On Mon, Oct 19, 2009 at 09:17:30AM -0400, Dave wrote:
> Are the direction arrows actually used by the Net command? IE => <=
> <=> etc?
>
> I thought that I had read that they do not matter but are simply for
> human clarification..
That's correct. You can remove the arrows from a net or linkXX command
and it has exactly the same effect.
> I also don't understand the format where a signal name has to be placed
> after the Net command even though it seems to be irrelevant oftentimes.
The net command needs a signal name plus at least one pin name.
If you're familiar with linkxx, then you should understand that
net s p q r
is equivalent to
newsig s hal-type-of-pin-p
linksp s p
linksp s q
linksp s r
> But if you have two read/write pins how do you know which way the
> read/write is going?
By enforcing the rule for the number of writers.
Each signal may be linked to any number of readers. (Hal pins with
direction "IN")
A signal may be linked to 0 or 1 writers (hal pins with direction "OUT").
If there is no writer, then a signal may be linked to any number of
bidirectional pins (hal pins with direction "I/O").
Because of the second rule (the at-most-one-writer rule), you know the
direction the information is flowing: from the single writer to all of
the readers. (the writer is free to read the value from the pin as well,
but there's little point to it)
The third rule is for a few special-purpose pins; the only one of note
is index-enable. It allows a "one line handshake". Suppose that
component C wants to request an action from component D, and be notified
when the action is complete. You can do this by having an I/O pin in
both C and D, and linking them together on a signal.
When C wants to request the action, it sets the pin value from 0 to 1.
When D sees the pin value change from 0 to 1, it begins the action.
When D completes the action, it sets the pin value from 1 to 0.
When C sees the pin value change from 1 to 0, it knows the action is
complete.
Jeff
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