JTAG interfaces


There are several different types of cables that are popular for hooking up to JTAG headers inside consumer electronic equipment. Most of these rely on a regular PC's parallel port to drive the JTAG signal lines. There are vendors of commercial JTAG cables that sell them at extravagent prices. For the home user or hobbyist, however, a better choice is usually to construct a cable at home from commonly available parts.

JTAG interfaces on the d-link 30xT


There are 2 JTAG interface tested on the 30xT models.

The buffered interface


TODO


The poorman interface


Even if it's very cheap a lot of people were able to succesfully use it with the CICLaMaB.
This is the picture:

PoorJTAG_B.gif

The VERY poorman interface


This is the simplest type of JTAG cable, the easiest to construct and the cheapest to make. The original cable was introduces by Xilinx and has a full name "Xilinx DLC5 JTAG Parallel Cable III". Someone removed a buffer and changed it with a four 100 Ohm resistor. Popularized by the Hairydairymaid de-brick utility software for Linksys routers, many people have successfully built their own unbuffered JTAG cable. It consists of only a few cheap resistors, a 25-pin parallel port connector and a ribbon-cable with a 12-pin connector that slides onto a header soldered onto the PCB found inside the cases of Linksys WRT54G and WRT54GS routers. The chief limitation of this type of cable is that it must be very short; the length must be 6 inches or less (15 cm) to avoid problems with electrical noise.
NOTE: This was not tested on the d-link 30xT but is likely to work

JTAGunbuffered.png


Speed issues


Driving a JTAG interface through the parallel port on a PC is a slow proposition. Really slow. This is due more to the nature of the parallel port connection than an inherent limit of the JTAG specification. In fact, the JTAG spec allows for up to 25 milliion bits-per-second transfers. With a parallel port cable, however, you will be lucky o achieve more than about 400,000 bits-per-second. With these speeds it is not unusual to spend 25 minutes writing a mere 256 KB of data over a JTAG cable. Programming an entire 2 MB or 4 MB flash chip can literally take hours. It's worth it, however, if you have an otherwise worthless device on your hands and JTAG is the only way to revive it. The Macraigor Raven and USB JTAG adaptors are much faster, but there are no known schematic to implement it.

However on the 30xT router is usually necessary just to upload the ADAM2 bootloader that is 64Kbytes. It does not take more than 15 minutes


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