Schematic decisions

Purpose

This file describes design decisions around the schematic editor and its files.

Connection Data

Since the wiring tool requires knowlege of wires connected together in real time the in memory schematic must have information on what pin is connected to what.

There are 2 ways to handle this

• CPTR - Each wire/pin had a pointer to the object it is connected to
is not very close to the netlist format and therefore must be regenerated every time a file is loaded or a netlist exported.
• NLSTMEM - A separate list is maintained in memory
Very similar to a netlist file and would allow eash loading and changing. The data in memory is basically a binary image of the ASCII NETLIST file.

Comparison of Methods

Operation	CPTR	NLSTMEM
Loading Schematic file	Complicated	Easy
Loading Netlist file	Complicated	Easy - just readthe ASCII file and load into the binary data structure in memory
Saveing Netlist file	Complicated- must check every part for connections avoiding ovrlaps	Easy - just write an ASCII version of the binary netlist in memory
DRC Checking	Must check from every component, checking for overlapped paths as we go	Can perform most checking by one pass though the memory image
Wire Dragging	Each wire knows about its adjacent connections so knows which wires to drag and which wires to stretch.	Each wire can check the NLSTMEM for all its connections so can determin very quickly all connected wires and pins.
Netlist renaming	Must be handled in separate code	Kept in the central NLSTMEM list.

The NLSTMEM option sounds better in terms of handling files requred for the schematic editor. It provides:

• Easilly translatable to/from NETLIST file
• Central location of all connection data
• More efficient for DRC checking

Currently the CPTR option is implemented in the wiring tool and connection tool for testing but it is apparent that switiching to the NLSTMEM option is very adventagous