Re: [Openocd-development] Getting rid of default jtag clock rate

[Laurent G. <lau...@am...>]

>
> 1MHz is just as bad as as default as 30MHz. It may work in more cases,
> but it is still broken in some.
>   
As I said before the 1Mhz startup frequency could be 10kHz. The 
important is not the value of the startup frequency, the important is to 
know we have a startup frequency !!!
> IMHO, there are two possibilities:
>  - we find a method to auto-configure a default that works in *every* case
>   
That's the best to do at the end.
>  - or we require a configuration value that works (that may be chip- or
> board-specific).
>   
Will be never good. A JTAG frequency is not only board specific, but 
wire (cable length) dependent, buffer dependend, dongle dependent ....
Too many parameters !!!
> Having a default that works most of the time, but causes strange effects
> in some cases, is really bad useability.
>   
The question is what is 'most of time' 80% 90% 99%
Fixing at a default 10KHz frequency will help user to auto-detect their 
JTAG chain ... and this is REALLY ever a GOOD thing to know the JTAG 
chain is auto-detected -> my JTAG Chain is working, the JTAG transport 
layer is working, user are already happy ;-)
Next, is to find the max. safe frequency :-)
>
> >/ THE SAFE CORRECT ROBUST frequency can never be known as static ! A
> />/ good JTAG software has to find this frequency by itself
> />/ (auto-detecting the max safe JTAG frequency );-)
> /Then how do you propose to implement such an auto-detection?
>   
Work on 10KHz startup frequency.
READ the TAPID_old (the first TAP ID is enough)
READ the TAPID_new (the first TAP ID is enough)
  WHILE TAPID_old  = TAPID_new old
     up/scale the frequency
    TAPID_old <= TAPID_new;
    READ the TAPID_new (the first TAP ID is enough)
  DO
max safe frequency is TAPID_old

> >/ The default frequency could be 10KHz instead 1MHZ ...
> /Until some new really-low-power chip comes along that starts at an even
> lower internal clock - this does not sound robust.
>   
I am almost sure at 10KHz it will be 99% robust -> not enough !

Laurent
Amontec JTAGkey-2 Hi-SPEED USB JTAG CABLE
http://www.amontec.com