Big Boy
Joined: 20 Jan 2004
Posts: 253
Helped: 10
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 22 Oct 2004 17:32 Re: Device Select or Device Select bar(NOT)? |
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Active-low mainly come from TTL days.
It is easy to use pull-ups resistors, and TTL, when having floating input, also saw a logic 1. So, when having, for example, a 3-state output, or an input going to a connector, it is possible that, when no output is driving a line, or when a cable is removed, that the chip input is left floating (ou pulled-up).
In this case, you would want control signals to not be asserted. So, this is why the standard is active-low (or inactive high). Because for TTL chip, it was easy to leave the input floating, which did let the input see a logic 1. Note that floating inputs only apply to TTL. CMOS inputs must not be left floating.
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