Specifications

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Communications Device Class (CDC)
 Bulk transfers used for data transport
 Any device can use the CDC drivers. It does not need to be
a modem device.
 Volt meters
 Bar code readers
 Thermometers
 Anything that requires a serial port input to the host uses the Abstract
Control Model (ACM)
 What do I need?
 Firmware side
 Firmware to detect and respond to Standard requests and the Class
specific requests
 Standard and class specific descriptors defined.
 Host side
 To use CDC, the host application opens the comm port assigned by
the operating system. MSCOMM is typical in Windows.
The communications device class (CDC) is used for quite a few applications.
Where it is most beneficial to embedded designers is the implementation of the
Abstract Control Model (ACM) which is a sub part of the CDC specification. The
ACM allows a USB device to operate as a USB to RS232 bridge. It requires the
descriptors to call out the CDC class and the firmware to be compliant to the USB
2.0 and the CDC specification. In addition the firmware is required to respond to
the class specific requests. An example would be the Set_Line_Coding which sends
the baud rate information in order to configure the integrated UART. So does a
CDC class device have to use a UART on the other end? No. We can just use the
class specific enumeration and the built in driver to the OS to provide the means to
pass data back and forth to the host. Once we receive it we can do anything we
want with it.