C8051F970-A-GM Datasheet, PDF (244/454 Pages) Silicon Laboratories – Low Power Capacitive Sensing MCU with up to 32 kB of Flash

C8051F97x
f
=
1_----._-2_----3_-----_----1_----0_---³_-
RC
Where
f = frequency of clock in MHz
R = pull-up resistor value in k
V_DD = power supply voltage in Volts
C = 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 frequency desired
is 100 kHz, let R = 246 k, and C = 50 pF:
f
=
-¹---^.--²---³-----^----¹----⁰---³-
RC
=
1_----._--2_---3_-----_----1_----0_---³_-
246  50
=
100
kHz
Where
f = frequency of clock in MHz
R = pull-up resistor value in k
V_DD = power supply voltage in Volts
C = capacitor value on the XTAL2 pin in pF
Referencing Table 24.2, the recommended XFCN setting is 010.
XFCN
000
001
010
011
100
101
110
111
Table 24.2. Recommended XFCN Settings for RC and C Modes
Approximate Frequency K Factor (C Mode)
Range (RC and C Mode)
f 25 kHz
K Factor = 0.87
Typical Supply Current/ Actual
Measured Frequency
(C Mode, VDD = 2.4 V)
3.0 µA, f = 11 kHz, C = 33 pF
25 kHz f 50 kHz
K Factor = 2.6
5.5 µA, f = 33 kHz, C = 33 pF
50 kHz f 100 kHz
K Factor = 7.7
13 µA, f = 98 kHz, C = 33 pF
100 kHz f 200 kHz
K Factor = 22
32 µA, f = 270 kHz, C = 33 pF
200 kHz f 400 kHz
K Factor = 65
82 µA, f = 310 kHz, C = 46 pF
400 kHz f 800 kHz
K Factor = 180
242 µA, f = 890 kHz, C = 46 pF
800 kHz f 1.6 MHz
K Factor = 664
1.0 mA, f = 2.0 MHz, C = 46 pF
1.6 MHz f 3.2 MHz
K Factor = 1590
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
245