C8051F99X_10 Datasheet, PDF (189/322 Pages) Silicon Laboratories – Ultra Low Power, 8-2 kB Flash, Capacitive Sensing MCU

C8051F99x-C8051F98x
19.3.2. External RC Mode
If an RC network is used as the external oscillator, the circuit should be configured as shown in
Figure 19.1, 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:
f = ^{1---.--2---3----}-----1---0----³
RC
where
f = frequency of clock in MHzR = pull-up resistor value in k
V_DD = power supply voltage in VoltsC = capacitor value on the XTAL2 pin in pF
To determine the required External Oscillator Frequency Control value (XFCN) in the OSCXCN Register,
first select the RC network value to produce the desired frequency of oscillation. For example, if the fre-
quency desired is 100 kHz, let R = 246 k and C = 50 pF:
f = ^{1----.-2---3----}-----1---0---³- = ^{1---.--2---3----}-----1---0----³ = 100 kHz
RC
246  50
where
f = frequency of clock in MHz
V_DD = power supply voltage in Volts
R = pull-up resistor value in k
C = capacitor value on the XTAL2 pin in pF
Referencing Table 19.2, the recommended XFCN setting is 010.
XFCN
000
001
010
011
100
101
110
111
Table 19.2. Recommended XFCN Settings for RC and C modes
Approximate
Frequency Range (RC
and C Mode)
f 25 kHz
25 kHz f 50 kHz
50 kHz f 100 kHz
100 kHz f 200 kHz
200 kHz f 400 kHz
400 kHz f 800 kHz
800 kHz f 1.6 MHz
1.6 MHz f 3.2 MHz
K Factor (C Mode)
K Factor = 0.87
K Factor = 2.6
K Factor = 7.7
K Factor = 22
K Factor = 65
K Factor = 180
K Factor = 664
K Factor = 1590
Typical Supply Current/ Actual
Measured Frequency
(C Mode, VDD = 2.4 V)
3.0 µA, f = 11 kHz, C = 33 pF
5.5 µA, f = 33 kHz, C = 33 pF
13 µA, f = 98 kHz, C = 33 pF
32 µA, f = 270 kHz, C = 33 pF
82 µA, f = 310 kHz, C = 46 pF
242 µA, f = 890 kHz, C = 46 pF
1.0 mA, f = 2.0 MHz, C = 46 pF
4.6 mA, f = 6.8 MHz, C = 46 pF
When the RC oscillator is first enabled, the external oscillator valid detector allows software to determine
when oscillation has stabilized. The recommended procedure for starting the RC oscillator is as follows:
1. Configure 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.
Rev. 1.0
189