Lpc2000

Jaya,

On Wednesday 08 February 2006 15:32, Jayasooriah wrote:
> Dominic,
>
> I believe you are not distinguishing between JTAG reset and LPC reset.
I am distinguishing between JTAG reset and LPC reset. I wouldn't say nSRST if 
I meant nTRST, or vice versa. nTRST is hardly of any use (my OpenOCD works 
fine without it).

> JTAG operations are carried out using the TAP (Test Access Port)
> controller.  This TAP controller can be forced to its "reset" state by
> driving signal nTRST LOW or by pushing a sequence through TMS for a given
> number (I recall 5) of TCKs.
Yes.

> To use the boundary scan interface, nTRST must be driven LOW and then HIGH
> again. You appear to have held it low, and hence your observations.
No. This behaviour has been confirmed by someone from Rowley.

> Driving nTRST HIGH does not mean you have also to drive nRESET HIGH.  The
> TAP controller can be accessed with nRESET held low, and this how "boundary
> scanning" is used for debugging in the days before ETM (Embedded Trace
> Macrocell).
Debugging doesn't use the ETM. Debugging uses the EmbeddedICE macrocell.
On the LPCs, driving nRESET low keeps the TAP controller in reset, too. 
Period.

> ... snip ...
> PS:  I bet you can find ways to reduce the 79 TCK requirements by taking in
> to account the state the shift registers before you shift but this is not
> why I would not take the 79 TCK requirement seriously.  If you needed this
> many cycles then it does not make sense to reduce the window of opportunity
> by one jump instruction at the expense of exception handling, as was done
> on the 2292.
The scan chain select register is set to 0 after reset, and the instruction 
register is set to the IDCODE instruction. That means you have to:
- select the SCAN_N instruction
- shift in b0010 (Scan chain 2 is the EmbeddedICE macrocell)
- select the INTEST instruction
- shift in data for the EmbeddedICE control register to request a debug entry
Just because you believe that this is the reason why Philips dumped the 
exception vectors doesnt necessarily mean that there's a way to cut the 
number of TCK cycles down.

Regards,

Dominic