Re: gEDA-user: Test pads in PCB

[Marvin Dickens <marvindickens@xxxxxxxxxxxxx>]

DJ Delorie wrote:
> > > It's a 20 pin chip, and you want to use four of them for JTAG?
> > >       
>
>   
> > I thought I saw you were looking at doing something with an ARM9 chip.
> >     
>
> JTAG is only fully useful when all the chips support it - you'd chain
> all the chips together to do continuity testing of the board after
> assembly.  The R8C/17 doesn't support JTAG.
>
> I think ARM9 chips expect you to use JTAG for downloading and
> debugging, yes.
>
>   
This is both true and false and neither true or false :)

For instance, a lot of designs have jtag only for flash or for a single FPGA
or for the processor only. Some of the equipment that I work with has 
multiple
JTAG busses (Up to 6 independent JTAG busses) on one card. This is
particularly true of designs that incorporate FPGAs and/or other devices
from muliple vendors. The only constant with JTAG is that one size does
not fit all - It all goes back to testabilty and upgrade paths.

Best

Marvin