Some questions about conventions
werner at openmoko.org
Fri May 22 19:41:56 CEST 2009
?lvaro Lopes wrote:
> - Should power pins (GND/VCC) be placed at bottom/top, respectively,
> or can be arbitrary placed on the layout ?
If possible, I would put them at top and bottom. That way, they're
easier to spot and, in cases of unusual naming, it's easier to guess
> - Should the component name be placed inside the main rectangle,
> or can be placed outside ?
If the component is big, I would put component reference and part
name near the center.
If a rectangular component is small and/or its inside is crowded,
I would move the component reference to the top left and the part
name or specification on the bottom left.
For non-rectangular components (resistors, transistors, etc.), I'd
put both together somewhere safe or follow general conventions if
> - Shall pins be always the same length (0.3) or can be
> shortened/lengthened ?
300 mil seems to be a reasonable default, but it's not a hard rule.
In what cases would you think more or less would be better ?
> - Shall we depict logical "active-low" using both 'n' prefix and inverted symbol (circle) ?
Yes, except if the pin is multi-purpose (e.g., a GPIO) and some of
the functions are non-inverted. In this case, the pin would not
get the circle.
> - Output pins that might be used for power (like a mosfet switch)
> shall be made "power out" ? This might invalidate usage of same
> symbol for logic switching.
That's a tricky one :-) If it's really something that only make
sense for power, such as an LDO output or a switch designed for
high current, I'd make it "Power Out". If it may have non-power
uses, I'd just make it "Passive".
There's a broad grey area for this kind of decision, but we can
always fix things if we notice conflicts in the schematics.
> - Shall we draw right away the footprints, if not available ?
Footprints are also tricky. Let me address them in a separate
> My first attempt at SN47AUP1G125 follows in attach.
Looks good, thanks ! One idea: for these simple components, I
think it would help to make their function more obvious if
their inside logic could be drawn as well.
For example, something like the logic symbol in Figure 1 of
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