Here is how to interface a PIC18F452 to a PC via the USB port. The PIC captures the ambient temperature with a SPI sensor TC77. Its value is then transferred to the USB-interface.
The windows-pc (98, Me, 2000, XP, 2003) runs a program (C# application, Microsoft Visual Studio .NET 2003) to communicate with the USB interface, which transfers data to and from the PIC18F452.
The USB interface will have to be recognized as a device by Windows. Basically, there are two ways of doing this. Either with a VCP (Virtual COM Port) or with a D2XX (Direct) driver. We will use the D2XX driver here for its high speed. It is more complicated to start up than VCP, but why not going the extra mile?
The FT245BM chip comes with a fixed VID (0403) and PID (6001). We can override this with our own VID (ECDE), PID (0001), our own Product Description (TechDesign USB Device 017) and serial number (FEDC0001). These settings are stored in an external SPI EEPROM 93C46. To program these settings, FTDI made a utility called MProg EEPROM programming utility.
When plugging in for the first time and with a blank (or without) EEPROM, you will notice that the interface is seen as “USB device” and after its driver is installed, it will be recognized as a “FTDI FT8U2XX Device”. Now we can program the EEPROM with our custom settings. To do this, run MProg, open the file 017_USB_id.ept and program (choose “Device”, then “Program”). After this, unplug the interface briefly and plug it in again. It will now be recognized as a “TechDesign USB Device 017”. Acknowledge a couple of times to install it and you are done.
Note that this project will also work without an EEPROM, but in that case you will have to make do with the fixed VID & PID.
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