What is the meaning of a symbol consisting of a diamond with four boundary triangles in a schematic?
Re. the following from the Atmega328 datasheet:
Answer
That is a transmission gate: https://en.wikipedia.org/wiki/Transmission_gate.
It is constructed out of a P-channel and a N-channel MOSFET so when ON it is bi-directional. It is often used as a analog switch but that is not the case here.
It is bi-directional when 'SLEEP' is low (0, or OFF), and high-impedance when 'SLEEP' is high (1, or ON). Also the input to the Schmidt trigger is grounded when in sleep mode, so it is not 'floating'.
It is an analog switch but only expecting a logic 1 or 0 as an input to the Schmidt trigger, which cleans up noisy digital inputs.
It is followed by a synchronizing stage so that the data accepted is in step with the local clock for that port.
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