On Sun, 04 Feb 2001 15:05:00 -0800, Abd ul-Rahman Lomax wrote:
... snip ...
>I'll agree. Multiple power *output* pins is a relatively unusual
>circumstance and it is easy enough to pop down No-ERCs. The fact that power
>nets are not checked for a power source is a routine problem. I've seen
>plenty of schematics without one.
>
I actually see multiple output pins that need to be tied together quite
frequently, especially in switching regulator circuits. Sometimes they have
multiple output drive pins for lower output impedance in a small package. In
fact, my current design has one such part that has a drive output tied to a
synchronous rectifier output pin. The choice is to call the rectifier pin an
input (which technically it *sort of* is), or to call it an output and live
with a ERC error. I don't mind a couple errors like this personally.
>However, my modification of the idea is simply to change power-input pins
>to inputs. Power outputs would be left as power * or changed to outputs. If
>there is any power pin or output pin in a net, the net will be recognised
>as correct.
>
>It seems to me that the difference between power and output is that
>multiple power pins may coexist in the same net. The other difference is
>that the net may be expressed with a power port. If any power port is used,
>or any power pin our output pin is in the net, (or certain other
>combinations as determined by the error matrix), it is considered driven,
>and inputs will not report an error.
I've been defining power output pins as outputs and power input pins as inputs
since my Orcad SDT days in the early 80's. I am not a fan of the power pin
type, especially hidden pins. I prefer to show all pins of all devices (even NC
pins) on all components. It seems to eliminate a lot of questions later in the
project from technicians and other engineers who might be reviewing the
documentation. Redefining the power pins in this manner has caused me the least
amount of grief and still allows me to make sure I have a driving source for
all my power inputs on the design. It also allows for "non-power" pins (like
the amplifier output you cited) to drive the power input pins of other devices
and still generate a clean ERC.
Matt Pobursky
Maximum Performance Systems
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