jtag cable for cpld xc9572
By implementing the Xilinx cable from the website, I am assuming that you built the 74HC125 circuit shown on the web. A couple of things to check. This is the same circuit Xilinx sells in their Parallel Port III adapter. Lots of people have built this and gotten it to work, so the design is solid.
A: The circuit MUST get its power from the CPLD host. Verify that 3-5V from pin 14 to 7 of the 74HC125. No power means no work.
B: The Impact software has a quirk in that if you attempt to access a CPLD or FPGA without powering up the device, the software will go into a bad state and will refuse to work even after you correct the power problem. The solution is to exit Impact and restart it. Also, be careful not to click the icon too many times. The program is slow to show a splash screen. If you click it again, it launches two copies and they will fight each other for control. Best method is to connect cable, power up the board, and then launch Impact.
C: Later versions of Impact have a debug mode that you can use to toggle TCK, TMS and TDI. This this is helpful in checking out your circuit connections. Look through the menus on the top banner and try using the debug mode. To describe JTAG in a couple of sentences. TCK is the clock. TMS is a signal used to differentiate between data and commands. TDI is a data into the CPLD or FPGA. TDO is data from the CPLD or FPGA back to the programmer. TCK and TDI toggle quite often. TMS toggles less frequently. TDO only toggles when the part is giving back data. TDO is open drain and must have an external pullup to VCC (see schematic).
D: This circuit ONLY works with a parallel ports that are connected to the motherboard or a plugin PCI card. It does NOT work with USB to parallel port converters.
E: Parallel port settings in BIOS can affect operation on some computers. If all else fails, try changing the BIOS settings.
F: Intermittent failures are possible in Windows XP. The problem is that XP periodically polls the ports looking for new Plug and Play devices. If XP polls the port during the time you are using it, failures can occur.
---- Steve