Rev. 1.4 119
C8051F320/1
13.2. 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 crys-
tal/resonator must be wired across the XTAL1
and XTAL2 pins as shown in Option 1 of Figure 13.1. A
10 Mr
esistor also must be wired across the XTAL1 and XTAL2 pins for the crystal/resonator configura-
tion. In RC, capacitor, or CMOS clo
ck configuration, the clock source should be wired to the XTAL2 pin as
shown in Option 2, 3, or 4 of Figure 13.1. The type of external oscillator must
be selected in the OSCXCN
register, and the frequency control bits (XFCN) must be selected appropriately (see Figure 13.3)
Important Note on External Oscillator Usage: Po
rt pins must be configured when using the external
oscillator circuit. When the external oscillator drive circuit is enabled in crystal/resonator mode, Port pins
P0.2 and P0.3 are used as XTAL1 and XTAL2 respectively. When the external oscillator drive circuit is
enabled in capacitor, RC, or CMOS clock mode, Port pin P0.3 is used as XTAL2. The Port I/O Crossbar
should be configured to skip the Port pins used by the oscillator circuit; see Section “14.1. Priority Crossbar
Decoder” on page 128 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 ana
log inputs.
In CMOS clock mode, the associated pin should be configured as a digital input. See Section “14.2. Port
I/O Initialization” on page 130 for details on Port input mode s
election.
13.2.1. Clocking Timers Directly Through the External Oscillator
The external oscillator source divided by eight is a clock option for the timers (Section “19. Timers” on
page 209) and the Programmable Counter Array (PCA) (Section “20. Programmable Counter Array
(PCA0)” on page 227). 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.
13.2.2. 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 13.1, Option 1. The External Oscillator Frequency
Control value (XFCN)
should be chosen from the Crystal column of the table in Figure 13.3 (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 c
hecking 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.
S
tep 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.
