JTAG: First of 4 Spartan-3E always UNKNOWN

Andrew Greensted <ajg112@ohm.york.ac.uk> wrote:

>Andrew Greensted wrote:
>> Looks like I've got a similar problem. After removing the oscillator 
>> module the JTAG identification works fine.
>
>I've dropped in a lower frequency oscillator module, that seems to have 
>improved things. It's not a massive problem, I'll just use a DCM to 
>multiply things up.
>
>I'm guessing the oscillator was causing noise on VCCO. However, isn't 
>the JTAG TAP and IO powered by VCCAUX?

SOmehow JTAG is extremely sensitive to electrical parameters. IIRC the
method used to generate the waveforms by Impact is not the best around
(data and clk are changed simultaneously). I've also had these sort of
problems on boards with multiple FPGAs. Sometimes adding a small RC to
the clock (TCK) line may help. You can use an oscilloscope to very
setup & hold timing on the JTAG bus.

-- 
Programmeren in Almere?
E-mail naar nico@nctdevpuntnl (punt=.)

>removing the 100mhz oscillator :)

Didn't that defeat the purpose of the pcb ..? :)

>that PCB proto was SO BAD that no powersupply bypassing helped.. i
>tried and looked with the scope

So a new pcb was made..?, what was explicitly changed..?

>BTW it was weird, i was also using lab power supply, so while sweeping
>the VCC i could see the JTAG ID to change from virtex to spartan and
>then failing completly

sweeping Vcc?, iee overlaying a variable amplitude and frequency signal onto
Vcc?

By the way.. is switch regualator ripple a big issue for spartan fpga?

Nico Coesel wrote:
> SOmehow JTAG is extremely sensitive to electrical parameters. IIRC the
> method used to generate the waveforms by Impact is not the best around
> (data and clk are changed simultaneously). I've also had these sort of
> problems on boards with multiple FPGAs. Sometimes adding a small RC to
> the clock (TCK) line may help. You can use an oscilloscope to very
> setup & hold timing on the JTAG bus.

That's interesting. I did wonder how the JTAG bus is driven. I've had a 
small project running in the background for a while to make my own usb 
JTAG module. Perhaps it's time to finish it off.

Andy