Re: JTAG debuggers - a couple of questions

Markus Zingg wrote: 
http://www.embedded.com/1999/9911/9911feat3.htm

Which explains very well how products work which use THIS aproach.
Still leaves a bit open how the "el cheapo" cable/dongle ones operate
though.

Didn't Grant already say it? :) Raven, Wiggler etc just pass the JTAG signals (clock, data out, data in, test mode select) between your chip and the PC software, with no fancy target-device specific command conversion firmware doing any magic in between.

Essentially all that the generic JTAG allows, is a boundary scan of the chip pin states. Programming, memory readout and debugging functions are proprietary, vendor or part/series specific extensions to JTAG commands.

The el-cheapos handle the extensions in the PC software. The more expensive ones do it inside the dongle in firmware (but the PC software then must also cope with the protocol the dongle firmware uses, this /may/ be a very vendor or device specific protocol!! ;-)

With el-cheapo cables (Raven, Wiggler) you're potentially able to use for a lot more of different devices. The only problem I can think of is that they are "a bit" slow. And they aren't supported by all debugger, programmer, IDE tools - one obvious reason is that these vendors want to sell you their own expensive proprietary interface dongles, so you won't mistakenly use your own too cheap $5 interface cable... ;-)

 - Jan
.

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