Update history

Date	Version	Author	Update content
2025-08-08	1.2.0	Neucrack	Added MaixCAM2 support
2024-08-01	1.1.0	Neucrack	Optimized documentation with more details
2024-03-07	1.0.0	Neucrack	Initial version

Prerequisite Knowledge

Please first learn to use the pinmap module to set pin functions.

To use a pin for UART functionality, you must first set its function to UART using pinmap.

Serial Port Overview

A serial port is a communication method that includes both hardware and communication protocol definitions.

• Hardware includes:

  • 3 pins: GND, RX, and TX. Communication between two devices is cross-connected for RX and TX, meaning one device’s TX connects to the other’s RX, and both GND pins are connected together.
  • A controller, usually inside the chip, also called a UART peripheral. A chip usually has one or more UART controllers, each with corresponding pins.

• Serial communication protocol: To ensure proper communication, a protocol defines timing, baud rate, parity bits, etc. The baud rate is the most commonly used parameter.

Through the board’s serial port, you can communicate with other microcontrollers or SoCs. For example, MaixCAM can perform human detection and send the detected coordinates to an STM32/Arduino via the serial port.

Choosing the Appropriate I2C to Use

First, we need to know which pins and I2C interfaces are available on the device, as shown below:

Device Model	Pin Diagram	Pin Multiplexing Description
MaixCAM		The board’s silkscreen shows the pin name (e.g., A19) and function name (e.g., UART1_TX).
MaixCAM-Pro		The first label (e.g., A19) is the pin name, corresponding to the function name (e.g., UART1_TX).
MaixCAM2		The first label (e.g., A21) is the pin name, corresponding to the function name (e.g., UART4_TX).

Note: Pins may be used for other purposes by default. It’s best to avoid these pins—see the pinmap documentation.

Notes for MaixCAM/MaixCAM-Pro Serial Port Usage:

• By default, a UART0 serial port is routed from the USB port. You can use the matching Type-C adapter board to directly access the serial pins, or you can use the onboard A16 (TX) and A17 (RX) pins, which are equivalent to the USB-exposed serial pins.
• When using the USB-exposed serial port on MaixCAM, note that the Type-C plug’s orientation affects the adapter board’s RX and TX pins (swapped if reversed; silkscreen matches when the Type-C female port faces forward). If communication fails, try flipping the Type-C connector.
• UART0 on MaixCAM prints boot logs during startup and prints serial ready when boot completes. If communicating with a microcontroller, ignore this initial output. Boot logs can also help diagnose startup issues.
• The TX pin of UART0 is also a boot mode detection pin. It must not be pulled low during power-on, or the device won’t boot. If using a 3.3 V to 5 V level shifter, ensure it doesn’t pull TX low by default (use a level-shifting chip or keep it floating). If the board won’t start, check if TX is being pulled low.
• If UART0 causes issues, consider using another UART such as UART1.
• UART0 is also the system’s default maix protocol port.

Notes for MaixCAM2:

• MaixCAM2 has multiple serial ports: UART0 / UART1 / UART2 / UART3 / UART4—don’t mix them up.
• UART0 is the system terminal and log port.

Baud Rate Limitations

Not all baud rates are supported. Unless necessary, use 115200 (universally supported). Other baud rates may have high error rates or be unsupported.

Common tested baud rates (contributions welcome):

• MaixCAM / MaixCAM-Pro: 115200.
• MaixCAM2: 115200. Theoretical max: 4000000 bits/s. Formula:
  baud = uart_clk / (fractional_div * 16)
  Default uart_clk: 200000000. Integer part: uart_clk / (baud * 16). Fractional part: round((uart_clk % (baud * 16)) * 16 / (baud * 16)) / 16.
  Example: For 115200, divisor = 108.5, precision = 0.0064%.

Serial Port Hardware Wiring

For two devices to communicate, connect three pins: GND, RX, TX. Connect TX of one to RX of the other, and connect both GNDs together.

Using Serial Port in MaixPy

Once the two boards are connected (crossed RX/TX, common GND), you can use the software.

Basic MaixPy code:

from maix import uart

serial_dev = uart.UART("/dev/ttyS0", 115200)
serial_dev.write_str("Hello MaixPy")

/dev/ttyS0 is the serial device. Use print(uart.list_devices()) to list all devices.

For pins that are already mapped to UART, you can use them directly. For others, set their function via pinmap before creating the UART object:

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

# ports = uart.list_devices() # list all UARTs

device_id = sys.device_id()
if device_id == "maixcam2":
    pin_function = {
        "A21": "UART4_TX",
        "A22": "UART4_RX"
    }
    device = "/dev/ttyS4"
else:
    pin_function = {
        "A16": "UART0_TX",
        "A17": "UART0_RX"
    }
    device = "/dev/ttyS0"

for pin, func in pin_function.items():
    err.check_raise(pinmap.set_pin_function(pin, func), f"Failed set pin{pin} function to {func}")

serial_dev = uart.UART(device, 115200)
serial_dev.write_str("Hello MaixPy")

Connecting Serial Port to a Computer

• Why doesn’t a serial device appear on my computer when I plug in USB?
  The board’s USB port is for USB functions (e.g., USB network adapter), not USB-to-UART. For terminal access, use SSH.

• How to communicate between the computer and board via UART?
  Use a USB-to-UART adapter (e.g., this one). Connect USB to the PC and UART to the board.

• How to view boot logs or interact with the board via UART terminal?
  SSH is recommended for terminal interaction. For serial terminal access:

  • MaixCAM/MaixCAM-Pro: Connect USB-to-UART adapter to UART0 (A16 TX, A17 RX). In /boot/uEnv.txt, comment or remove the consoledev line to enable UART0 terminal, then reboot. You’ll see boot logs and have terminal access.
  • MaixCAM2: Connect USB-to-UART adapter to UART0 (U0T/U0R). You’ll see boot logs and have terminal access after boot.

Sending Data

There are mainly two functions for sending data: write_str and write.

The write_str function is used to send strings, while write is used to send byte streams, i.e., str and bytes types, which can be converted to each other. For example:

• "A" can be converted to b"A" using the encode() method, and vice versa, b"A" can be converted back to "A" using the decode() method.
• str cannot display some invisible characters, such as the ASCII value 0, which is generally \0 in strings and serves as a terminator. In bytes type, it can be stored as b"\x00".
• This is more useful for non-ASCII encoded strings. For example, the Chinese character 好 in UTF-8 encoding is represented by three bytes \xe5\xa5\xbd. We can use "好".encode("utf-8") to get b"\xe5\xa5\xbd", and b'\xe5\xa5\xbd'.decode("utf-8) to get "好".

So if we need to send byte data, we can use the write() method to send it. For example:

bytes_content = b'\x01\x02\x03'
serial.write(bytes_content)

Therefore, for the str type, you can use serial.write(str_content.encode()) instead of write_str to send it.

If you have other data types that you want to convert into a string to send, you can use Python string formatting to create a string. For example, to send I have xxx apple, where xxx is an integer variable, you can do:

num = 10
content = "I have {} apple".format(num)
content2 = f"I have {num} apple"
content3 = "I have {:04d} apple".format(num)
content4 = f"I have {num:d} apple"
print(content)
print(content2)
print(content3)
print(content4)
print(type(content))
serial.write_str(content)

Additionally, you can encode the data into a binary stream to send. For example, the first 4 bytes are hexadecimal AABBCCDD, followed by an int type value, and finally a 0xFF at the end. You can use struct.pack to encode it (if this is unclear, you can read the explanation later):

from struct import pack
num = 10
bytes_content = b'\xAA\xBB\xCC\xDD'
bytes_content += pack("<i", num)
bytes_content += b'\xFF'
print(bytes_content, type(bytes_content))
serial.write(bytes_content)

Here, pack("<i", num) encodes num as an int type, which is a 4-byte signed integer. The < symbol indicates little-endian encoding, with the low byte first. Here, num = 10, the 4-byte hexadecimal representation is 0x0000000A, and little-endian encoding puts the low byte 0x0A first, resulting in b'\x0A\x00\x00\x00'.

Here, we use i to encode int type data as an example. Other types, such as B for unsigned char, etc., can also be used. More struct.pack formatting options can be searched online with python struct pack.

In this way, the final data sent is AA BB CC DD 0A 00 00 00 FF as binary data.

Receiving Data

Use the read method to read data directly:

while not app.need_exit():
    data = serial.read()
    if data:
        print(data)
    time.sleep_ms(1)

Similarly, the data obtained by the read method is also of the bytes type. Here, read reads a batch of data sent by the other party. If there is no data, it returns b'', which is an empty byte.

Here, time.sleep_ms(1) is used to sleep for 1ms, which frees up the CPU so that this thread does not occupy all CPU resources. 1ms does not affect the program's efficiency, especially in multithreading.

In addition, the read function has two parameters:

• len: Represents the maximum length you want to receive. The default is -1, meaning it will return as much as there is in the buffer. If you pass a value >0, it means it will return data up to that length.
• timeout:
  • The default 0 means it will return immediately with whatever data is in the buffer. If len is -1, it returns all data; if a length is specified, it returns data not exceeding that length.
  • <0 means it waits until data is received before returning. If `

lenis-1, it waits until data is received and returns (blocking read for all data); if a length is specified, it waits until it reacheslen` before returning.

• >0 means it will return after this time, regardless of whether data is received.

It may seem complex, but here are some common parameter combinations:

• read(): Which is read(-1, 0), reads the data received in the buffer, usually a batch of data sent by the other party. It returns immediately when the other party has stopped sending (within one character's sending time).
• read(len = -1, timeout = -1): Blocking read for a batch of data, waits for the other party to send data and returns only when there is no more data within one character's sending time.
• read(len = 10, timeout = 1000): Blocking read for 10 characters, returns when 10 characters are read or 1000ms has passed without receiving any data.

Setting a Callback Function for Receiving Data

In MCU development, a serial port interrupt event usually occurs when data is received. MaixPy has already handled the interrupt at the bottom layer, so developers don't need to handle the interrupt themselves. If you want to call a callback function upon receiving data, you can use set_received_callback to set the callback function:

from maix import uart, app, time

def on_received(serial : uart.UART, data : bytes):
    print("received:", data)
    # send back
    serial.write(data)

device = "/dev/ttyS0"

serial = uart.UART(device, 115200)
serial.set_received_callback(on_received)

serial0.write_str("hello\r\n")
print("sent hello")
print("wait data")

while not app.need_exit():
    time.sleep_ms(100) # sleep to make CPU free

When data is received, the set callback function will be called in another thread. Since it's called in another thread, unlike an interrupt function, you don't have to exit the function quickly. You can handle some tasks in the callback function before exiting, but be aware of common multithreading issues.

If you use the callback function method to receive data, do not use the read function to read it, or it will read incorrectly.

Using Other Serial Ports

Each pin may correspond to different peripheral functions, which is also known as pin multiplexing. As shown below, each pin corresponds to different functions. For example, pin A17 (silkscreen identification on the board) corresponds to GPIOA17, UART0_RX, and PWM5 functions. The default function is UART0_RX.

By default, you can directly use UART0 as shown above. For other serial port pins, they are not set to the serial peripheral function by default, so you need to set the mapping to use other serial ports. Use pinmap.set_pin_function to set it.

Let's take UART1 as an example. First, set the pin mapping to choose the serial port function, then use the device number /dev/ttyS1. Note that uart.list_devices() will not return manually mapped serial ports by default, so you can directly pass the parameters manually:

from maix import app, uart, pinmap, time

pinmap.set_pin_function("A18", "UART1_RX")
pinmap.set_pin_function("A19", "UART1_TX")

device = "/dev/ttyS1"

serial1 = uart.UART(device, 115200)

Application Layer Communication Protocol

Concept and Character Protocol

Serial ports only define the hardware communication timing. To let the receiver understand the meaning of the character stream sent by the sender, an application communication protocol is usually established. For example, if the sender needs to send coordinates containing two integer values x, y, the following protocol is established:

• Frame Header: When I start sending the $ symbol, it means I'm about to start sending valid data.

Content: Designing a start symbol is because serial communication is stream-based. For example, sending 12345 twice may result in receiving 12345123 at some moment. The 45 from the second frame has not been received. We can determine a complete data frame based on start and end symbols.

• The value range of x, y is 0~65535, i.e., an unsigned short integer (unsigned short). I'll first send x then y, separated by a comma, such as 10,20.
• Frame Tail: Finally, I'll send a * to indicate that I've finished sending this data.

In this way, sending a data packet looks like $10,20* as a string. The other party can receive and parse it using C language:

// 1. Receive data
// 2. Determine if the reception is complete based on the frame header and tail, and store the complete frame data in the buff array
// 3. Parse a frame of data
uint16_t x, y;
sscanf(buff, "$%d,%d*", &x, &y);

Thus, we have defined a simple character communication protocol with a certain degree of reliability. However, since we usually use parameters like 115200 8 N 1 for serial ports, where N means no parity check, we can add a checksum to our protocol at the end. For example:

• Here, we add a checksum value after x, y, ranging from 0 to 255. It is the sum of all previous characters modulo 255.
• Taking $10,20 as an example, in Python, you can simply use the sum function: sum(b'$10,20') % 255 --> 20, and send $10,20,20*.
• The receiver reads the checksum 20, calculates it in the same way as $10,20, and if it is also 20, it means no transmission error occurred. Otherwise, we assume a transmission error and discard the packet to wait for the next one.

In MaixPy, encoding a character protocol can be done using Python's string formatting feature:

x = 10
y = 20
content = "${},{}*".format(x, y)
print(content)

Binary Communication Protocol

The character protocol above has a clear characteristic of using visible characters to transmit data. The advantage is simplicity and human readability. However, it uses an inconsistent number of characters and larger data volumes. For example, $10,20* and $1000,2000* have varying lengths, with 1000 using 4 characters, which means 4 bytes. We know an unsigned short integer (uint16) can represent values ranging from 0~65535 using only two bytes. This reduces the transmission data.

We also know visible characters can be converted to binary via ASCII tables, such as $1000 being 0x24 0x31 0x30 0x30 0x30 in binary, requiring 5 bytes. If we directly encode 1000 in binary as 0x03E8, we can send 0x24 0x03 0xE8 in just 3 bytes, reducing communication overhead.

Additionally, 0x03E8 is a 2-byte representation with 0xE8 as the low byte, transmitted first in little-endian encoding. The opposite is big-endian encoding. Both are fine as long as both parties agree on one.

In MaixPy, converting a number to bytes is simple with struct.pack. For example, 0x03E8 (decimal 1000):

from struct import pack
b = pack("<H", 1000)
print(b)

Here, <H indicates little-endian encoding, with H denoting a uint16 data type, resulting in b'\xe8\x03' as bytes.

Similarly, binary protocols can have a frame header, data content, checksum, frame tail, or a frame length field instead of a frame tail, based on preference.

Built-in MaixPy Communication Protocol

MaixPy also includes a built-in communication protocol.

Using this protocol, it is possible to implement application switching, application control, and data retrieval via serial communication or even TCP.

For example, the coordinates detected by an AI detection application after identifying an object can be parsed using this protocol.

Other Tutorials

• 【MaixPy/MaixCAM】Visual Tool -- MaixCAM Beginner Tutorial 2 Watch the serial port explanation section
• How to Communicate via Serial Port between Visual Module and STM32
• [MaixCam] Experience 2: UART Serial Communication
• For more, search online for resources.

Key

I2C