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Physical / electrical concepts

Push-pull

An output type that can actively drive both high and low. SPI lines are push-pull.

How it looks

Drawing
A PUSH-PULL OUTPUT3.3 Vground 0 VP switchN switchoutput
Two switches inside the chip: one to the supply, one to ground.

How it works

A PUSH-PULL OUTPUT3.3 Vground 0 Vtop onbottom offoutputHIGH, driventop offbottom onoutputLOW, driventop offbottom offoutputneither: Hi-Z

Where it fits

The two switches are a CMOS pair. SPI uses push-pull, which is why two outputs fighting is a collision. Compare open-drain.

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