Re: [Openocd-development] adapter_khz ...

[David B. <da...@pa...>]

On Wednesday 24 March 2010, Laurent Gauch wrote:
> Electrically, the JTAG serial port as a SPI serial port will ever have a 
> faster frequency capability than SWD,

Yet ... the same wire is used for JTAG/TCK -or- SWD/SWCLK.  And the
same pin on the target/chip uses that as an input, and gates that clock
against the same CPU clock.  So "Electrically" there's no reason to think
they'd be very different.


> since the JTAG has all signals as  
> unidirectional, but the SWD play with bidirectional signal (push-pull 
> mode),

And there's an explicit turnaround delay as part of the SWD protocol,
after switching directions on the SWDIO signal.


> the I2C play with bidirectional signal too (open-drain mode) ... 

 I2C has rise times as part of the protocol, and uses both clock stretching
and arbitration to slow things down.  Maybe the high speed 3.4 MBit/sec mode
works a bit differently, but that's hardly common.  (Even "Fm+", at 1 Mbit/sec
isn't as widespread as classic 100 KHz or 400 KHz style...


> with adapter_khz only, we will not be able to specify a JTAG frequency 
> faster than SWD frequency, and what about we will have I2C protocol ...

See above for reasons that clocking JTAG very differently from than SWD seems
somewhat foolish at a basic electrical level.

If the adapter can't obey I2C clock stretching and arbitration rules,
it shouldn't claim to support I2C.  (Not that we have such a debug
transport defined yet... I expect SWD will come first!)