Specifications

C8051F330/1
Rev. 1.1 109
13.3. 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 13.1. A 10 MΩ resistor also must be wired
across the XTAL2 and XTAL1 pins for the crystal/resonator configuration. In RC, capacitor, or CMOS clock config-
uration, 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 13.5).
Important Note on External Oscillator Usage: Port pins must be configured when using the external oscillator cir-
cuit. 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 115 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. See
Section “14.2. Port I/O Initialization” on page 117 for details on Port input mode selection.