C8051F93X Datasheet, PDF (194/330 Pages) Silicon Laboratories – Pipelined intstruction architecture executes 70 of instruction in 1 or 2 system clocks

C8051F93x-C8051F92x
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
10k. 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 MHz
V_DD = power supply voltage in Volts
R = pull-up resistor value in k
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-R--3---}_-^----C-1---0---³- = ^{1---2.--2-4--3-6---}-_----1-5--0-0---³ = 100 kHz
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.
Table 19.2. Recommended XFCN Settings for RC and C modes
XFCN
000
001
010
011
100
101
110
111
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
194
Rev. 1.3