On Fri, 2007-06-08 at 15:01 -0700, Harold Hallikainen wrote:
> I'm working on my first project with JTAG. I managed to use onTap to find
> some problems between a PowerPC and flash memory today. That was great!
> Anyway, there is concern that if we make one big JTAG loop, it will not
> work if there is anything wrong anywhere on the loop. I'm thinking we
> could stick resistors between the serial data out of one chip and the
> serial data in of the next, then put a single JTAG header for the whole
> board and also headers for each chip. When driving an individual chip, the
> serial data out of the previous chip would be "overpowered" by the JTAG
> "probe" serial data out.
> 
> Is this workable? Is anyone doing anything like this? What value resistor?

I don't like it, there are too many areas to "go wrong" IMHO (i.e.
what's the output impedance of the chip, what's the output impedance of
your JTAG cable, what about a different JTAG cable?).

I've worked with boards where 0ohm resistor were used to "configure" the
board with regards to the "shape" of the loop. Jumpers can also be used.

Personally I don't see the point. I've never seen a problem with a JTAG
loop, and this is on board with 6 seperate JTAG devices on it, in one
big loop. 

The only issue I have seen is clock distribution, since the data is a
loop, but the clock is a "star" type config, if you're not careful with
the routing of your clock you can get into trouble. 

TTYL

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