[SOLVED] Bus wiring question, fan or no fan, that is the question?

[jgk2004]

Hello all,

I am questioning if I should fan or not fan a signal bus which is relatively slow and has to travel a somewhat long distance on chip. See attached. If I mux then fan out, I feel that number of wires is to large. If I just wire all bus lines to each cell then locally select, it should be a better choice. Or is there something wrong with my thinking? What would you do? Is wiring to all and selecting a bad idea?

JGK

 

[erikl]

What would you do? Is wiring to all and selecting a bad idea?

Not at all, at least for a fast bus: gives you better symmetry, on cost of more buffering.
But for a slow bus the 1^st method saves a lot of routing and buffers.

 

[jgk2004]

Hello Erikl,

Let me get this straight. For a fast bus, the second option is better due to better symmetry but at the cost of buffering. While the first option would saving buffering at the cost of routing and be better for slower busing??

Is this correct?

JGK

 

[erikl]

Let me get this straight. For a fast bus, the second option is better due to better symmetry but at the cost of buffering.

Yes, I think so.

While the first option would saving buffering at the cost of routing and be better for slower busing??

It saves buffering, but not at the cost of routing: it needs less routing, I think, because you need to route only the selected signals - if I understand your 1^st method correctly. Or do you always need the full bus behind the MUX (as shown)? If so, it doesn't save routing, of course, just saves buffering - at cost of the MUX. And the timing symmetry still could be sustained - so would be appropriate for a fast bus, too.