PIC18F2450/4450
2.2.5
INTERNAL OSCILLATOR
2.3
Oscillator Settings for USB
The PIC18F2450/4450 devices include an internal RC
oscillator (INTRC) which provides a nominal 31 kHz out-
put. INTRC is enabled if it is selected as the device clock
source; it is also enabled automatically when any of the
following are enabled:
When the PIC18F2450/4450 is used for USB
connectivity, it must have either a 6 MHz or 48 MHz
clock for USB operation, depending on whether Low-
Speed or Full-Speed mode is being used. This may
require some forethought in selecting an oscillator
frequency and programming the device.
• Power-up Timer
The full range of possible oscillator configurations
compatible with USB operation is shown in Table 2-3.
• Fail-Safe Clock Monitor
• Watchdog Timer
• Two-Speed Start-up
2.3.1
LOW-SPEED OPERATION
These features are discussed in greater detail in
The USB clock for Low-Speed mode is derived from
the primary oscillator chain and not directly from the
PLL. It is divided by 4 to produce the actual 6 MHz
clock. Because of this, the microcontroller can only use
a clock frequency of 24 MHz when the USB module is
active and the controller clock source is one of the
primary oscillator modes (XT, HS or EC, with or without
the PLL).
Section 18.0 “Special Features of the CPU”.
2.2.5.1
Internal Oscillator Modes
When the internal oscillator is used as the micro-
controller clock source, one of the other oscillator
modes (External Clock or External Crystal/Resonator)
must be used as the USB clock source. The choice of
USB clock source is determined by the particular
internal oscillator mode.
This restriction does not apply if the microcontroller
clock source is the secondary oscillator or internal
oscillator.
There are four distinct modes available:
1. INTHS mode: The USB clock is provided by the
oscillator in HS mode.
2.3.2
RUNNING DIFFERENT USB AND
MICROCONTROLLER CLOCKS
2. INTXT mode: The USB clock is provided by the
oscillator in XT mode.
The USB module, in either mode, can run
asynchronously with respect to the microcontroller core
and other peripherals. This means that applications can
use the primary oscillator for the USB clock while the
microcontroller runs from a separate clock source at a
lower speed. If it is necessary to run the entire
application from only one clock source, full-speed
operation provides a greater selection of microcontroller
clock frequencies.
3. INTCKO mode: The USB clock is provided by an
external clock input on OSC1/CLKI; the OSC2/
CLKO pin outputs FOSC/4.
4. INTIO mode: The USB clock is provided by an
external clock input on OSC1/CLKI; the OSC2/
CLKO pin functions as a digital I/O (RA6).
Of these four modes, only INTIO mode frees up an
additional pin (OSC2/CLKO/RA6) for port I/O use.
© 2006 Microchip Technology Inc.
Advance Information
DS39760A-page 27
MICROCHIP [ MICROCHIP ]
