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

[Michael S. <rin...@di...>]

On 12/07/2010 10:39 AM, Laurent Gauch wrote:
> Will be never good. A JTAG frequency is not only board specific, but
> wire (cable length) dependent, buffer dependend, dongle dependent ....
> Too many parameters !!!
Chip, dongle, board configs could supply max values (and OpenOCD would
have to select the lowest of them) - however, the user will have to
select the frequency he wants to use, the config values can only limit
the user's choice in case he sets a frequency that is too hight for some
component.

>> 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 ;-)
I am not sure 10kHz is good. On a core where the max. JTAG frequency is
cpu_clk / 6, with a 32.768kHz crystal, 10kHz is already too high.


>> /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
Probably not. On cores where the JTAG frequency is limited to some
fraction of the core frequency due to internal synchronization, you may
get a broad area where probing works some of the times, so you would
choose a frequency that fails from time to time.

cu
Michael