Problem with USB in virtual port com

[fab04]

Hi Everybody,

I've got a few problems with my USB. I have configured the USB port of my STM32L476 in communication device class (virtual port com).

I can send and receive data on my terminal.

What I'm doing is the following :

sprintf(buf,BYTE_TO_BINARY_PATTERN,BYTE_TO_BINARY(micro_switch));
	strcpy(usb_buf,"Etats des microswitchs : ");
	strcat(usb_buf,buf);
	strcat(usb_buf,"\r\n\0");
	CDC_Transmit_FS(usb_buf,strlen(usb_buf));
 
	/* Valeur du 0/10V channel 1 */
	strcpy(usb_buf,"0-10V - channel 1 : ");
	gcvt(calcul_tension(tab_val_ana_moy[0]),4,buf);
	strcat(usb_buf,buf);
	strcat(usb_buf," Volts \r\n\0");
	CDC_Transmit_FS(usb_buf,strlen(usb_buf));
 
	/* Valeur du 0/10V channel 2 */
	strcpy(usb_buf,"0-10V - channel 2 : ");
	gcvt(calcul_tension(tab_val_ana_moy[1]),4,buf);
	strcat(usb_buf,buf);
	strcat(usb_buf," Volts \r\n\0");
	CDC_Transmit_FS(usb_buf,strlen(usb_buf));
 
	/* Nombre d'impulsions du compteur d'énergie */
	sprintf(buf,"%d",comptage_impulsion);
	strcpy(usb_buf,"nombre d'impulsions du compteur d'énergie : ");
	strcat(usb_buf,buf);
	CDC_Transmit_FS(usb_buf,strlen(usb_buf));

So, the terminal is supposed to display four lines of data, but only two (sometimes three) are displayed.

Is there a problem of size of buffer or maybe of timing ?

If you have somes ideas, I'll be very happy.

Thank you.

[fab04]

I've found a solution by adding HAL_Delay(10) after each CDC_Transmit_FS.

[Pavel A. · Accepted solution]

Hi,

Before calling CDC_Transmit, you need to check that the previous transmit is gone. This was discussed here many times.

The CDC driver informs you about TX completion via TransmitCplt callback:

https://github.com/STMicroelectronics/stm32_mw_usb_device/blob/2022e75b01a499b17acd17d28691b1ed5bbef2dc/Class/CDC/Src/usbd_cdc_if_template.c#L227

The speed (bytes/sec) of TX is managed by the host. Normally the host is a powerful computer, and it won't cause unexpected delays, but things happen. Device cannot "push" data to host unless the host is ready to receive.

Device should be prepared to situation when the host does not receive, and behave reasonably.

[fab04]

Hi,

Thank you for your explanation, I've modified my code to take care of this point.