I have simple code which writes to the USART on a Nucleo board that is connected to ST-Link, and appears on my workstation via VCP over USB.
I'm trying now to port it to a STM32H735G-DK Discovery but see no output on my VCP on my workstation.
According to the board manual
The serial interface USART3 (PD8/PD9) that supports the bootloader is directly available as a Virtual COM port of the PC connected to the CN15 STLINK-V3E USB connector. The VCP configuration is the following: • 115200 bps • 8-bit data • No parity • One-stop bit • No flow control
So I moved my RCC and GPIO code to use USART3, PD8, PD9 (instead of the values on the Nucleo board):
#define MY_USART USART3
...
rcc_periph_clock_enable(RCC_USART3);
// and wires USART2 to PD8 (TX) and PD9 (RX)
rcc_periph_clock_enable(RCC_GPIOD);
// Set both pins to AF7
gpio_mode_setup(GPIOD, GPIO_MODE_AF, GPIO_PUPD_NONE, GPIO8 | GPIO9);
gpio_set_af(GPIOA, GPIO_AF7, GPIO8 | GPIO9);
...
usart_set_baudrate(MY_USART, 115200); // The other defaults are correct: 8N1, etc.
usart_set_flow_control(MY_USART, USART_FLOWCONTROL_NONE);
usart_set_mode(MY_USART, USART_MODE_TX_RX);
usart_enable(MY_USART);
usart_send_blocking(MY_USART, b);
but no output ("0 bytes received").
What further perplexes me is that this example by ST themselves, for the very same board uses USART1 (not USART3).
I can't find any explicit docs stating the correct Alternate Function for the pins, but I believe it is AF7.
My question is: How do I debug this? I've verified via gdb that my code is being called, including usart_send_blocking(MY_USART, b). I've verified my VCP works via the Nucleo board. The most likely culprit would be incorrectly setting up RCC, GPIO pins, or simply using the wrong pins or UART - but how do I debug that? There's no error or exception in that case, just a no-op.
