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Physical / electrical concepts
Bus contention / collision
When two push-pull outputs try to drive the same wire in opposite directions. Shorts VCC to GND. The reason SPI can't be multi-drop.
How it looks
Drawing
One chip pushes high while the other pulls low: a short through both chips.
How it works
- Chip A drives the wire HIGH; chip B drives it LOW at the same moment.
- Current rushes from A's supply through the wire into B's ground: a short circuit through both outputs. The level is undefined and the chips heat up.
- That is why SPI gives each chip its own select line, and why shared lines use open-drain or Hi-Z.
Where it fits