Crystal/Ceramic Oscillator Basic Circuit

Crystal/Ceramic Oscillator Basic Circuit

Below are two application circuits for crystal/ceramic oscillators.

Description 1. The internal bias resistor RINB is used to generate the operating point of the oscillator.
2. The internal oscillator capacitors CIN1 and CIN2 form a Pierce oscillator with an external crystal/ceramic oscillator. When oscillating, the crystal/ceramic
oscillator can be regarded as an equivalent inductor, and this circuit can also reduce EMI.
3. The external bias resistor REXB is used for special applications of low-voltage stop control. It needs to be matched with CEX1 so that REXB×CEX1 is greater than 2πfSYS. The main
principle is to increase the load of the oscillation circuit and stop the oscillator at low voltage, so that the MCU will not have a low-voltage operation error. If
the low voltage situation is not used in the actual application, this resistor can be omitted.
4. The external oscillator capacitors CEX1 and CEX2 are used for fine-tuning the oscillation frequency or matching the crystal/ceramic oscillator, and can be used to adjust the start-up time. They can be omitted in
normal applications.
5. In the application circuits of the HOLTEK MCU datasheet and handbook, matching resistor and capacitor values ​​are provided for user reference.

Crystal/Ceramic Oscillator Warm-up Time
• The warm-up time required for the crystal/ceramic oscillator before starting.
• The length of time is related to the characteristics of the crystal/ceramic oscillator and the length of the stop (cooling) time. Generally, in the cold state, the warm-up time is about 3~5ms.
System Start-up Timer
• The delay time required for the oscillator to start oscillating stably.
• The time is 1024 oscillator oscillation cycles.
EMI/EMS (EMC) Considerations
• The crystal/ceramic oscillator should be placed closest to the oscillator pin of the MCU, that is, its connection should be the shortest.
• To reduce EMI, the pins of the crystal/ceramic oscillator should be shielded by a VDD or GND (VSS) loop.
• The VDD or GND (VSS) connected to CEX1 and CEX2 should have the shortest connection to the VDD or GND (VSS) of the MCU.