External oscillator drive circuit, External crystal example – Silicon Laboratories C8051F347 User Manual
Page 135
Rev. 1.3
135
C8051F340/1/2/3/4/5/6/7/8/9/A/B/C/D
14.3. External Oscillator Drive Circuit
The external oscillator circuit may drive an external crystal, ceramic resonator, capacitor, or RC network. A
CMOS clock may also provide a clock input. For a crystal or ceramic resonator configuration, the crystal/
resonator must be wired across the XTAL1 and XTAL2 pins as shown in Option 1 of Figure 14.1. A 10 M
resistor also must be wired across the XTAL1 and XTAL2 pins for the crystal/resonator configuration. In
RC, capacitor, or CMOS clock configuration, the clock source should be wired to the XTAL2 pin as shown
in Option 2, 3, or 4 of Figure 14.1. The type of external oscillator must be selected in the OSCXCN register,
and the frequency control bits (XFCN) must be selected appropriately (see SFR Definition 14.4)
Important Note on External Oscillator Usage:
Port pins must be configured when using the external
oscillator circuit. When the external oscillator drive circuit is enabled in crystal/resonator mode, Port pins
P0.6 and P0.7 (C8051F340/1/4/5/8) or P0.2 and P0.3 (C8051F342/3/6/7/9/A/B) are used as XTAL1 and
XTAL2 respectively. When the external oscillator drive circuit is enabled in capacitor, RC, or CMOS clock
mode, Port pin P0.7 (C8051F340/1/4/5/8) or P0.3 (C8051F342/3/6/7/9/A/B) is used as XTAL2. The Port I/
O Crossbar should be configured to skip the Port pins used by the oscillator circuit; see
“15.1. Priority Crossbar Decoder” on page 144
for Crossbar configuration. Additionally, when using the
external oscillator circuit in crystal/resonator, capacitor, or RC mode, the associated Port pins should be
configured as analog inputs. In CMOS clock mode, the associated pin should be configured as a digital
input
Section “15.2. Port I/O Initialization” on page 147
for details on Port input mode selection.
14.3.1. Clocking Timers Directly Through the External Oscillator
The external oscillator source divided by eight is a clock option for the timers (
) and the Programmable Counter Array (PCA) (
Section “22. Programmable Counter Array
). When the external oscillator is used to clock these peripherals, but is not used as
the system clock, the external oscillator frequency must be less than or equal to the system clock fre-
quency. In this configuration, the clock supplied to the peripheral (external oscillator / 8) is synchronized
with the system clock; the jitter associated with this synchronization is limited to ±0.5 system clock cycles.
14.3.2. External Crystal Example
If a crystal or ceramic resonator is used as an external oscillator source for the MCU, the circuit should be
configured as shown in Figure 14.1, Option 1. The External Oscillator Frequency Control value (XFCN)
should be chosen from the Crystal column of the table in SFR Definition 14.4 (OSCXCN register). For
example, a 12 MHz crystal requires an XFCN setting of 111b.
When the crystal oscillator is first enabled, the oscillator amplitude detection circuit requires a settling time
to achieve proper bias. Introducing a delay of 1 ms between enabling the oscillator and checking the
XTLVLD bit will prevent a premature switch to the external oscillator as the system clock. Switching to the
external oscillator before the crystal oscillator has stabilized can result in unpredictable behavior. The rec-
ommended procedure is:
Step 1. Enable the external oscillator.
Step 2. Wait at least 1 ms.
Step 3. Poll for XTLVLD => ‘1’.
Step 4. Switch the system clock to the external oscillator.
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.