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.

Thursday, July 3, 2014

via-rhine in Debian Wheezy

This blog post is only of interest to a small handful of people,  but since you found it, it might be you :). As always with system changes, there's a chance of damage or data loss. So check your backups first.

I have an old Averatec notebook that I use for travel and when I don't want to take risks with my main PC. I'm planning a trip so it's time to update to the latest debian distribution. After the update, everything seems to work, except the via-rhine wired ethernet.
The symptoms appear similar to Debian bug report 708757 . After a lot of hacking and experimenting, I finally upgraded to the latest kernel in wheezy-backports and the problem went away.


lspci shows the device as:
00:12.0 Ethernet controller: VIA Technologies, Inc. VT6102 [Rhine-II] (rev 74)

It does work with Debian kernel packages:
 linux-image-2.6.32-5-686  # Old, from previous Debian distribution.
 linux-image-3.14-0.bpo.1-686-pae # Very latest, bleeding edge.

And does not work with
linux-image-3.2.0-4-686-pae # debian stable kernel for my system.


To get the very latest kernel, you need the backports repository. Best instructions I found were:
1) add wheezy-backports to /etc/apt/sources.list
2) apt-get update
3) apt-get -t wheezy-backports install linux-image-3.14-0.bpo.1-686-pae
4) done. reboot to use the new kernel.


One side effect of the bug is that it returns a wrong and somewhat random MAC address for the ethernet controller. Recent versions of Debian try to ensure that each MAC address is always associated with a consistent eth* number. So each random mac address gets assigned to a new eth* number. If this is happening the output of dmesg will show messages like:
udev: renamed network interface eth0 to eth7
Every time udev sees a new MAC address it adds it to /etc/udev/rules.d/70-persistent-net.rules,
so the fix is to edit that file and remove all the junk MAC addresses.