Re: Use of TRST on ST vs. Luminary ARM JTAG?


On Sat, 11 Feb 2012, Tim Wescott wrote:

I'm using an ST ARM Cortex part for the first time. I've been using
Luminary parts until now.

I notice that on the Olimex demo board for the STM32F103RBT6, they have
the JTAG TRST pin connected, but on the Luminary eval board for the
LM3S811 the TRST pin is left open.

Does anyone know why this is? Is it just that the TRST pin is sorta
optional, and the Luminary folks weren't in a mood to dot all their 'i's
and cross all their 't's?

TRST is sorta ptional. It's the IC designer's decision whether they implement it and user's decision whether to use it. You get to the same TAP state with just 5 (IIRC) clocks when TMS=1.

Is there some reason that you might _not_ want to connect TRST?

Some ICs don't have it. Also if you want less signals in a JTAG header/cable.

Perhaps there might be reasons _to_ connect it.

I also see that the Olimex board has a solderable bridge to connect TRST
to the processor reset. Again -- why, do you know the advantages and
disadvantages?

I'd guess a configuration for a certain cable/software which would use it for hard MCU reset. It won't be TRST anymore, just using the cable wire.

Some cables have separate MCU reset signal - perhaps this covers some that do not.
.

Relevant Pages

  • Re: Use of TRST on ST vs. Luminary ARM JTAG?
    ... LM3S811 the TRST pin is left open. ... I also see that the Olimex board has a solderable bridge to connect TRST ... The Keil Ulink interface supports JTAG and the ARM SW debug interface. ... Keil suggest connecting the reset pin which can be used to reset the processor from the debugger. ...
    (comp.arch.embedded)
  • Re: Use of TRST on ST vs. Luminary ARM JTAG?
    ... I notice that on the Olimex demo board for the STM32F103RBT6, ... the JTAG TRST pin connected, ... LM3S811 the TRST pin is left open. ... Processor reset is usually tied to high capacitive ...
    (comp.arch.embedded)