Rtc Clock Source (Optional) - Espressif Systems ESP32-S3 Hardware Design Manuallines

Chapter 3. Schematic Checklist
where the value of C (load capacitance) can be found in the crystal's datasheet, and the value of C
L
PCB's stray capacitance. The values of C1 and C4 need to be further adjusted after an overall test as below:
1. Select TX tone mode using the
2. Observe the 2.4 GHz signal with a radio communication analyzer or a spectrum analyzer and demodulate it to
obtain the actual frequency offset.
3. Adjust the frequency offset to be within ±10 ppm (recommended) by adjusting the external load capacitance.
• When the center frequency offset is positive, it means that the equivalent load capacitance is small, and the
external load capacitance needs to be increased.
• When the center frequency offset is negative, it means the equivalent load capacitance is large, and the external
load capacitance needs to be reduced.
• External load capacitance at the two sides are usually equal, but in special cases, they may have slightly different
values.
Note:
• Defects in the manufacturing of crystal (for example, large frequency deviation of more than ±10 ppm, unstable
performance within the operating temperature range, etc) may lead to the malfunction of ESP32-S3, resulting
in a decrease of the RF performance.
• It is recommended that the amplitude of the crystal is greater than 500 mV.
• When Wi-Fi or Bluetooth connection fails, after ruling out software problems, you may follow the steps men-
tioned above to ensure that the frequency offset meets the requirements by adjusting capacitors at the two sides
of the crystal.

3.4.2 RTC Clock Source (Optional)

ESP32-S3 supports an external 32.768 kHz crystal or an external signal (e.g., an oscillator) to act as the RTC clock.
The external RTC clock source enhances timing accuracy and consequently decreases average power consumption,
without impacting functionality.
Figure ESP32-S3 Schematic for 32.768 kHz Crystal
Please note the requirements for the 32.768 kHz crystal:
• Equivalent series resistance (ESR) ≤ 70 kΩ.
• Load capacitance at both ends should be configured according to the crystal's specification.
The parallel resistor R is used for biasing the crystal circuit (5 MΩ < R ≤ 10 MΩ). In general, you do not need to
populate the resistor.
If the RTC clock source is not required, then the pins for the 32.768 kHz crystal can be used as GPIOs.
The external signal can be input to the XTAL' s P end through a DC blocking capacitor (about 20 pF). The XTAL' s
N end can be floating. Figure
The signal should meet the following requirements:
Espressif Systems
Fig. 7: ESP32-S3 Schematic for External Crystal
Certification and Test
shows the schematic for the external 32.768 kHz crystal.
ESP32-S3 Schematic for External Oscillator
Submit Document Feedback
Tool.
shows the schematic of the external signal.
15
refers to the
stray
Release master