communication method that includes definitions for both hardware and communication protocols.

Hardware includes:
- 3 pins: GND, RX, TX. The communication parties connect RX and TX in a cross configuration, i.e., the TX of one side is connected to the RX of the other side, and both GND pins are connected together.
- Controller: Usually inside the chip, also known as a UART peripheral. A chip generally has one or more UART controllers, each with corresponding pins.
Communication protocol: To ensure smooth communication between both parties, a set of protocols is defined. You can learn the specifics on your own. Common parameters include baud rate and parity bit, with baud rate being the most frequently used parameter.

Using the board's UART, you can communicate with other microcontrollers or SoCs. For instance, you can implement a human detection function on MaixCAM and send the detected coordinates to an STM32 microcontroller via UART.

Using UART in MaixPy

For MaixCAM, a UART is by default exposed through the USB port. By connecting a Type-C adapter board, you can directly use the UART pins on it. Alternatively, you can use the A16 (TX) and A17 (RX) pins on the board directly, which are equivalent to the ones exposed through the USB port.

When using the UART exposed through the USB port on MaixCAM, note that the RX and TX pins on the Type-C adapter board will switch when you flip the Type-C connector. So, if communication fails, it might be due to reversed RX and TX. Try flipping the Type-C connector and check if the communication works properly. This is a design flaw, but it’s manageable with minimal adjustments.

After connecting the communicating boards (connecting RX to TX in a cross configuration and both GND pins together), you can use the software.

Using UART in MaixPy is simple:

from maix import uart

devices = uart.list_devices()

serial = uart.UART(devices[0], 115200)
serial.write_str("hello world")
print("received:", serial.read(timeout = 2000))

Here, we list all UART devices in the system and use the first one, which is the one exposed through the Type-C port.

For more UART APIs, please refer to the UART API Documentation

Other Usage and Using Additional UARTs

from maix import app, uart, pinmap, time
import sys

# pinmap.set_pin_function("A16", "UART0_TX")
# pinmap.set_pin_function("A17", "UART0_RX")
device = "/dev/ttyS0"

serial0 = uart.UART(device, 115200)

serial0.write("hello 1\r\n".encode())
serial0.write_str("hello 2\r\n")

while not app.need_exit():
    data = serial0.read()
    if data:
        print("Received, type: {}, len: {}, data: {}".format(type(data), len(data), data))
        serial0.write(data)
    time.sleep_ms(1) # sleep 1ms to make CPU free

Here, we set up pin mapping, especially if you want to use UARTs other than UART0. Use pinmap.set_pin_function based on the pin numbers and UART in the diagram:

Additionally, a sleep_ms function is added in the loop to briefly release the CPU, ensuring that the program does not occupy the CPU fully. There are other methods to achieve this, but this is the simplest and most straightforward approach.

GPIO

I2C