Start-up time is the interval from power applied (or enable released) to stable oscillation. In a Pierce oscillator, noise at the resonant frequency is amplified until nonlinearity limits gain to unity. The build-up rate depends on loop gain (transconductance vs. crystal loss), ESR, load capacitance, Q, and environmental factors.

For low‑frequency tuning‑fork crystals (e.g., 32.768 kHz), start‑up typically spans 1–5 s, while crystals in the MHz range usually start in a few milliseconds. For details, see vendor app notes linked below.

What is the start-up time of a crystal oscillator?

Short answer: it’s the time from power-on (or enable) until the output reaches a stable, in‑spec amplitude and frequency. Typical values:

• 32.768 kHz tuning‑fork: ~1–5 s depending on ESR, CL, and temperature.
• AT‑cut 8–40 MHz: usually a few milliseconds.
• TCXO modules: enable‑to‑stable‑out often 1–10 ms (check datasheet).
• OCXO: oscillation begins quickly, but oven warm‑up to accuracy spec takes seconds.

How to think about start-up time

Amplitude grows approximately exponentially: V(t) \u2248 V\u2092 e^{t/\u03c4} until limited by the amplifier. A useful heuristic is Tstart \u2248 (2Q/\u03c90) \u22c5 ln(Vtarget/Vnoise). In practice, designers ensure sufficient gain margin (negative resistance or G_m headroom) and correct CL to guarantee timely start.

Common causes of slow start-up

• Insufficient loop gain / gain margin: oscillator G_m too low vs. crystal ESR → marginal negative resistance.
• Incorrect CL (load capacitors): too large reduces effective loop gain; too small shifts frequency and stresses stability.
• High ESR crystals (very small packages, low-drive parts) or excessive series resistance / damping.
• Poor PCB layout: long traces, stray capacitance, ground return not tight around the crystal loop.
• V_DD ramp & enable sequencing: slow rise or gating clocks before the loop is ready; PLL lock adds extra delay.
• Temperature & humidity: cold starts and contamination increase ESR / leakage → slower start.

Fixes that actually work (checklist)

1. Verify gain margin: for MCU Pierce oscillators, target gain margin ≥ 5 (or adequate negative‑resistance safety factor) with your chosen crystal (ESR, C₀, C_L).
2. Right C_L network: match the crystal’s specified load capacitance; include stray capacitances; keep CL1=CL2 unless the datasheet says otherwise.
3. Prefer lower‑ESR crystals: within the MCU/ASIC’s drive limits; avoid extra series resistance unless required for drive control.
4. Tight layout: crystal and caps close to pins; symmetric loop; short guard ground; route XIN/XOUT away from fast digital lines.
5. Use vendor start‑up aids when available: e.g., on‑chip Oscillator Start‑up Timer (OST), drive‑strength settings, or firmware kick‑start procedures.
6. Consider higher‑integration timing when start‑up time is critical: TCXO (temperature‑compensated) or OCXO (oven‑controlled; note warm‑up) modules with optimized drive and control.

Typical start‑up ranges (illustrative)

Need very fast lock on wake? Consider clock gating from a running reference, or a small MHz crystal/TCXO for the main domain and keep 32 kHz for RTC only.

How to measure and verify start‑up

• Use the SoC’s “clock ready / OST” signals (if provided) and log TOSCD + TOST.
• Observe the amplitude envelope at XIN/XOUT with a high‑impedance active probe; avoid loading the loop.
• Firmware kick‑start (if supported): briefly toggle gain/drive modes to inject broadband noise and reduce start‑up time.
• Corner testing: hot/cold, V_DD min/max, multiple crystal lots and PCB variants; watch for marginal designs that occasionally fail to start.

Explore FCom models for faster, reliable start‑up

FVT‑7S‑WT — ±0.1 ppm TCXO (–40~+105℃)

7.0×5.0 mm TCXO designed for wide temperature. Optimized drive and fast enable start‑up for comms & GNSS.

FVT‑9S‑LN — Ultra Low‑Noise TCXO

Sub‑ps jitter TCXO for radios and test gear. Low ESR crystal and tuned loop for dependable start‑up.

FOC‑4D — ±5 ppb SC‑cut OCXO

36.3×27.2 mm OCXO; oven warm‑up seconds, but rock‑solid holdover for base stations & timing servers.

FCO‑3K — 32.768 kHz CMOS Oscillator

Drop‑in CMOS output module for RTC; predictable start behavior and tri‑state enable.

FAQ: start‑up time in crystal oscillators

Why does a 32.768 kHz crystal start much slower than an 8–40 MHz crystal?

Lower frequency and higher Q increase the time constant; tuning‑fork ESR and required CL also reduce effective loop gain. MHz crystals typically build amplitude much faster.

Will a TCXO start faster than a bare crystal?

Often yes. Many TCXOs integrate an optimized oscillator and control loop that starts quickly and delivers a CMOS/Clipped‑Sine output. Always check the device enable‑to‑output spec.

My design sometimes fails to start at cold temperature—what should I adjust?

Increase gain margin (choose lower ESR crystal or adjust drive/gain per datasheet), re‑calculate CL including strays, and review layout. Validate again at V_DDmin and cold soak.

Does an OCXO have the longest “start‑up”?

Oscillation itself can begin quickly, but the oven warm‑up and control loop stabilization add seconds before frequency accuracy is within spec. Check warm‑up time in the OCXO datasheet.

How do I measure start‑up time without loading the crystal?

Prefer on‑chip status (clock‑ready/OST) or buffered reference nodes. If probing, use a very high‑impedance active probe and watch the envelope rather than forcing the loop with large capacitance.

Is there a rule of thumb for negative resistance / gain margin?

Many vendors recommend a safety factor around 5× under worst‑case conditions. Always use the specific methodology and limits in your MCU/ASIC datasheet.

Further reading

• STMicroelectronics — AN2867: Guidelines for oscillator design
• Texas Instruments — SLAA322: MSP430 32‑kHz Crystal Oscillators
• Microchip — Oscillator Start‑up Timer (OST) overview

Related reading on our site: What Are Timing Devices? · TCXO vs OCXO · Crystal vs MEMS Oscillator

Contact FCom

Need help selecting a crystal/TCXO/OCXO with predictable start behavior? Talk to our engineers with your target frequency, supply, C_L, ESR, temperature, and start‑up requirement.