c8051t617 Silicon Laboratories, c8051t617 Datasheet - Page 113

• Manufacturer Part Number: c8051t617
• Description: Mixed Signal Byte-programmable Eprom Mcu
• Manufacturer: Silicon Laboratories

C8051T610/1/2/3/4/5/6/7
13.2. External Oscillator Drive Circuit
The external oscillator circuit may drive an external capacitor or RC network. A CMOS clock may also pro-
vide a clock input. In RC, capacitor, or CMOS clock configuration, the clock source should be wired to the
EXTCLK pin as shown in Option 1, 2, or 3 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 SFR
Definition 13.4).
Important Note on External Oscillator Usage: The EXTCLK pin must be configured when using an
external oscillator circuit. The Port I/O Crossbar should be configured to skip the Port pin used by the oscil-
lator circuit; see Section “14.1. Priority Crossbar Decoder” on page 118 for Crossbar configuration. Addi-
tionally, when using the external oscillator circuit in capacitor or RC mode, the associated Port pin should
be configured as an analog input. In CMOS clock mode, the associated pin should be configured as a
digital input. See Section “14.2. Port I/O Initialization” on page 120 for details on Port input mode selec-
tion.
13.3. System Clock Selection
The CLKSL0 bit in register CLKSEL selects which oscillator is used as the system clock. CLKSL0 must be
set to ‘1’ for the system clock to run from the external oscillator; however the external oscillator may still
clock certain peripherals (timers, PCA) when the internal oscillator is selected as the system clock. The
system clock may be switched on-the-fly between the internal and external oscillator, so long as the
selected oscillator is enabled and has settled. The internal oscillator requires little start-up time and may be
selected as the system clock immediately following the OSCICN write that enables the internal oscillator.
An external CMOS clock may require a certain amount of settling time or power-on time, as specified in the
manufacturer's data sheet. RC and C modes typically require no startup time.
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