C8051F93X Datasheet, PDF (195/330 Pages) Silicon Laboratories – Pipelined intstruction architecture executes 70 of instruction in 1 or 2 system clocks

C8051F93x-C8051F92x
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:
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.
19.3.3. External Capacitor Mode
If a capacitor is used as the external oscillator, the circuit should be configured as shown in Figure 19.1,
Option 3. The capacitor 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 oscillation frequency and the required
External Oscillator Frequency Control value (XFCN) in the OSCXCN Register can be determined by the
following equation:
f
=
-------K-----F---------
C  V_DD
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
Below is an example of selecting the capacitor and finding the frequency of oscillation Assume V_DD = 3.0 V
and f = 150 kHz:
f
=
-------K-----F---------
C  V_DD
0.150 MHz
=
------K----F-------
C  3.0
Since a frequency of roughly 150 kHz is desired, select the K Factor from Table 19.2 as KF = 22:
0.150 MHz = _{C------}---2--3-2--.-0-----V---
C
=
---------------------2---2----------------------
0.150 MHz  3.0 V
C = 48.8 pF
Therefore, the XFCN value to use in this example is 011 and C is approximately 50 pF.
The recommended startup procedure for C mode is the same as RC mode.
Rev. 1.3
195