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Author Topic: Inverted Output Enable  (Read 4005 times)

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Offline insurgentTopic starter

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Inverted Output Enable
« on: April 18, 2011, 03:12:52 am »
Hi All,

I've noticed that IC's with an "output enable" pin of some kind are often inverted (1 = disabled, 0 = enabled). I was curious as to whether there is some historical or technical reasoning behind this.

Take care,

John
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Offline Zad

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Re: Inverted Output Enable
« Reply #1 on: April 18, 2011, 03:20:17 pm »
It always puzzled me too, I guess it is because they are easy to drive with open-collector outputs.
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Offline Mechatrommer

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Re: Inverted Output Enable
« Reply #2 on: April 18, 2011, 09:59:16 pm »
imo the software IDE is made by the hardware guy, they are not good at encapsulating things, no offense! ;D
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Offline Psi

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Re: Inverted Output Enable
« Reply #3 on: April 19, 2011, 02:26:39 am »
it may have something to do with driving the IC's on a system bus, but i'm not sure how exactly.
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Offline logictom

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Re: Inverted Output Enable
« Reply #4 on: April 19, 2011, 12:28:41 pm »
We were taught that it's to do with/came from when a system isn't functioning or loses power, you can detect that a high has gone low when it's not working/loses power but if it's already low then it's undetectable. In general for active low, not specifically for CS/OE.
That's a pretty crap explanation but I'm sure you can get the jist :P
« Last Edit: April 19, 2011, 01:06:47 pm by logictom »
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Offline scrat

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Re: Inverted Output Enable
« Reply #5 on: April 19, 2011, 01:02:28 pm »
Quote from: logictom on April 19, 2011, 12:28:41 pm
We were taught that it's to do with/came from when a system isn't functioning or loses power, you can detect that a high has gone low when it's not working/loses power but if it's already low then it's undetectable. In general for active low, not specifically for CS/OE.
That's a pretty crap explanation but I'm sure you can get this jist :P

This can be also reversed if you think of safety issues, where you don't want a controlled IC to work if the controller isn't powered up...
It still is a strange thing
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Offline FreeThinker

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Re: Inverted Output Enable
« Reply #6 on: April 19, 2011, 01:04:22 pm »
I Think it's a 'fail to safety' type of thing. Unconnected gates will tend to float high and most fail conditions will produce a non zero voltage of some sort. Also driving a pin low is a more positive act than driving one high (noise etc).
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Online mikeselectricstuff

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Re: Inverted Output Enable
« Reply #7 on: April 19, 2011, 01:22:27 pm »
The convention of active-low selects and enables dates back to the days of RTL and TTL in the early 1970's or maybe a little earlier. These early logic families pulled low harder and faster than they pulled high, and the input structure would go high if unconnected.
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Offline insurgentTopic starter

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Re: Inverted Output Enable
« Reply #8 on: April 20, 2011, 11:56:45 pm »
Thanks guys,

The safety/failsafe & older electronic family reasons definitely makes sense.
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