Pull-up & Pull-down
Pull resistors give an input a defined default state when no external device actively drives the line. They are common in buttons, open-drain buses and control signals.
CORE IDEA
Floating inputs can change unpredictably
ENGINEERING VIEW
Pull-up biases the line HIGH
WHAT TO CHECK
Pull-down biases the line LOW
REMEMBER
Internal pulls may be available in an MCU
Engineer’s checklist
- Floating inputs can change unpredictably
- Pull-up biases the line HIGH
- Pull-down biases the line LOW
- Internal pulls may be available in an MCU
KEY TAKEAWAY
Digital behavior is physical behavior. When firmware and expectations disagree, measure the signal instead of guessing.