PIC18F2450_08 Datasheet, PDF (25/324 Pages) Microchip Technology – 28/40/44-Pin, High-Performance, 12 MIPS, Enhanced Flash, USB Microcontrollers with nanoWatt Technology

2.0 OSCILLATOR
CONFIGURATIONS
2.1 Overview
Devices in the PIC18F2450/4450 family incorporate a
different oscillator and microcontroller clock system
than the non-USB PIC18F devices. The addition of the
USB module, with its unique requirements for a stable
clock source, make it necessary to provide a separate
clock source that is compliant with both USB low-speed
and full-speed specifications.
To accommodate these requirements, PIC18F2450/
4450 devices include a new clock branch to provide a
48 MHz clock for full-speed USB operation. Since it is
driven from the primary clock source, an additional
system of prescalers and postscalers has been added
to accommodate a wide range of oscillator frequencies.
An overview of the oscillator structure is shown in
Figure 2-1.
Other oscillator features used in PIC18 enhanced
microcontrollers, such as the internal RC oscillator and
clock switching, remain the same. They are discussed
later in this chapter.
2.1.1 OSCILLATOR CONTROL
The operation of the oscillator in PIC18F2450/4450
devices is controlled through two Configuration registers
and two control registers. Configuration registers,
CONFIG1L and CONFIG1H, select the oscillator mode
and USB prescaler/postscaler options. As Configuration
bits, these are set when the device is programmed and
left in that configuration until the device is
reprogrammed.
The OSCCON register (Register 2-1) selects the Active
Clock mode; it is primarily used in controlling clock
switching in power-managed modes. Its use is
discussed in Section 2.4.1 “Oscillator Control
Register”.
PIC18F2450/4450
2.2 Oscillator Types
PIC18F2450/4450 devices can be operated in twelve
distinct oscillator modes. In contrast with the non-USB
PIC18 enhanced microcontrollers, four of these modes
involve the use of two oscillator types at once. Users
can program the FOSC3:FOSC0 Configuration bits to
select one of these modes:
1. XT
Crystal/Resonator
2. XTPLL Crystal/Resonator with PLL Enabled
3. HS
High-Speed Crystal/Resonator
4. HSPLL High-Speed Crystal/Resonator
with PLL Enabled
5. EC
External Clock with FOSC/4 Output
6. ECIO External Clock with I/O on RA6
7. ECPLL External Clock with PLL Enabled
and FOSC/4 Output on RA6
8. ECPIO External Clock with PLL Enabled,
I/O on RA6
9. INTHS Internal Oscillator used as
Microcontroller Clock Source, HS
Oscillator used as USB Clock Source
10. INTXT Internal Oscillator used as
Microcontroller Clock Source, XT
Oscillator used as USB Clock Source
11. INTIO
Internal Oscillator used as
Microcontroller Clock Source, EC
Oscillator used as USB Clock Source,
Digital I/O on RA6
12. INTCKO Internal Oscillator used as
Microcontroller Clock Source, EC
Oscillator used as USB Clock Source,
FOSC/4 Output on RA6
© 2008 Microchip Technology Inc.
DS39760D-page 23