The latest version of this firmware is available in the latest Micropendous Distribution or via SVN in the /Micropendous/Firmware/USBVirtualSerial directory.

Purpose: USBVirtualSerial is a demonstration of USB Virtual Serial Port communication. Data sent to a device loaded with this firmware will be echoed back.

Source Code: View or Download

Targets: Micropendous1, Micropendous2, Micropendous3, or Micropendous4, or other USB AVR boards

USB Mode: Device-mode enumerating as USB Virtual Serial Port

Status: Working. Please post any problems you encounter to the Micropendous Group.

Usage

Assuming you have completed all setup tasks in the QuickStart tutorial, simply compile and upload the USBVirtualSerial firmware onto your Micropendous board by following the ProgramAndTest tutorial.

USBVirtualSerial programmed devices will enumerate as a Virtual Serial Port and any data sent to the device using Serial will be echoed back.

Firmware Design

This firmware is based on LUFA's USBtoSerial. The main difference is that UART communication was replaced with loopback code. All UART settings can be reused for your own firmware's parameters by editing the Reconfigure() function.

Standard stdin and stdout/stderr streams are created in main() and linked to the USB Virtual Serial Port. This means you can use standard printf and getchar functions to read and write data over the USB connection. Just place your code in Main_Task() which runs periodically. See avr-libc stdio for detailed instructions on using these functions.

Note that your code should not busy-loop or block as this will interfere with USB functionality. To avoid this issue, use USBVirtualSerial_FreeRTOS.

On the host side, serialpy.py is an example of serial communication and is explained in Serial.

The following is the core of this firmware and where your code should be placed.

/* Main_Task will run periodically once initialization is complete */
void MainTask(void)
{
  int count = 0;

  // If the host has sent data then process it

  // Throughput is maximized if the full EP buffer is read and sent each time
  // Throughput approaches CDC_TXRX_EPSIZE kbytes/second and depends on transfer size from host 
  if ((count = fread(&buffer, 1, CDC_TXRX_EPSIZE, &USBSerialStream)) > 0) {
    fwrite(&buffer, 1, count, &USBSerialStream);
  }


  if (count == 14) {
    // Examples of using formatted standard IO functions

    // 1) send a string that is constant and stored in FLASH
    fprintf_P(&USBSerialStream, PSTR("\r\nUSBVirtualSerial\r\n"));

    // 2) send a string that is dynamic and stored in SRAM
    fprintf(&USBSerialStream, "Received count = %3d\r\n", count);
  }
}

The above example is very simple. haveData() checks if data has been received from the host and then stdio functions getchar() and putchar(...) are used to read and resend the data over the USB serial connection.

You can also use printf(...) functions but note that printf() is stored entirely in SRAM so it is best to use printf_P(PSTR("..."); to store and print any constant strings. See the printf tutorial for more information.