C8051F02X Datasheet, PDF (137/272 Pages) Silicon Laboratories – 8K ISP FLASH MCU Family

C8051F020/1/2/3
Figure 14.3. OSCXCN: External Oscillator Control Register
R/W
R/W
R/W
R/W
R/W
XTLVLD XOSCMD2 XOSCMD1 XOSCMD0 -
Bit7
Bit6
Bit5
Bit4
Bit3
R/W
XFCN2
Bit2
R/W
XFCN1
Bit1
R/W
XFCN0
Bit0
Reset Value
00000000
SFR Address:
0xB1
Bit7:
Bits6-4:
Bit3:
Bits2-0:
XTLVLD: Crystal Oscillator Valid Flag
(Valid only when XOSCMD = 11x.)
0: Crystal Oscillator is unused or not yet stable
1: Crystal Oscillator is running and stable
XOSCMD2-0: External Oscillator Mode Bits
00x: Off. XTAL1 pin is grounded internally.
010: System Clock from External CMOS Clock on XTAL1 pin.
011: System Clock from External CMOS Clock on XTAL1 pin divided by 2.
10x: RC/C Oscillator Mode with divide by 2 stage.
110: Crystal Oscillator Mode
111: Crystal Oscillator Mode with divide by 2 stage.
RESERVED. Read = undefined, Write = don't care
XFCN2-0: External Oscillator Frequency Control Bits
000-111:
XFCN Crystal (XOSCMD = 11x) RC (XOSCMD = 10x) C (XOSCMD = 10x)
000
f < 12 kHz
f < 25 kHz
K Factor = 0.44
001
12 kHz < f ≤ 30 kHz
25 kHz < f ≤ 50 kHz
K Factor = 1.4
010
30 kHz < f ≤ 95 kHz
50 kHz < f ≤ 100 kHz
K Factor = 4.4
011
95 kHz < f ≤ 270 kHz
100 kHz < f ≤ 200 kHz
K Factor = 13
100
270 kHz < f ≤ 720 kHz 200 kHz < f ≤ 400 kHz
K Factor = 38
101
720 kHz < f ≤ 2.2 MHz 400 kHz < f ≤ 800 kHz
K Factor = 100
110
2.2 MHz < f ≤ 6.7 MHz 800 kHz < f ≤ 1.6 MHz
K Factor = 420
111
f > 6.7 MHz
1.6 MHz < f ≤ 3.2 MHz
K Factor = 1400
CRYSTAL MODE (Circuit from Figure 14.1, Option 1; XOSCMD = 11x)
Choose XFCN value to match the crystal or ceramic resonator frequency.
RC MODE (Circuit from Figure 14.1, Option 2; XOSCMD = 10x)
Choose oscillation frequency range where:
f = 1.23(10³) / (R * C), where
f = frequency of oscillation in MHz
C = capacitor value in pF
R = Pull-up resistor value in kΩ
C MODE (Circuit from Figure 14.1, Option 3; XOSCMD = 10x)
Choose K Factor (KF) for the oscillation frequency desired:
f = KF / (C * AV+), where
f = frequency of oscillation in MHz
C = capacitor value on XTAL1, XTAL2 pins in pF
AV+ = Analog Power Supply on MCU in volts
Rev. 1.4
137