PDP-8 on FPGA project
hachti at hachti.de
Sat Dec 20 22:43:55 CST 2008
>>> You can emulate that in std_logic with a single driver driving an "H"
>>> (the "pull-up"), and everyone else drives either "0" or "Z".
>> Yes. You can write that. But the synthesis tool - if it eats that -
>> will generate
>> many ORs... So avoid it. In the company I have worked a bit for, MANY
>> were simply forbidden. Thinks like Z, H, L in std_logic are suspicious
> What you say is true for FPGA-internal signals but the output drivers
> _can_ be switched to high-impedance.
> The open-collector driver you were asking about originally can be
> emulated by assigning "Z" directly to the IOB, switching it to high-z.
> That will only work if the signal you use is routed directly to the IOB
> => use only in top level entity to be sure.
Yes, of course... IOBs can also be controlled "manually" using the output enable...
The discussion was about the on-chip (!!) wiring of the OMNIBUS. the original OMNIBUS is an
open collector thing.
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