Lpc2000

From: "Bertrik Sikken" <bertrik@...>
> Perhaps we can improve upon the existing designs and create
> the ultimate low-cost ARM JTAG cable, yet keeping it simple.
> Some ideas:
> * provide proper current limiting for all lines
> * add readback of the RTCK signal (return clock)
> * add readback of the targets' VCC (like the Olimex clone does)

Also:

* use proper level shifting buffers or separate buffers for the LPT port
  side(5V logic) and the target side (target voltage).
* use low-pass filters (RC) + schmidttrigger receivers on the LPT port side.
* series termination resistor on each driven line, to LPT and to target.
* a (jumper selectable) nTRST drive, separate from the nRESET drive.
* optional: a 1mA LED indicator on the target side VCC.

The problems occur not just with building the thing - it's the wiggler
design itself that is a bit flakey, especially the lack of a noise filter
and schmidttrigger receiver on the CLK input at the LPT side. A 100...400
nsec RC filter should do it while still allowing up to 1 MHz clocking (above
LPT port speed levels). If this is not present, long LPT cables or improper
grounding will cause glitches that upset the JTAG state machine.

I fail to see the actual use of the RTCK signal - is this to be tested by SW
on the LPT status bits? Which CPU is so slow that this would make a
difference?

Regards,
Arie de Muijnck