C8051F970-A-GM Datasheet, PDF (243/454 Pages) Silicon Laboratories – Low Power Capacitive Sensing MCU with up to 32 kB of Flash

C8051F97x
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 Crystal
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 Control value
(XFCN) must also be specified based on the crystal frequency. The selection should be based on Table 24.1. For
example, a 25 MHz crystal requires an XFCN setting of 111b.
Table 24.1. Recommended XFCN Settings for Crystal Mode
XFCN
000
001
010
011
100
101
110
111
Crystal Frequency
f  20 kHz
20 kHz f 58 kHz
58 kHz  f 155 kHz
155 kHz  f 415 kHz
415 kHz  f 1.1 MHz
1.1 MHz  f 3.1 MHz
3.1 MHz  f 8.2 MHz
8.2 MHz  f 25 MHz
Bias Current
0.5 µA
1.5 µA
4.8 µA
14 µA
40 µA
120 µA
550 µA
2.6 mA
Typical Supply Current
(VDD = 2.4 V)
3.0 µA, f = 32.768 kHz
4.8 µA, f = 32.768 kHz
9.6 µA, f = 32.768 kHz
28 µA, f = 400 kHz
71 µA, f = 400 kHz
193 µA, f = 400 kHz
940 µA, f = 8 MHz
3.9 mA, f = 25 MHz
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. Configure and enable the external oscillator.
3. Poll for XTLVLD => 1.
4. Switch the system clock to the external oscillator.
24.3.2. External RC Mode
If an RC network is used as the external oscillator, the circuit should be configured as shown in Figure 24.1, Option
2. The RC network should be added to XTAL2, and XTAL2 should be configured for analog I/O with the digital
output drivers disabled. XTAL1 is not affected in RC mode.
The capacitor should be no greater than 100 pF; however for very small capacitors, the total capacitance may be
dominated by parasitic capacitance in the PCB layout. The resistor should be no smaller than 10 k. The
oscillation frequency can be determined by the following equation:
244
Rev 1.0