Forums

JTAG RTCK question

Started by seangra September 29, 2005
I see that people are saying that the RTCK line for JTAG debugging
isn't used. Does this mean that I can use this as a GPIO even when
using the JTAG debugger?


An Engineer's Guide to the LPC2100 Series

Not really.

While using the JTAG debugger this pin is controlled by the JTAG
interface. Also in the LPC2103 which was the device in discussion, the
RTCK is not multiplexed with GPIO at all.

Robert

--- In lpc2000@lpc2..., "seangra" <sgraham@o...> wrote:
> I see that people are saying that the RTCK line for JTAG debugging
> isn't used. Does this mean that I can use this as a GPIO even when
> using the JTAG debugger?



Alas,

it is just the other way around.
The RTCK line is not used by a lot of the JTAG pods around, the ARM7
TDMI-S does use the RTCK.
Look at page 5-3 (105) in the ARM datasheet
(http://www.arm.com/pdfs/DDI0084F_7TDMIS_R3.pdf)
There is a TCK synchronizer in between the 'real' JTAG signals going
into the processor core and the pins
on the LPC2xxx, this circuit makes sure that the JTAG signals are
synchronized to the processor clock.

Any JTAG debugger pod used should make sure that TDO is read with RTCK
used as a clock,
otherwise it may read wrong data.

On the LPC2104/5/6 the RTCK is a seperate pin, not shared with a GPIO
pin and on the LPC213/2/4/6/8
there is only one bit in PINSEL2 to determine if P31-26 are used as JTAG
or GPIO lines.
So even if your JTAG debug pod does not use the RTCK there is no way to
get the extra GPIO pin.

Regards,
Rob

seangra wrote:

> I see that people are saying that the RTCK line for JTAG debugging
> isn't used. Does this mean that I can use this as a GPIO even when
> using the JTAG debugger?