Change STLINK-V3EC VCP Baud rate on NUCLEO-H7S3L8

[Basti_]

Hello,

is it possible to change the baud rate VCP to USART bridge of the STLINK-V3EC on the Nucleo STM32H7S evalboards?

The documentation states the following:
Virtual COM port
The serial interface UART3 (PD8/PD9) that supports the bootloader is directly available as a Virtual COM port of
the PC connected to the STLINK-V3EC USB connector (CN5). The VCP configuration is the following:
• 115200 bps
• 8-bit data
• No parity
• One-stop bit
• No flow control

Since the stand-alone STLINKs have the feature to change the baud rate of the VCP to UART bridge I was wondering if this is also possible on the STLINK-V3EC.

 

 

Best regards

Basti

 

[Andrew Neil]

It would be easy to try it - just change the baud rate setting in your terminal app, and see what happens ...

[Basti_]

Hello Andrew,

assuming I would do so, what UART Baudrate is exactly set? The baud rate of the MCU is derived from the MCO clock and the prescaler settings. So I cannot set any baud rate value. What will happen if I set something like 1.2345 MBaud?

Normally VCP ignores baudrate settings anyway.

Best regards

Basti

[Andrew Neil]

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That's the baud rate of the VCP; ie, the baud rate of the UART in the ST-Link on which the signals emerge.

EDIT - Here:

 

So you set your MCU's baud rate to match that.

 

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@Basti_ wrote:

Normally VCP ignores baudrate settings anyway.

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No, it defines the baudrate of the physical connection emerging from the ST-Link.

[Mike_ST]

Hello

The baudrate that you set in the PC application (600,1200..., 921600) is transmitted to the ST-Link on the VCP interface, it is automatic. From that baudrate, the ST-Link will compute its prescaler (see below) to use for the UART interface that communicates with the MCU.

 

The ST-Link V3 Documentation section 14.2 tries to explain what baurates are possible:

in "high perf" mode (default configuration of the ST-Link):

"384 MHz / prescaler with prescaler = [24 to 31]"
i.e. 12 387 096, ..., 16 000 000 bauds

or

"192 MHz / prescaler with prescaler = [16 to 65535]"
i.e. 2929, ..., 12 000 000 bauds

So, to reuse your example of 1.2345 MBaud (assuming you have an app that allow to configure such a baurate), the ST-Link firmware should compute a prescaler of 192000000/1234500 ~= 155.

 

The level shifters on the Nucleo boards can introduce frequency limitation, I think this is the case for the H7S one.

 

For the baudrate to use to communicate with the bootloader please see AN2606 

[Andrew Neil]

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@Mike_ST wrote:

The baudrate that you set in the PC application (600,1200..., 921600) is transmitted to the ST-Link on the VCP interface, it is automatic.

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Indeed.

So it is misleading of the document to say, "The VCP configuration is the following..."

because that is set by the host application - not the target.

[Mike_ST]

Hi Andrew,

>> So it is misleading of the document to say, "The VCP configuration is the following..."

Yes, I know.

The intention of the writer was to say that the "out of the box" demonstration software, when there is one, is configured that way.