UART

This guide covers the physical, transport, and link layers for UART interfaces used by the YOS communication system (UMSG protocol). The physical layer defines baud rate and serial port mode. The transport layer determines the datagram byte ordering, redundancy checks and start/end of frame recognition mechanisms. The link layer defines the device addressing and service identification. The UMSG link layer omits the sender address due to the implicit 1:n topology. The construct can be used for the utility driver and common I/O API communication.

Physical layer configuration

The physical layer for UART interfaces is configured through cfg0-cfg3 of the msgconf command (interface number 1 for UMSG/UARTCOM, 2 for UMSG2, 5 for UMSG3/UARTEXT, 6 for UMSG4).

Baud rate (cfg0)

cfg0 sets the baud rate of the selected UART interface:

cfg0 value	Baud rate
8	1200 bps
9	2400 bps
10	4800 bps
11	9600 bps
12	19200 bps
13	38400 bps
14	57600 bps
15	115200 bps
16	230400 bps
17	460800 bps
18	921600 bps

Mode and timing (cfg1, cfg2, cfg3)

cfg1, cfg2, and cfg3 configure the UART mode and timings. The default values are 0 which denotes 8 data bits, no parity, and 1 stop bit. Other values are reserved for further definition.

Link and transport layer — UMSG datagram

UART Frame Structure

Byte	Description
0	session | 0x80
1	(session | 0x80)^UMSG_VERSION
2	Message length - 1
3	Paylaod byte 0 (SID - link layer)
4	Payload byte 1 (Receiver address - link layer)
5	Payload byte 0
6	Payload byte 1
7	Payload byte 2
5 + n	Payload byte n
Last byte	CRC

CRC Calculation

CRC is calculated by XORing all bytes in the UART message, excluding the first two bytes.

crc = Byte2 ^ Byte3 ^ Byte4 ^ ... ^ Last_byte

Session increment

The sender must increment the session number each time a new datagram is composed. If the receiver gets multiple times a message with the same session, it is discarded.

UART Frame Creation Example (C Code)

/*!
    \brief Create the UART frame (umsg)

    \param[in] payload_len - Number of bytes to send
    \param[in] payload - payload to send
                0 byte of data = SID
                1 byte of data = Receiver addr
                2-end bytes = Payload
    \param[in] output_buf - Output buffer
    \param[in] output_buf_len - Number of bytes in output buffer

    \return int32_t
            0 - success
            -1 - output (tx) buffer too small
*/

#define UMSG_VERSION 0x55

int32_t
umsg_compose_datagram(uint8_t payload_len, const uint8_t* payload, uint8_t* output_buf, uint8_t output_buf_len)
{
    static uint8_t session = 0;

    if (payload_len + 4 > output_buf_len)
        return -1;

    session |= 0x80;
    *output_buf++ = session;
    *output_buf++ = session ^ UMSG_VERSION;
    *output_buf++ = payload_len - 1;

    uint8_t crc = 0;
    crc ^= payload_len - 1;

    for (int i = 0; i < payload_len; i++) {
        crc ^= *payload;
        *output_buf++ = *payload++;
    }

    *output_buf++ = crc;
    session++;

    return 0;
}