Resistors
The most common part on any board. The value usually tells you the job — experienced designers read "10 k" as "pull-up" almost automatically.
Common use cases (read the value)
| Value | Typical job | Notes |
|---|---|---|
| 10 kΩ | Pull-up / pull-down on GPIO, reset, boot/strap, enable | The default pull value |
| 4.7 kΩ / 2.2 kΩ | I²C bus pull-ups (SDA/SCL) | Lower value for faster/longer buses |
| 1 kΩ | LED current (with 3.3/5 V), general pull, base resistor | |
| 330 Ω – 1 kΩ | LED series resistor | Set per LED current & supply |
| 22 Ω – 33 Ω | Series termination on USB D+/D−, fast digital lines | Tames reflections/ringing |
| 0 Ω | Jumper / link — option, net tie, layout fix | Cheap configurability |
| m Ω (shunt) | Current sense | Use Kelvin (4-wire) connection |
| 100 kΩ – 1 MΩ | High-impedance pull, bleeder, dividers | Watch leakage/noise |
Pull-ups & pull-downs
- A pull-up ties a line to V+ so it has a defined level when nothing drives it; a pull-down ties to GND.
- Needed on: floating inputs, open-drain outputs (I²C), reset lines, boot/strap pins, enable pins, buttons.
- Value trade-off: lower R = stronger pull, faster edges, more current; higher R = lower power but slower and noise-prone. 10 k is the everyday compromise; I²C wants ~2–4.7 k.
Voltage dividers
- Two resistors set a fraction:
Vout = Vin × R2/(R1+R2). - Used for ADC scaling, feedback, ID pins. Keep impedance low enough for the ADC's input.
Ratings to check
- Power:
P = I²R(orV²/R). A 0603 handles ~0.1 W — size up for current-sense / bleed. - Tolerance: 1 % (E96) is standard and cheap; use tighter only where it matters (dividers, feedback, sense).
See also
- Common Practices — where pulls are required
- MCU & Digital Circuits