Re: [ukvac] TECH: PCB's
Quoting shepg2002 <lists@ntlworld.com>:
> Why on many PCB's (e.g. Pacman) do some tracks lead to both
> inputs of an AND gate? You get the same thing out as what
> you put in! There must be a reason I'm sure.
It's usually one of two reasons. Sometimes it's buffering
for extra output drive, eg a gate's output can only drive a
specified number of gate inputs before it starts to approach
it's maximum output drive capacity. If you need to drive
more inputs than this, then you can use the gate as a buffer
for some of the inputs and drive the rest directly from the
unbuffered signal (or if the gate doing the buffering has
a higher output drive capability than the original output,
then it may be able to drive all of the inputs).
Second reason is timing. As there is always a propagation
delay between a signal going into the input of a gate, and
it appearing on the output, the circuit designer could use
this to their advantage whenever they need a small delay.
This can be very useful if you need to ensure a data bus
has had time to settle before it is latched, or the delay
can also be used in clock multiplier circuits.
There are other reasons, for example I've used a spare gate
to avoid adding a wire link when laying out a single sided
circuit board, but most cases it should be covered by the
two reasons above.
Cheers,
Dave.
DATA Imported from archives: originally posted by Dave Roberts (daverob@cwcom.net)
