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Jun 24, 2014

Implementing USB communication device class (CDC) on EFM32GG MCUs

1. Introduction

USB revolutionized the PC peripheral space by making a very simple plug-and-play interface for users. As a result, many modern computers no longer support RS-232 serial COM ports, opting for the slimmer USB alternative. This can be an issue for the developer who needs a COM port for communication between a peripheral and host PC. A subset of the USB Communication DeviceClass (CDC) can be used to emulate a serial port providing a virtual COM port UART interface. This allows developers to use legacy applications with new products using the same COM port interface as before, with few hardware and software modifications.
USB CDC Intro
Figure 1 USB CDC Virtual COM Port System
This application note describes the USB communications device class driver (or USB CDC) in detail and includes an implementation example for the Silicon Labs EFM32 Giant Gecko MCU.

1.1. Assumptions

This document assumes the following:
  • A working knowledge of the C programming language.
  • Familiarity with the USB 2.0 specification and terms and abbreviations defined by the USB specification.
  • Familiarity with Silicon Labs EFM32GG development environment.

1.2. Features and Limitations

The CDC firmware implemented with this application note includes the following features:
  • Emulates a serial COM port on PC that supports the CDC Abstract Control Model (ACM).
  • Provides an abstract communication interface for data transfers between the host and the device.
  • Handles standard Chapter 9 USB device requests.
  • Handles CDC-specific requests from USB host.
  • Notifies the USB host of status using an interrupt endpoint.
  • Provides data communication with the USB host using a bulk endpoint.
  • The following baud rates are supported: 600, 1200, 2400, 4800, 9600, 14400, 19200, 38400, 57600, 76800, 115200 and 230400 bps.
The example does not implement the following:
  • No CTS/RTS control is performed, so flow control must be set to nonein the terminal program.
  • RTS/DTR control is not implemented.

2. Relevant Documentation

EFM32 Giant Gecko Application Notes are listed on the following website: www.silabs.com/32bit-appnotes.
  • AN758 IMPLEMENTING USB COMMUNICATION DEVICE CLASS (CDC) ON SiM3U1XX MCUs -- provides an implementation example on porting LUFA USB CDC on SiM3U1xx MCUs.
  • AN0822 SIMPLICITY STUDIO USER’S GUIDE -- provides a description of the Simplicity Studio IDE features and environment.
  • AN0065 EFM32 as USB Device -- provides a description of the EFM32 USB Device stack.

3. USB CDC Class

The USB communications device class (CDC) is a composite USB device class, and the class may include more than one interface. The CDC is used primarily for modems, but also for ISDN, fax machines, and telephony applications for performing regular voice calls. The Abstract Control Model subclass of CDC and bridges the gap between legacy modem devices and USB devices, enabling the use of application programs designed for older modems.

3.1. Class Requests

The class requests and class notifications supported are listed in Table 1.
Table 1. Abstract Control Model Requests
RequestCodeDescription
SET_LINE_CODING20hConfigures baud rate, stop-bits, parity, and numberof-character bits.
GET_LINE_CODING21hRequests current DTE rate, stop-bits, parity, and number-of-character bits.
SET_CONTROL_LINE_STATE22hRS232 signalused to tell the DCE device the DTE device is now present.
These class-specific requests are used for device and call management.

3.1.1. Set Line Coding

This request allows the host to specify typical asynchronous line-character formatting properties.
bmRequestTypebRequestwValuewIndexwLengthData
00100001bSET_LINE_CODING0interfacesize of structureline coding structure
Table 2 defines the line coding properties.
Table 2. Line Coding Format
OffsetFieldSizeValueDescription
0dwDTERate4NumberData terminal rate, in bits per second.
4bCharFormat1Number0: 1 Stop bit
1: 1.5 Stop bits
2: 2 Stop bits
5bParityType1NumberParity:
0:None
1: Odd
2: Even
3: Mark
4: Space
6bDataBits1NumberData bits (5, 6, 7, 8 or 16).

3.1.2. Get Line Coding

This request allows the host to find out the currently configured line coding. Table 2 defines the line coding properties.
bmRequestTypebRequestwValuewIndexwLengthData
10100001bGET_LINE_CODING0interfacesize of structureline coding structure

3.1.3. Set Control Line State

This request generates RS-232/V.24 style control signals. Table 3 defines control signal bitmap.
bmRequestTypebRequestwValuewIndexwLengthData
00100001bSET_LINE_CONTROL_STATEcontrol signal bitmapinterface0none
Table 3. Control Signal Bitmap
Bit PositionDescription
15:2Reserved (Reset to zero).
1Carrier control for half duplex modems. This signal corresponds to V.24 signal 105 and RS232 signal RTS.
0: Deactivate carrier.
1: Activate carrier.
The device ignores the value of this bit when operating in full duplex mode.
0Indicates to DCE if DTE is present or not.This signal corresponds to V.24 signal 108/2 and RS232 signal DTR.
0: DTE is not present.
1: DTE is present

3.2. Class Notifictions

Table 4 shows the class notifications supported by the Abstract Control Model.
Table 4. Abstract Control Model Notifications
NotificationCodeDescription
SERIAL_STATE20hReturns the current state of the carrier detects, DSR, break, and ring signal.

Serial State

This notification sends an asynchronous message containing the current UART status.
bmRequestTypebRequestwValuewIndexwLengthData
10100001bSERIAL_STATE0interface2UART state bitmap
The data field for this notification is a bitmapped value that contains the current state of detects transmission carrier, break, ring signal, and device overrun error. These signals are typically found on a UART and are used for communication status reporting. A state is considered enabled if its respective bit is set to 1.
Note: The firmware example included with this application does not currently support state change
Table 5. UART State Bitmap
Bit PositionFieldDescription
15:7Reserved (future use).
6bOverRunReceived data has been discarded due to overrun in the device.
5bParityA parity error occurred.
4bFramingA framing error occurred.
3bRingSignalState of ring signal detection of the device.
2bBreakState of break detection mechanism of the device.
1bTxCarrierState of transmission carrier. This signal corresponds to V.24 signal 106 and RS232 signal DSR.
0bRxCarrierState of receiver carrier detection mechanism of device. This signal corresponds to V.24 signal 109 and RS232 signal DCD

3.3. Endpoint Configuration

Table 6 illustrates the endpoint configuration for the Abstract Control Model.
Table 6. UART State Bitmap
EndpointDirectionTypeMax Packet SizeDescription
EP0In/OutControl64Standard requests, class requests.
EP1InInterrupt16State notification from device to host.
EP2InBulk64Data transferfrom device to host.
EP3OutBulk64Data transfer from host to device.
Figure 2 shows a standard CDC communication flow.
CDC USB LOG
Figure 2. USB CDC Communication FLow

4. LUFA USB Stack

The USB CDC firmware example is based on the LUFA open-source project. LUFA is an open-source complete USB stack released under the permissive MIT License. It includes support for many USB classes, both for USB Hosts and USB Devices. For USB Devices, the LUFA stack includes support for Audio Class, CDC Class, HID Class, Mass Storage Class, MIDI Class, and RNDIS Class. More information about the LUFA project can be found on the official website: http://www.fourwalledcubicle.com/LUFA.php
  • The USB CDC project contains a prebuilt LUFA USB stack documentation which locate at .\LUFA\Documentation\html. Double click on the index.html, the documentations shows in your default browser.
LUFA documentation
Figure 3. USB LUFA Libary Documentation
  • This implementation support two boards of EFM32GG, STK3700 and DK3750. Default setting is DK3750, To change board selection, just modify macro definition in .\LUFA\Common\BoardTypes.h.
#if !defined(__DOXYGEN__)
    #define BOARD_                 BOARD_DK3750
    #if !defined(BOARD)
        #define BOARD              BOARD_DK3750
    #endif
#endif
For STK3700 board, there is no UART socket on board. UART signals are connected to EXP Header.
USART on EXP
Figure 4. USART signals on STK3700 Expansion Header

5. USB CDC Driver

The CDC class is implemented in all releases of Windows, and the operatingsystem needs an INF file for the CDC driver. This INF file contains the Vendor ID and Product ID. If the VID/PID of the USB devices matches the INF file, Windows will load the driver described in the file. The VirtualSerial.inf file can be found in the.\Demos\Device\LowLevel\EFM32Demos\VCP directory.

Installing the Driver

To install the driver on Windows 7:
  1. Build the project and download firmware to the EFM32GG DK3750 board.
  2. Connect the USB cable between the Device MCU plugin board USB connector and the PC.
  3. Open Device Manager. The device will appear under Other devices as the EFM32GG CDC DeviceDevice Manager
  4. Right-click on the EFM32GG CDC Device and select Update Driver SoftwareDevice Manager Update
  5. Select Browse my computer for driver softwareUpdata Driver Software
  6. Select Browse my computer for driver softwareBrowse Driver Software
  7. Windows will display a warning. Select Install this driversoftware anywayWindows Sercurity
  8. When the driver finishes installing,Windows will report the installation results.
  9. Open Device Manager and observethe device. It will now appear under Ports (COM & LPT) with an assigned COM port number. Device Manager Done

Source code

Source code can be found in https://github.com/MarkDing/lufa-efm32. 

Github page in http://markding.github.io/lufa-efm32/. 

CONTACT INFORMATION

Silicon Laboratories Inc. 400 West Cesar Chavez
Austin, TX 78701
Tel: 1+(512) 416-8500
Fax: 1+(512) 416-9669
Toll Free: 1+(877) 444-3032
Please visit the Silicon Labs Technical Support web page:
https://www.silabs.com/support/pages/contacttechnicalsupport.aspx
and register to submit a technical support request.
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