Friday, July 11, 2014

Parallel port jtag

I've been using a homebrew version of the Xilinx DLC5 parallel port jtag adapter for programming my fpga boards. The DLC5 is no longer supported by Xilinx, but some Linux jtag programmers still support it.
Recently I moved to a new PC, and my DLC5 hack no longer works with that parallel port. I can see on the scope that the right pins are moving, but the chip won't program. I suspect that either the rise and fall times are too slow, or there's noise on the signal lines.
Anyway, I had a sample of the SiLabs si8663 digital isolator, so I decided it's time to build a new interface. The Si8663 is a hex isolator (3 signals each direction), that's spec'ed to operate up to 150 Mbps bit rate (WAY more than needed for this app). Another neat feature is that the drivers on each side work from 2.5 to 5.5 volts, providing a level shifter at no extra charge.
Board is available on OshPark ( for anyone interested. Project name is "isopartag 140513". Circuit's simple: chip + 4 bypass caps on the bottom. The design includes resistors to tap power from unused pins of the parallel port, but they haven't been added yet, to that feature is untested. For now I just tap 5 volts from an unused keyboard or mouse port.
Caution:The Si8663 comes in two versions: a wide body for safety isolation, and a narrow body intended more for electrical noise isolation. I'm using the narrow version, and the board is not designed with wide separation between the input and output sides. In other words, this design is for low voltages only, don't use it where safety is an issue.

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