Ah! I see.
I guess it's fine as long as OpenOCD either tristates or drive high the
reset lines when it's not doing anything.
Is that the case?
Thanks!
Matthew
On 7/8/2011 9:51 AM, Phil Fong wrote:
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**
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?
Since you are embedding the JTAG interface with the target, you can
tell if buffering is needed. We just went through a similar exercise
and you can pretty much hook everything up directly and use the FTDI
MiniModule layout (see commit from Rodrigo Rosa) if you target circuit
is right.
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 is optional since you can reset the JTAG TAP state machine with
clocking TCK which TMS=1.
SRST might be needed for other reasons. For example some chips need
it as part of some kind of flash / code protection enable/disable.
Phil
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