Passives
Crystal
A precisely cut sliver of quartz that vibrates at one exact frequency. Quartz bends when a voltage is applied and makes a voltage when it bends, so with a small amplifier it rings like a tuning fork and gives a chip an accurate clock.
Think of it as: A tuning fork for electronics.
Pins
- Either end
- : No polarity. Some crystal cans have a third pin for the case, which goes to ground.
Metal cans printed with the frequency (16.000 = 16 MHz), small cylinders for 32.768 kHz watch crystals, and tiny SMD packages.
What to look for inside
- The quartz slab flexes back and forth between its two metal electrodes.
- Each flex makes a small voltage on the electrodes (the trace). The chip amplifies it and feeds it back, which keeps the crystal ringing.
- Its size sets the frequency so exactly that a cheap watch crystal drifts less than a second a day.
The numbers that matter
| Spec | Typical |
|---|---|
Frequency 32.768 kHz = 2¹⁵ ticks per second, ideal for clocks. | 32.768 kHz · 8 MHz · 16 MHz |
Tolerance / stability 20 ppm is about 1.7 s per day. | ±10 to ±50 ppm |
Load capacitance C_L The two capacitors next to it must match this, or it runs slightly off frequency. | 6 pF to 20 pF |
Where you’ll use it
- Clocks for microcontrollers, USB and radios, which need accurate timing.
- Real-time clocks (32.768 kHz).
Common mistakes
- Wrong load capacitors. The crystal starts slowly, drifts or doesn’t start at all.
- Long PCB traces to the crystal. Keep it right next to the chip.
Lessons that use it
A crystal sets the clock that times serial protocols like the one in lesson 4.3.
Go deeper
Choosing the two load capacitors (C_stray ≈ 3 to 5 pF).
Turning a ppm rating into seconds per day.