USART2 runs at 115200 8N1. The parser is uartcom::Protocol. It is host-tested with a fake USART, and the same object runs on the board.
Lines end with \n. \r is ignored. Empty lines are ignored. The Python driver strips the newline before returning a reply. The C++ tests compare the reply including \n.
A line longer than the parser buffer answers ERR line. A non-ASCII byte in a line answers ERR bad_char. An unknown command answers ERR unknown. Extra arguments on a command that takes none answer ERR args.
After reset, before it reads a command:
UARTCOM STM32F446RE USART2 ready
| Command | Reply |
|---|---|
PING |
PONG |
ECHO |
ECHO |
ECHO hello |
ECHO hello |
ECHO hello |
ECHO hello |
HEX |
HEX |
HEX 00ff10 |
HEX 00FF10 |
HEX 0 or HEX 0g |
ERR bad_hex |
CRC32 |
CRC32 00000000 |
CRC32 313233343536373839 |
CRC32 CBF43926 |
CRC32 00FF10 |
CRC32 71D23404 |
STAT |
STAT rx= tx= pe= fe= ne= ore= swore= rxq= txq= |
BAUD |
BAUD pclk= baud= actual= brr= ppm= |
RESET |
OK |
HELP |
HELP PING INFO ECHO HEX CRC32 STAT BAUD RESET |
ECHO keeps the remainder of the line, including a leading space. ECHO with nothing after the word replies ECHO.
HEX reads an even number of hex digits and prints them in uppercase. An empty payload is legal. An odd length or a non-hex digit is ERR bad_hex.
CRC32 is the ISO-HDLC / zlib polynomial (0xEDB88320, reflected, init 0xFFFFFFFF, final XOR 0xFFFFFFFF). The argument is hex, same rules as HEX. The three vectors above are the empty buffer, the ASCII string 123456789, and the bytes 00 FF 10.
STAT counts are since the last RESET (or since boot). rx and tx are bytes through the driver. pe, fe, ne, and ore are USART line errors. swore is bytes dropped because the software RX ring was full. rxq and txq are the current depths of those rings. RESET clears the counters and then replies OK, so that OK is not part of the cleared snapshot.
INFO is one line of key=value fields:
INFO board=NUCLEO-F446RE mcu=STM32F446RE usart=USART2 src=hse_bypass pll=1 sysclk=180000000 pclk=45000000 baud=115200 actual=115089 brr=391 ppm=-959 cpp=202302 ctors=1 data=1
src is hse_bypass, hse_crystal, or hsi. On this Nucleo the expected path is hse_bypass with pll=1, sysclk=180000000, and pclk=45000000. cpp is the firmware __cplusplus. ctors=1 means a static constructor ran. data=1 means the .data copy put 0x446 into RAM. The Python suite requires the 180 MHz path and compares cpp with build/firmware_cxx.txt.
A frame may start only at column 0 of a line (the parser is idle, not mid-command).
A5 5A | uint16le length | payload | uint32le crc32
length is the payload size, 0 through 256. The CRC is the same ISO-HDLC CRC over the payload only. A good frame is echoed with a freshly computed CRC. A bad CRC answers ERR crc and the parser returns to line mode. length above 256 answers ERR length and the parser discards the rest of the payload and the CRC. A lone A5 that is not followed by 5A answers ERR sync.
The Python helper Port.transact_frame writes a frame and reads one back. The first two bytes ER mean the device answered with an ASCII error line instead of a frame.