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Old 05-15-2006, 04:18 PM
Marco
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Default pull-ups and jtag questions

Hi,
I have to establish a 50MHz serial communication with a Spartan3, so:
1) should I place termination resistors on data_in, data_out or clock
pins? Value?
2) could I use somehow DCIs (I'm working with LVCMOS33 standard)?
then on the pull-up/down:
3) which JTAG pins should I pull-up? With 10KOhms?
4) I'll need to use serial mode (slave or master) also on the
configuration stage, so I could harware fix the mode pins as I'll
always be able to connect with the JTAG, no matter how the pins mode
are set, right?
Thanks,
Marco

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Old 05-15-2006, 04:27 PM
Antti
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Default Re: pull-ups and jtag questions

3) usually doesnt matter at all
4) yes and no. JTAG config overrides the mode settings, but there are
cases where JTAG configuration fails unless tricks are made (changing
mode as example). Basically there is some 'critical time slot' in the
JTAG config sequence, if in that time the FPGA sees a valid SYNC on
serial config then the JTAG config gets confused and weird things
happen (can happen).

Antti

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Old 05-15-2006, 04:36 PM
Ad
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Default Re: pull-ups and jtag questions

Marco

The need for termination resistor depends upon alot of things, though
it is not that complicated. you will need to terminate the signal if
the rise time divided by the propagation speed (rise time /
propagation speed) is greater than one sixth of the pcb track length
(different people may use different ratios but i will stick with what i
learnt and know to work). the termination resistor value will depend
upon the track imedance, when laying out the pcb try and ensure the
tracks all have same impedance regardless of the layer they are on
(this involves varying the trace widths on different layers during
layout). You then have a choice of termination schemes A.C termination
is good and can save power, while series (source) termination is also
useful.
There are packagesavailable that will determine if you need termiantion
for your traces hyperlinx is one of the best.
If you are ussure you could always add the pads and not fit the
resistors, it easier that way

hope this helps

Ad

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Old 05-15-2006, 05:02 PM
Marco
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Default Re: pull-ups and jtag questions

Antti,
3) why it doesn't matter? I saw several schemes with pull-ups,
sometimes with different configurations (someone placed on TCK and TMS,
others on TDI... that's why I ask)
4) for the prototype I should go with a 3-element-dip-switch toward GND
to be safe?

Ad,
I think I'll place on the 3 pins a 0 Ohm resistance and I'll replace it
with a 22 Ohm or other values if I'll see I need it. Do you agree?
Would you place terminations only on 1 or 2 of these 3 pins?

Thanks,
Marco

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Old 05-15-2006, 06:04 PM
Ad
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Default Re: pull-ups and jtag questions


Marco wrote:
> Antti,
> 3) why it doesn't matter? I saw several schemes with pull-ups,
> sometimes with different configurations (someone placed on TCK and TMS,
> others on TDI... that's why I ask)
> 4) for the prototype I should go with a 3-element-dip-switch toward GND
> to be safe?
>
> Ad,
> I think I'll place on the 3 pins a 0 Ohm resistance and I'll replace it
> with a 22 Ohm or other values if I'll see I need it. Do you agree?
> Would you place terminations only on 1 or 2 of these 3 pins?
>
> Thanks,
> Marco


Marco

placing the 0 ohm links is a good idea make sure you place them
correctly for source termination these resistors should be placed
inline with the signal and near the output driver, for ac termination
the resistors need to be placed near the destination of the signal
to ensure you get the correct resistor / schematic i would suggest a
quick google search on termination schemes.

regards

Ad

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