C8051F388 Datasheet, PDF (144/285 Pages) –

C8051F388/9/A/B
Figure 19.2. External Crystal Example
Important Note on External Crystals: Crystal oscillator circuits are quite sensitive to PCB layout. The
crystal should be placed as close as possible to the XTAL pins on the device. The traces should be as
short as possible and shielded with ground plane from any other traces which could introduce noise or
interference.
When using an external crystal, the external oscillator drive circuit must be configured by software for Crys-
tal Oscillator Mode or Crystal Oscillator Mode with divide by 2 stage. The divide by 2 stage ensures that the
clock derived from the external oscillator has a duty cycle of 50%. The External Oscillator Frequency Con-
trol value (XFCN) must also be specified based on the crystal frequency (see SFR Definition 19.6).
When the crystal oscillator is first enabled, the external oscillator valid detector allows software to deter-
mine when the external system clock is valid and running. Switching to the external oscillator before the
crystal oscillator has stabilized can result in unpredictable behavior. The recommended procedure for start-
ing the crystal is:
1. Configure XTAL1 and XTAL2 for analog I/O.
2. Disable the XTAL1 and XTAL2 digital output drivers by writing 1s to the appropriate bits in the Port
Latch register.
3. Configure and enable the external oscillator.
4. Wait at least 1 ms.
5. Poll for XTLVLD > 1.
6. Switch the system clock to the external oscillator.
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