MCP23008 IO Expander - Demultiplexer

http://www.blogger.com/blogger.g?blogID=1840459474737789074#editor/target=post;postID=2871360714213861663

The first time I used HC573, I found it LE Latch Enable not compatible to that of HC137.  For HC573, LE High means input is transparent to output.  Only when LE Low then the input is latched.

So I need to use an inverter to invert HC137's low active select signal first.  Instead of using a simple hex inverter HC04, I take the opportunity to use a HC02, Quad input NOR gate which give one input to act as an HC573 buffer enable/diesable option.

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