For customers designing STMicroelectronics

We are pleased to introduce you to our crystal unit

Items to Confirm (designing of oscillation circuit)

Please note: to design oscillation circuit, you need to confirm the electronic characteristics as follows:

Oscillation allowance and Negative resistance (-R):
You need to confirm oscillation capability.
We recommend the condition: (-R) / ESR > 5
* -R: Negative resistance, ESR: Equivalent Series Resistance
Please refer to our web site:
Support information > Precautions in Designing oscillation circuits > 2. Allowance for oscillation

Drive Level:
You need to confirm if oscillation is stable, and if the drive level is within the specification.
Please refer to our web site:
Support information > Precautions in Designing oscillation circuits > 1. Drive level

Load Capacitance:
Load capacitance affects frequency stability, oscillation allowance, negative resistance, and start-up time of oscillation.
In addition the load capacitance is determinative factor of crystal unit's load capacitance (CL value) directly.
Please refer to our web site:
Support information > Precautions in Designing oscillation circuits > 3. Load capacitance

* frequency stability: frequency deviation out of circuit conditions.

Selection Guidance for crystal unit

When Customer selects crystal unit, lload capacitance (CL value) is an important factor of the selection.
For instance, crystal with small CL(7.0 pF) makes larger oscillation allowance, smaller current consumption and worse frequency stability, like below table.
Oppositely, crystal with large CL(12.5 pF) makes smaller oscillation allowance, larger current consumption and better frequency stability.
Please select it in consideration of these characteristics.

Selection Guidance for crystal unit

Epson Crystal Product Lineups

Product Size [mm] Applications Images
FC-135R 3.2x1.5x0.8t Portable equipment FC-135R
FC-12M 2.05x1.2x0.6t FC-12M
FC1610AN 1.6x1.0x0.5t FA1610AN

Oscillation Circuit and Selection Guidance of Crystal Unit for STM32F401

Pin number of OSC32_IN/OSC32_OUT
For STM32F401xB/STM32F401xC

UQFP48 3 4
LQFP64 3 4
LQFP100 8 9
UFBGA100 D1 E1

32kHz Crystal Unit Evaluation result *4 Vdd of STM32F4
Name CL*1
Part number *2 R1*6
[kΩ] Max.
External parts *3 |-R| *5
Cg[pF] Cd[pF]
FC-135R 7.0 X1A0001410001 50 7 7 270 3.3V
270 2.4V
300 1.8V
6.0 Click here 50 6 6 300 3.3V
330 2.4V
360 1.8V
FC-12M 5.0 Click here 90 4 3 400 3.3V
430 2.4V
470 1.8V
TBD 2.4V
TBD 1.8V
Please use the digital calibration function of STM32F401 for RTC purpose.For the detail, please refer to the application note "AN3371" from STMicroelectronics.


  1. Load capacitance.
  2. Two digits of the product code is packing specihication,please refer here for the detail.
  3. Above recommendations are based on actual evaluation results and intended to support users in picking the right components. And the results are only according to the sample set that received from our customers, so it is not including the dispersion trend of IC and the external parts.
    As the actual board layout and choice of external components influences the best suitable crystal load capacitance, We do not assume any responsibility and grant warranty for above recommendations. Users design must be verified and decided by own and individual evaluation.
  4. The evaluation board is the STM32F401Discovery Board (32F401CDISCOVERY).
  5. Negative resistance. Our recommend value is five times or more R1.
  6. Equivalent Series Resistance (ESR).

Precautions for designing of PCB Board

<Layout of pins, Crystal unit, Capacitor and Resistor>
-Please lay out crystal unit, capacitor and resistor near STM32F401 as far as possible.
-The length of signal patterns in oscillatiton circuit should be as short as possible, and do not cross other signal lines.

TOP View
For 32kHz Crystal unit

<GND Line Pattern and Interconnection of PCB>

-Please lay out GND line pattern under crystal unit.
-In case of multi-layered PCB board, do not lay out other signal lines under crystal unit.

Cross-seection View(Multi-layered PCB Board)

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