Setting up Serial USB communication between STM32 and PC with Mbed library - c++

Question

I hava an STM32 f401RE. I am using Mbed library for setting up a conexion from STM32 to PC. I want to send via serial a char sequence to the board. As an answer i expect a blinking LED. E.g: led1 results in LED ON, led2 results in LED OFF.
The problem is that i don't know how to set the port for the connection.
#include "mbed.h"
#include "USBSerial.h"
//Virtual serial port over USB
USBSerial serial;
int main(void) {
while(1)
{
serial.printf("I am a virtual serial port\r\n");
wait(1.0);
}
}
You can use the USBSerial interface to emulate a serial port over USB. You can use this serial port as an extra serial port or as a debug solution. It also communicates between Mbed and a computer.
I would like to do all the above(even thogh i don't know what does emulate a serial port over USB. What is that Virtual USB?).
I see that USBSerial constructor takes
USBSerial (bool connect_blocking=true, uint16_t vendor_id=0x1f00, uint16_t product_id=0x2012, uint16_t product_release=0x0001). And i think i need to modify some of this adresses. The problem is that on Windows the ports are represented in Device Manager with COMxx and on Linux like ttyACMxx. How would i transform this in hexa adresses - is this what i have to do?

Answer 1

You should not have to transform anything or mess with the USB product_id or vendor_id, an mbed serial port should show as any other serial port so if it doesn't for you it means you are having driver issues.
On most recent Linux distros the device should show something similar to the following kernel messages:
cdc_acm 5-2:1.1: ttyACM0: USB ACM device
usbcore: registered new interface driver cdc_acm
cdc_acm: v0.26:USB Abstract Control Model driver for USB modems and ISDN adapters
On Windows, you will probably need to install drivers. After you do that, the serial port should show as mbed Serial Port (COMx) on your Device Manager. There are many places you can get troubleshooting help, see here, for instance.
The fact that you are getting nothing on both Windows and Linux makes one wonder if you are using the right cable (some USB cables work only for charging and are no good for your purposes, and some others simply fail after a while). I would first make sure your cable works with other devices (obviously not for charging only). There is also the possibility your board went (or came from the factory) bad, but that's quite unlikely.

Answer 2

I just found this approach and it is working. The thing that i don't understand is why on my pc i get this message: b'Hello World!\n'
#include "mbed.h"
Serial pc(USBTX, USBRX); // tx, rx
int main() {
pc.baud(9600);
while(1)
{
pc.printf("Hello World!\n");
wait(0.1);
}
}

Answer 3

Ignore that 'b'. Your device is not seeing that 'b'. It is just being printed by serial terminal utility. Also I would like to mention what I got from your question is, you want to send some data from PC to board over Serial and if device receives that data, it should start blinking the LED. If that is correct, use the code below:
#include "mbed.h"
Serial pc(USBTX, USBRX); // tx, rx
DigitalOut led(LED1); // If blinking doesn't work with LED1, Check the pin map for your board and pass the LED pin instead of LED1
char token = 'a'; // This is the character that you should send to trigger blinking
bool startBlinking = false;
int main() {
pc.baud(9600);
while(1)
{
if (pc.getc() == token) {
startBlinking = true;
}
if (startBlinking) {
led = 1;
wait(0.2);
led = 0;
wait(0.8);
}
}
}