Si1000/1/2/3/4/5
15 pF
XTAL1
XTAL2
10 M
25 MHz
15 pF
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 Crys-
tal 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 Con-
trol 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.
Table 19.1. Recommended XFCN Settings for Crystal Mode
XFCN
Crystal Frequency
Bias Current
Typical Supply Current
(VDD = 2.4 V)
000
001
010
011
100
101
110
111
f 20 kHz
0.5 µA
1.5 µA
4.8 µA
14 µA
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
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
40 µA
120 µA
550 µA
2.6 mA
3.9 mA, f = 25 MHz
When the crystal oscillator is first enabled, the external oscillator valid detector allows software to deter-
mine 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.
184
Rev. 1.0
SILICON [ SILICON ]
