Table 8.1. Recommended XFCN Settings for Crystal Mode
XFCN Field Setting
Crystal Frequency
Approximate Bias Current
000
f ≤ 20 kHz
0.5 µA
001
20 kHz < f ≤ 58 kHz
1.5 µA
010
58 kHz < f ≤ 155 kHz
4.8 µA
011
155 kHz < f ≤ 415 kHz
14 µA
100
415 kHz < f ≤ 1.1 MHz
40 µA
101
1.1 MHz < f ≤ 3.1 MHz
120 µA
110
3.1 MHz < f ≤ 8.2 MHz
550 µA
111
8.2 MHz < f ≤ 25 MHz
2.6 mA
When the crystal oscillator is first enabled, the external oscillator valid detector allows software to determine when the external system
clock has stabilized. Switching to the external oscillator before the crystal oscillator has stabilized can result in unpredictable behavior.
The recommended procedure for starting the crystal is as follows:
1. Configure XTAL1 and XTAL2 for analog I/O and disable the digital output drivers.
2. Disable the XTAL1 and XTAL2 digital output drivers by writing 1's 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 XCLKVLD set to 1.
6. Switch the system clock to the external oscillator.
EFM8SB2 Reference Manual
Clocking and Oscillators
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