Re: [SI-LIST] : Advice for long bus?

Stephen Peters (sjpeters@ichips.intel.com)
Wed, 28 Apr 1999 14:56:05 -0700

Hello Chris:

One major criteria for selecting the driving technology is the desired
style of signal switching. You can operate your bus as either incident wave
or reflected wave switched, and this choice in turn depends on the required
bus clock speed. For a 33Mhz bus in a standard card cage you can probably get
away with reflected wave switching. This implies relatively low drive
LVTTL devices, with no termination required. At 66Mhz and/or
long bus lengths, your timing budget may not allow for the two way
propagation delay, forcing you to rely on incident wave switching. This
implies a terminated bus, with stronger drive requirements and tight(er)
receiver thresholds -- i.e. GTL or BTL bus drivers.

For more information on reflected vs. incident wave switched busses I'd
suggest checking out the PCI bus electrical specs. Also, this is only one
criteria.
There are others, such as reliability, EMI considerations, etc. that I'm
sure others can speak to.

Regards,
Stephen Peters
Intel Corp.

> We are designing a board that has a micro with a 32 bit address/data bus
> which will ultimately go to 14 loads (memory, flash, various other devices).
> The bus speed hasn't been decided yet, but it will be either 33 or 66MHz.
> Most everything is 3V logic.
>
> Question: Would it be advantageous to use an alternative signaling scheme
> (such as GTL) instead of the LVTTL? There will probably be buffer/drivers
> anyhow since the micro won't be able to drive 14 loads.
>
> Does anybody have any other suggestions/advice? I realize you may need more
> info for this, but I want to keep this brief/high-level.
>
> Thank you,
>
> Chris
>
>
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