Do I need to write my own host side USB driver for a CDC device

Question

This may not be considered to be directly programming related but I am at a loss to know where else to ask. I have tried looking at a variety of websites but so far Google has not been my friend.

I am having trouble finding out whether I need to write my own device driver for the various windows/linux/mac platforms that the device I am developing may be connected to, or whether the functionality is provided by the standard drivers.

My device is a USB CDC (communications device) that appears as a COM: port. It also includes a battery charger that will, once the device has been enumerated require the full 5 unit load (500mA) supply current that can be drawn from the USB connector. My problem is that if the USB driver in the host decides that it cannot deliver the full supply current then it should fail to enumerate the device.

If, as a fallback, I provide a second configuration set that only allows the device to draw 1 unit load from the interface connector will the standard drivers enumerate the device using this configuration.

I believe that you first have to look into the USB specification to see if these things are allowed. If the specification allows it, then standard browsers will probably support it. — kgiannakakis
In the USB 2.0 specification the device descriptor allows for a number of configurations, each of which can specify that it requires a certain amount of power from the interface. It would then, I presume, be up to the driver to choose between these different configurations. — uɐɪ

Vicky Vicky · Accepted Answer · 2009-07-29T08:38:14

You need to write a .inf file for Windows that ties up your device VID and PID with the system usbser.sys. Mine looks like this (Replace YourCompany as necessary, put in your VID and PID (in hex), and change the DriverVer line to whatever date and version you want):

; -----------------------------------------------------------------------------
; XP/2000 USB Comms Port Setup
; -----------------------------------------------------------------------------

[Version] 
DriverVer=12/03/2008,1.0.0000.0000
Signature="$Windows NT$"
Class=Ports 
ClassGUID={4d36e978-e325-11ce-bfc1-08002be10318} 
Provider=%YourCompany% 

[DestinationDirs]
DefaultDestDir=10,system32\drivers
DriverCopyFiles=12

[ControlFlags]
ExcludeFromSelect = *

[Manufacturer] 
%YourCOmpany%=YourCompanySerialPort 

[YourCompanySerialPort] 
%YourCompanyUSBSerialPort%=YOURCOMPANYUSB,USB\VID_1234&PID_ABCD

; 
; Win 2000/XP
;
[YOURCOMPANYUSB]
Include=mdmcpq.inf
CopyFiles=FakeModemCopyFileSection

[YOURCOMPANYUSB.HW] 
AddReg=YOURCOMPANYUSBAddReg.HW 

[YOURCOMPANYUSBAddReg.HW] 
HKR,,DevLoader,0,*ntkern 
HKR,,NTMPDriver,,"usbser.sys" 

[YOURCOMPANYUSB.Services] 
AddService=usbser, 0x00000002, FuncDrv_Service_Inst 

[FuncDrv_Service_Inst] 
DisplayName=%USBFilterString% 
ServiceType= 1 
StartType = 3 
ErrorControl = 0 
ServiceBinary = %12%\usbser.sys 

[Strings] 
YourCompany="YourCompany" 
YourCompanySerialPort="Your Company USB Serial Port" 
USBFilterString = "USB Serial Service"

Note this works with 32 bit OSs only. It also works with Vista although the file header doesn't say so!

Be aware that some versions of usbser.sys have significant problems, including bluescreening, for example when transferring packets that are exact multiples of 64 bytes. If you're using XP SP2 or previous then install hotfix KB943198. XP SP3 and Vista are fine.

For the Mac you simply need to report your device class correctly and the driver scan picks up the correct drivers. (Windows ignores the device class which is why you need to supply the .inf file).

EDIT: Sorry, I should have been clearer. This will not fail to enumerate if it can't draw the full load - I'm not sure that's possible.

Do I need to write my own host side USB driver for a CDC device

5 Answers