How to use USB CDC ACM and MS composite Linux gadget driver

Introduction

This document explains the Communications Device Class (CDC) Abstract Control Model (ACM) + Mass Storage (MS) composite Linux gadget driver and how to use it with a Linux host PC and a Windows host PC.

• Linux device (gadget) driver framework (last updated 2005)
• USB Communication Device (CDC) and Mass Storage (MS) Device Class Specifications

CDC ACM + MS driver

The CDC ACM + MS driver is a USB composite gadget driver which implements the serial communication and mass storage functions. This allows the device to be registered on a host computer as a seral port (e.g. /dev/ttyACM0 or COM1:) and as a USB thumb drive (e.g. /dev/sdf or E:). The composite driver allows for simultaneous use of the serial and mass storage functions.

Driver support

The CDC ACM only driver that comes with the Linux kernel is supported on both Linux and Windows. For Windows it is necessary the use of the following INF file:

http://www.kernel.org/doc/Documentation/usb/linux-cdc-acm.inf

Our experience shows the CDC ACM + MS composite USB driver is not well supported on Windows with this INF file. We have tested with a propretary solutions from Thesycon, with a demo version available at

• Demo version of CDC ACM driver.

Linux host computers have full support for the CDC ACM + MS composite USB drivert without the need of additional files.

Using CDC ACM + MS device with Linux host

Linux host device file identification

The following description is based on the host computer running Linux Ubuntu 10.04 LTS.

Once you attached the device, inspect the log to obtain the device file names used on the host.

dmesg | tail -n 20

Jan 31 14:19:34 mmadrigal kernel: [109059.420096] usb 1-3: new high speed USB device using ehci_hcd and address 16
Jan 31 14:19:34 mmadrigal kernel: [109059.553321] usb 1-3: configuration #1 chosen from 1 choice
Jan 31 14:19:34 mmadrigal kernel: [109059.562570] cdc_acm 1-3:1.0: ttyACM0: USB ACM device
Jan 31 14:19:34 mmadrigal kernel: [109059.567214] scsi8 : SCSI emulation for USB Mass Storage devices
Jan 31 14:19:35 mmadrigal kernel: [109060.604592] usb 1-3: USB disconnect, address 16
Jan 31 14:22:58 mmadrigal kernel: [109262.944049] usb 1-3: new high speed USB device using ehci_hcd and address 17
Jan 31 14:22:58 mmadrigal kernel: [109263.077362] usb 1-3: configuration #1 chosen from 1 choice
Jan 31 14:22:58 mmadrigal kernel: [109263.088675] cdc_acm 1-3:1.0: ttyACM0: USB ACM device
Jan 31 14:22:58 mmadrigal kernel: [109263.123308] scsi9 : SCSI emulation for USB Mass Storage devices
Jan 31 14:23:13 mmadrigal kernel: [109278.133699] scsi 9:0:0:0: Direct-Access     Linux    File-CD Gadget   0316 PQ: 0 ANSI: 2
Jan 31 14:23:13 mmadrigal kernel: [109278.142902] sd 9:0:0:0: Attached scsi generic sg6 type 0
Jan 31 14:23:13 mmadrigal kernel: [109278.150041] sd 9:0:0:0: [sdf] Attached SCSI removable disk

The serial communication port has been registed as 'ttACM0', meaning it will show up as '/dev/ttyACM0'. The mass storage device have been registered as 'sdf', meaning it will show up as '/dev/sdf'.

How to test the serial driver in Linux

In order to test the serial function on Linux you should need to install picocom

sudo apt-get install picocom

and use the picocom terminal emulator to display data received over the USB serial connection:

picocom /dev/ttyACM0

In order to send data from the target device we need to use the USB serial gadget driver device file '/dev/ttyGS0'. We can echo command to send data from the device to the Ubuntu host computer:

echo foo > /dev/ttyGS0

After that you should see the text foo on picocom's output.

In a similar way, if we want to send data from the host computer to the device we should first send the data on the host by running

echo hello > /dev/ttyACM0

and read it on the device

cat /dev/ttyGS0

How to use the mass storage function in Linux

Once the driver has been installed correctly we just need to mount the SD card on the device in order to get the mass storage device ready. In order to do that we just need to run the following commands on the target device:

• Mount the MMC partition

echo /dev/mmcblk0p2 >  sys/devices/platform/musb_hdrc/gadget/lun0/file

• On the Ubuntu PC open the external media device, now you have access to the MMC card on your device.

Using CDC ACM + MS device with Windows 7 host

When using the device on Windows, you need to use a INF file in order to load the appropriate driver. There is an INI file avaiable (see above), but it didn't work for use on Windows 7 with a CDC ACM + MS device. Instead we used the proprietary driver from Thesycon.

Thesycon provides a demo version of the driver [1] that can be installed on Windows. By default it creates a directory on C:\Thesycon\CdcAcm\V1.96.0_Demo\. There is an INI file and a SYS driver in the idisk sub-directory you can use to exchange USB serial data with the target device.

In order use the INF file provided it is necessary to add the VID (Vendor ID) and PID (Product ID) of the target device to the file; look for the following piece of code into the file:

[_Devices.ntamd64]
; enter your VID (VVVV) and PID (PPPP) here
%S_DeviceDesc1%=Install,USB\VID_VVVV&PID_PPPP

Replace the USB\VID_VVVV&PID_PPPP string with the corresponding VID and PID. In addition we need to specify the interface on which the serial function is located since it is a composite device. For example, the following uses the Linux Foundation (VID = 0x1D6B) device with a PID = 0x0104 (Multifunction Composite Gadget). We assume the serial interface is located in the interface 1, so the piece of code shown above should be replaced by:

[_Devices.ntamd64]
; enter your VID (VVVV) and PID (PPPP) here
%S_DeviceDesc1%=Install,USB\VID_1D6B&PID_0104&MI_01

where MI_01 means the serial interface is on interface 1.

Once the edited INF file is read, just plug the device to the Windows host computer. Windows will try to locate a usable driver. When it asks for the driver just point it to the folder where the modified INF file resides and Windows will do the rest.

If Windows doesn't ask for it but still fails on the installation you can install the driver by:

1. Go to Start -> Control panel -> Devices and printers"
2. Right click on the composite device.
3. Properties -> Hardware.
4. Select the CDC ACM Data function.
5. Properties
6. Change configuration
7. Update firmware
8. Point it to the folder containing the INF file and then wait to the driver to be installed.

Once Windows announces the driver was installed correctly, the device is ready to use.

Using the Windows 7 serial port

In order to use the serial function you can install TeraTerm on your Windows machine and launch the TeraTerm application. On first dialog TeraTerm will ask for the connection properties as is shown on figure 1, be sure to select Serial and the port corresponding to your composite device.

Figure 1. TeraTerm initial screen.

After selecting the appropriated port, TeraTerm will open the connection with the device.

To send data from the device to your host PC run the following command on the target device:

echo foo > /dev/ttyACM0

You should be able to see the text foo in the TeraTerm window.

To send data from Windows to the device write any word on your TeraTerm's window and press ENTER and then run the following command on the device:

cat /dev/ttyACM0

You should be able to see the string on your target device console.

How to use the mass storage function in Windows 7

Once the driver has been installed correctly we just need to mount the SD card on the device in order to get the mass storage device ready. In order to do that we just need to run the following commands on the target device:

• Mount the MMC partition

echo /dev/mmcblk0p2 >  /sys/devices/platform/musb_hdrc/gadget/lun0/file

• On the Windows PC open the external media device, now you have access to the MMC card on your device.

Endurance Test

A 4GB file was created using "dd". The file was transmitted from a LeopardBoard to a Ubuntu PC using the USB composite driver. The transmitted file and the file at the LeopardBoard where compared using the sha1sum, both numbers where the same indicating that the USB file transfer didn't corrupted the data.

The average speed of the transmission was approximately 3.5MB/s.