I see. Thanks for the clarifications! I understand the purposes of the
buffers much better now.
Matthew
On 7/8/2011 12:50 AM, Laurent Gauch wrote:
Hello!
I'm trying to embed a FT2232D based programmer into my board with a
STM32 (Cortex-M3 MCU).
I want the programmer to be compatible with jtagkey, so I looked at
schematics of compaible designs.
I noticed that while the JTAG signals (TCK, TDI, TDO, TMS) are only
buffered when translation is necessary, the SRST and TRST signals are
always tri-state buffered, with OE going into the FTDI chip.
Is there a reason for that? Can I omit the buffers?
On the JTAGkey http://www.amontec.com/jtagkey.shtml , the output
buffers have 3 basic functions:
- remove any pull-down or pull-up from the internal of JTAGkey (The
pull-up and pull-down value must be controlled by the target board and
not by the emultator)
- pass from 8mA output to 32mA output driver (JTAGkey and JTAGkey-2
are tested to support more than 12 daisy-chain devices on the same
JTAG Chain)
- possibility to tristate the JTAG port at any time
- provide an ultra large IO voltage tolerance from 1.5V to 5V ;-)
The Amontec JTAGkey was designed to be the most generic USB JTAG
adapter as possible. Also, the Amontec JTAGkey has the advantage to
provide a full control of the TRST and SRST line. In fact, the TRST
and SRST of the Amontec JTAGkey can be configured as push-pull buffer
and as open-drain buffer.
Note, SRST can be monitored from the JTAGkey Layout.
Also, I heard it's possible to omit the TRST signal and only keep the
SRST signal, because system reset will also reset the TAP controller.
Is that true? Are there problems with that?
TRST and SRST are optional, and their usage will depend on what is the
target need.
So you can try to customize the Amontec JTAGkey Layout for your need,
or you can use it as it is.
The advantage of using the original Amontec JTAGkey Layout is you make
to sure have a full compatibility with software supporting JTAGkey !
Regards,
Laurent http://www.amontec.com/jtagkey.shtml
Amontec JTAGkey-2 High-speed USB JTAG Adapter with RTCK feature.
Many thanks!
Matthew
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