3.0 APPLICATION NOTES
USB devices transfer data in packets. If data is to be sent from the PC, a packet is built up by
the application program and is sent via the device driver to the USB scheduler. This scheduler
adds a request to the list of tasks that the USB host controller will perform. This will typically
take at least 1 millisecond to execute because it will not pick up the new request until the next
USB frame (the frame period is 1 millisecond). There is, therefore, sizeable overhead
(depending upon your required throughput) associated with moving data from the application to
the USB device. If data is sent one byte at a time by an application, this will severely limit the
overall throughput of the system.
It must be stressed that in order to achieve maximum throughput, application programs should
send or receive data using buffers and not individual characters.
4.0 DRIVER SOFTWARE
FTDI's VCP (Virtual COM Port) driver-executable files are provided royalty free on the condition
that they are only used with designs incorporating an FTDI device (i.e. the FT245R and DLP-
USB245R). The latest version of the drivers can be downloaded from either
www.dlpdesign.com or www.ftdichip.com .
The CDM driver download file is a combined set of drivers for the Windows operating system
and contains both the VCP and D2XX driver versions. To download, simply unzip the file to a
folder on your PC. (The drivers can coexist on the same floppy disk or folder since the INF files
determine which set of drivers to load for each operating-system version.) Once loaded, the
VCP drivers will allow your application software—running on the host PC—to communicate with
the DLP-USB245R as though it were connected to a COM (RS-232) port.
In addition to VCP drivers, FTDI's D2XX direct drivers for Windows offer an alternative solution
to the VCP drivers that allow application software to interface with the FT245R device using a
DLL instead of a Virtual COM Port. The architecture of the D2XX drivers consists of a Windows
WDM driver that communicates with the FT245BR device via the Windows USB stack and a
DLL that interfaces with the application software (written in VC++, C++ Builder, Delphi, VB, etc.)
to the WDM driver.
The D2XX direct drivers add support for simultaneous access and control of multiple FT245R
devices. The extended open function (FT_OpenEx) allows the device to be opened either by its
product description or serial number, both of which can be programmed to be unique. The list
devices function (FT-ListDevices) allows the application software to determine which devices
are currently available for use, again by either product description or serial number.
Additional functions are provided to program the EEPROM (FT_EE_Program) and read the
EEPROM (FT_EE_Read). Unused space in the EEPROM is called the user area (EEUA), and
functions are provided to access the EEUA. FT_EE_UASize gets its size, FT_EE_UAWrite
writes data into it and FT_EE_UARead is used to read its contents.
Download FTDI Application Notes AN_103 and AN_104 for detailed instructions on how to
install the drivers on XP and Vista platforms.
Rev. 1.0 (November 2008)
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? DLP Design, Inc.