C8051F93X Datasheet, PDF (193/330 Pages) Silicon Laboratories – Pipelined intstruction architecture executes 70 of instruction in 1 or 2 system clocks

C8051F93x-C8051F92x
15 pF
XTAL1
25 MHz
10 M
15 pF
XTAL2
Figure 19.2. 25 MHz External Crystal Example
Important Note on External Crystals: Crystal oscillator circuits are quite sensitive to PCB layout. The
crystal should be placed as close as possible to the XTAL pins on the device. The traces should be as
short as possible and shielded with ground plane from any other traces which could introduce noise or
interference.
When using an external crystal, the external oscillator drive circuit must be configured by software for
Crystal Oscillator Mode or Crystal Oscillator Mode with divide by 2 stage. The divide by 2 stage ensures
that the clock derived from the external oscillator has a duty cycle of 50%. The External Oscillator
Frequency Control value (XFCN) must also be specified based on the crystal frequency. The selection
should be based on Table 19.1. For example, a 25 MHz crystal requires an XFCN setting of 111b.
XFCN
000
001
010
011
100
101
110
111
Table 19.1. Recommended XFCN Settings for Crystal Mode
Crystal Frequency
f  20 kHz
20 kHz f 58 kHz
58 kHz  f 155 kHz
155 kHz  f 415 kHz
415 kHz  f 1.1 MHz
1.1 MHz  f 3.1 MHz
3.1 MHz  f 8.2 MHz
8.2 MHz  f 25 MHz
Bias Current
0.5 µA
1.5 µA
4.8 µA
14 µA
40 µA
120 µA
550 µA
2.6 mA
Typical Supply Current
(VDD = 2.4 V)
3.0 µA, f = 32.768 kHz
4.8 µA, f = 32.768 kHz
9.6 µA, f = 32.768 kHz
28 µA, f = 400 kHz
71 µA, f = 400 kHz
193 µA, f = 400 kHz
940 µA, f = 8 MHz
3.9 mA, f = 25 MHz
When the crystal oscillator is first enabled, the external oscillator valid detector allows software to
determine when the external system clock has stabilized. Switching to the external oscillator before the
crystal oscillator has stabilized can result in unpredictable behavior. The recommended procedure for
starting the crystal is:
1. Configure XTAL1 and 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. 1.3
193