GTA04 ... JTAG

Joerg Reisenweber joerg at
Wed Apr 30 07:59:18 CEST 2008

Andy Green schrieb:
> Somebody in the thread at some point said:
> | Joerg Reisenweber wrote:
> |> Please, before we start to mux UARTs again, and get real bad headache
> |> like on GTA01
> |
> | Yeah, I also don't particularly like this. We've been bitten before
> | by this sort of multiplexing. Granted, it's more contained in this
> | case than on GTA01.
> Yeah, muxing something like that needs an excuse rather than getting
> waved through.  However the two modes are very distinct, you are either
> blowing flash in MSP430 or you are seeing Linux console traffic, and the
> modality is controlled by and affects only the FTDI interface.
> |> JTAG is designed _exactly_ to be chained up like this, no risk at all
> in hw,
> |
> | That's what I thought as well - before I learned the truth of what
> | TI have done to JTAG :-( Here's the thread on openocd-development:
> And indeed it was I that proposed tying them together from the start by
> JTAG.  We all agree all things being equal we should have them both on
> one scanchain.  But they ain't equal.
> Like Joerg says we need an eval board and write the applets to blow
> flash, etc.
> -Andy

So if this is MSP430 spoiling JTAG-*chainup*, can't we mux JTAG instead
of switching from JTAG to muxed serial access? After all JTAG also are
no more than 4 lines, probably we get away with muxing less than all the
4 of them.
Or is MSP430-JTAG such a crap it even won't serve for an emergency
backdoor when it's got a TDI-TDO-chain on it's own?

Saves us from wasting standard IO-functions on 430 chip for debug
purposes and thus need for a 'bigger' more expensive chip, while JTAG
pins just floating useless :-(

Anyway for the mux conditions themselves:
6400 relies on 430 doing the power management, so 6400 should be hold in
reset-state anyway, while flashing new code to 430 (means: reset to 6400
also is muxing JTAG-line to 430).
On releasing this debug-reset to 6400, we probably should assert a reset
pulse to 430 as well (flashing of 430 done, restart it).
When accessing 6400 via JTAG, we have to rely on proper power setup for
the 6400 CPU and support things (RAM etc), so we are not supposed to
mess around with 430-JTAG or bootloader/serial-IF meanwhile (means: JTAG
to 6400 mutually exclusive with flashing 430 -> 6400-reset not asserted
excludes meaningful flashing actions to 430.)


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