Lpc2000

Hi,

I found a really hard to track down hardware problem today
that was forcing the JTAG port to not work.

I am using an LCP2138 and JTAG quit working after a board rev.

We had pulled P0.31 down because it was the enable for a 485
transciever and needed to be low during reset.

Turns out there is an 'undocumented feature' on some of the LPC
parts including the 2138 that will disable JTAG if P0.31 is low during
reset. We all know about RTCLK needing to be high during reset
for JTAG because it's documented, but P0.31 functions exactly the
same way.

I think it all has to do with a obscure thing called the secondary JTAG
port.  I think for some of the series P0.31 functions like RTCLK on the
2138 and must be low at reset to enable the JTAG port.
Apperently it's still in the silicon even on the 2138 even though it's
not documented.

Anyhow it was very obscure and if it weren't for a single message 
buried deeply on the Keil site I would not have found until I removed 
every change on the new rev.


Landrum Haddix
lhaddix@...
http://web.qx.net/lhaddix