Unknown IO port electronic symbol

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#1
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Hi everyone,

I'm new to the Avr world and while I was reading about its datasheet I encountered I/O pin schematic attached below.

I understood how it works except for the electronic symbol I encircled in the figure .
Can anyone help please ? I would be very grateful

Thank you in advance.

 

Saad,

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Last Edited: Sat. Feb 8, 2020 - 05:02 AM
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A bidirectional transmission gate.

Ross McKenzie ValuSoft Melbourne Australia

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valusoft wrote:
A bidirectional transmission gate

 

Why did they use a bidirectional gate in that location?

It looks like it's for input only.

 

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I have no idea. Perhaps it was/is a left over from another part of the device that was pressed into service. In the distant past I have done things like that to avoid needing another gate package.

 

User01 said " I understood how it works ", so maybe s/he could explain.

Ross McKenzie ValuSoft Melbourne Australia

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It's interesting that someone asked this exact same question using this exact same picture, not long ago. Unfortunately I can't find the post, it's as if it was scrubbed from AVRFreaks (weird... normally I'm pretty good at websearchingangry).

 

Anyway, I did a search for the picture, leading me here:  http://easyelectronics.ru/avr-uchebnyj-kurs-ustrojstvo-i-rabota-portov-vvoda-vyvoda.html

This site seems to be a good resource, it's in Russian, but hey, google translate is your friendsmiley

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Found the following diagram in the ATmega328P datasheet.

Since it is an IC, using a bidirectional gate is not due to "using an extra gate in a package".

I'm obviously missing something, but I don't understand why they made that a bidirectional gate.

 

Edit: I suspect it has to do with minimizing power if the input voltage is close to half Vcc.

 

Last Edited: Sat. Feb 8, 2020 - 08:53 PM
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Now taking a closer look, it appears that the designer is using the very, very high impedance of the transmission gate during the sleep phase to minimise the current drawn on this input line and the fet to define the state of the schmit trigger device. Or something like that. cheeky

Ross McKenzie ValuSoft Melbourne Australia

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Agree.  Looks like the pin would be pulled low during sleep if the transmission gate were not there.

Letting the smoke out since 1978