32.768 kHz Quartz does not oscillate

Why is my 32,768 kHz crystal not resonating?

Root cause analysis and solutions for the most common problem in embedded development

The problem that every developer knows

The circuit is fully assembled, the microcontroller boots - but the real-time clock does not run. The 32,768 kHz crystal does not oscillate. Or even worse: sometimes it oscillates and sometimes not. Or it oscillates, but then stops sporadically.

This problem is one of the most frequent and at the same time most frustrating error patterns in embedded development. The 32,768 kHz clock crystal is an electrically sensitive component that works in conjunction with a weak oscillator circuit - and this interaction can be disrupted by numerous factors.

This article systematically analyses the most common causes of oscillation problems with 32,768 kHz quartz crystals and provides specific practical solutions.

1. the ESR of the quartz is too high for the oscillator circuit

Frequency: Very high - the No. 1 cause

The ESR (Equivalent Series Resistance) is the effective series resistance of the crystal at the resonant frequency. It is the most important - and most frequently underestimated - parameter when selecting a 32.768 kHz crystal.

The oscillator circuit in the microcontroller must generate enough energy to make the quartz oscillate. The value of the negative resistance (|-R|) of the oscillator circuit must be significantly greater than the ESR of the crystal. This ratio is known as the oscillation margin:

Oscillation Margin = |-R| / ESR

This factor should be at least 5, preferably 10 or higher. If it is below 3, the oscillation is unsafe. In the automotive sector, an SF >=10 is generally required.

Why is this particularly critical at 32,768 kHz?

In contrast to MHz crystals (typical ESR: 20-60 Ω), 32.768 kHz crystals have an ESR in the kiloohm range:

Housing size

Type. ESR (max.)

Rating

3.2 x 1.5 mm / 2-pad

70 kΩ

Uncritical for most MCUs

2.0 x 1.2 mm / 2-pad

80 kΩ

Limiting for weak drivers

1.6 x 1.0 mm / 2-pad

90 kΩ

Critical - only for MCUs with strong drivers

1.2 x 1.0 mm / 2-pad

100 kΩ

Very critical - carefully check the swing-back safety

At the same time, the 32,768 kHz oscillator stages in modern MCUs are deliberately optimised for minimum power consumption. The typical negative resistance in many low-power MCUs is only 200-500 kΩ.

Solution:

Use a crystal with the lowest possible ESR. Prefer the 3.2 x 1.5 mm housing with max. 50 kΩ. LRT resonant crystals (Low ESR Resonator Technology) offer significantly lower ESR values than standard crystals, even in smaller housings.

2. incorrect load capacity (load capacity mismatch)

Frequency: Very high

Each 32,768 kHz crystal is specified for a certain load capacitance (CL) - typically 4 pF, 6 pF, 7 pF, 9 pF, 12.5 pF or 18 pF. Mismatching is one of the most common causes of transient response problems.

The load capacitance is the total capacitance that the crystal "sees" at its terminals:

CL = (C1 × C2) / (C1 + C2) + Cstray

Where C1, C2 are the external load capacitors (if present) and Cstray is the parasitic capacitance (PCB leads, IC pins, typically 1-5 pF).

  • Load capacitance too low: The crystal does not receive sufficient energy feedback → oscillation can fail.
  • Load capacitance too high: The oscillation amplitude is damped, the frequency shifts downwards and the power consumption increases.

Solution:

Use a crystal with exactly the CL value recommended in the MCU data sheet. Calculate external load capacitors: C_external = 2 × (CL - Cstray). Example: CL = 7 pF, Cstray = 2 pF → C_external = 10 pF per side. (Calculation:102/20+2=10pF per C_ext.).

3. PCB layout error

Frequency: High - and often difficult to diagnose

The 32,768 kHz quartz operates with extremely low currents (nanoampere range). Any parasitic capacitance and any coupled interference can affect the oscillation.

  • Traces that are too long: Every millimetre adds parasitic capacitance (approx. 0.5-1 pF/cm).
  • Digital signals in the vicinity: Clock lines or SPI buses couple in interference.
  • Ground plane directly under the crystal: Increases the parasitic capacitance in multilayer PCBs.
  • Vias in the crystal area: Act as interference antennas.
  • Flux residues and moisture: Cause leakage currents - increased at low temperatures.

Solution:

Quartz directly next to the MCU pins (max. 5 mm), short symmetrical conductor tracks, guard ring with ground recess under the quartz, no signal lines between the quartz pins, clean PCB thoroughly after soldering.

4. missing or incorrect feedback resistor

Many MCU oscillator circuits require a high-impedance feedback resistor (Rf) in parallel with the crystal (typically 5-15 MΩ). It biases the inverter stage into its linear operating range. Some MCUs have this resistor internally (STM32, nRF52, ESP32), others require it externally (some MSP430 variants, certain 8-bit MCUs).

Solution:

Check the MCU data sheet to see whether an external Rf is required. If yes: typically 10 MΩ parallel to the quartz. If oscillation is problematic despite internal Rf: try external 15 MΩ.

5. overload of the quartz (drive level too high)

The 32,768 kHz tuning fork crystal is specified for a maximum drive power of typically 0.5-1.0 µW. Exceeding this leads to frequency drift, accelerated ageing and, in extreme cases, mechanical breakage of the resonator.

In practice, overloading occurs if there is no series resistor (Rd) for limitation.

Solution:

Check whether the MCU data sheet recommends a series resistance (Rd) (typically 47-470 kΩ). Measure the oscillation amplitude: it should be 200-600 mV peak-to-peak. Caution: Use 10:1 probes (10 MΩ) or better 100:1 - a 1:1 probe loads the oscillator so much that it can stop!

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