[Xcircuit-dev] RE: 7400 series library

[Stephen Hughes]

Dear Timothy,

>The chip numbering system was written before I came up with the 
>"libinst" idea, and I was already aware of problems with the auto-numbering, 
>which fails to work correctly under a number of situations. The condition 
>which you related, in which you got "U1-1-" and "U2-2-", highlights one of 
>these problems. The auto-numbering routine doesn't understand that the two 
>parts are supposed to be the same chip. On the whole, though, there is no 
>way to know which instances are supposed to go with which chip.

I always find it amazing how clear the answer to my questions become only 
SECONDS AFTER I click the "Send" button on my e-mail client, and with no way 
to quickly stop the transmission.

It was only then that I remembered that I needed to manually edit the U? for 
each instance that I placed on a schematic (as I had done for years on other 
systems and software).

>P.S.---There's still the issue of Vdd/GND pin connections. Apparently some 
>schematic capture systems put Vdd and GND pins on each instance. At least 
>one of the instances per chip should have explicit connections to Vdd and 
>GND in the schematic. A PCB netlist requires these pins, to be correct. 

>If you can think of a better method for Vdd/GND connections, though, please 
>let me know.

I guess a quick and dirty way to solve the issue of chips containing more 
than one type of gate and the issue of Vdd/GND pins would be as follows:

Create the base object such as 7400-1 with no parameterized labels and 
include the Vdd/GND connections on this object.

Next, create another base object 7400-2 with all necessary parameters and 
exclude the Vdd/GND connections.  The remaining 2 gates in this series would 
be an instance of the 7400-2 object.

Of course there are a few drawbacks to this as well.  The first being the 
doubling in size of the library file.  The second is the need to manually 
edit the .lps file to provide the correct pin numbering which excludes the 
Vdd/GND pins.  Of course I have been manually editing the "libinst" entries 
in this manner anyhow.

>At present, connecting multiple instances to Vdd and GND will generate 
>redundant entries in the PCB netlist, but I can get rid of that behavior. 

Perhaps this is the better way?

Sincerely,

Stephen Hughes
Honeycomb Electronics, Audio and Research, Inc.

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