Datasheet

PIC18F2450/4450
DS39760D-page 24 © 2008 Microchip Technology Inc.
2.2.1 OSCILLATOR MODES AND
USB OPERATION
Because of the unique requirements of the USB
module, a different approach to clock operation is
necessary. In previous PIC
®
microcontrollers, all core
and peripheral clocks were driven by a single oscillator
source; the usual sources were primary, secondary or
the internal oscillator. With PIC18F2450/4450 devices,
the primary oscillator becomes part of the USB module
and cannot be associated to any other clock source.
Thus, the USB module must be clocked from the
primary clock source; however, the microcontroller
core and other peripherals can be separately clocked
from the secondary or internal oscillators as before.
Because of the timing requirements imposed by USB,
an internal clock of either 6 MHz or 48 MHz is required
while the USB module is enabled. Fortunately, the
microcontroller and other peripherals are not required
to run at this clock speed when using the primary
oscillator. There are numerous options to achieve the
USB module clock requirement and still provide flexi-
bility for clocking the rest of the device from the primary
oscillator source. These are detailed in Section 2.3
“Oscillator Settings for USB”.
FIGURE 2-1: PIC18F2450/4450 CLOCK DIAGRAM
PIC18F2450/4450
FOSC3:FOSC0
Secondary Oscillator
T1OSCEN
Enable
Oscillator
T1OSO
T1OSI
Clock Source Option
for Other Modules
OSC1
OSC2
Sleep
Primary Oscillator
XT, HS, EC, ECIO
T1OSC
CPU
Peripherals
IDLEN
MUX
OSCCON<6:4>
WDT, PWRT, FSCM
Internal Oscillator
Clock
Control
OSCCON<1:0>
and Two-Speed Start-up
96 MHz
PLL
PLLDIV<2:0>
CPUDIV<1:0>
0
1
0
1
÷ 2
PLL Prescaler
MUX
111
110
101
100
011
010
001
000
÷ 1
÷ 2
÷ 3
÷ 4
÷ 5
÷ 6
÷ 10
÷ 12
11
10
01
00
PLL Postscaler
÷ 2
÷ 3
÷ 4
÷ 6
USB
USBDIV
FOSC3:FOSC0
HSPLL, ECPLL,
11
10
01
00
Oscillator Postscaler
÷ 1
÷ 2
÷ 3
÷ 4
CPUDIV<1:0>
1
0
Peripheral
FSEN
÷ 4
USB Clock Source
XTPLL, ECPIO
Primary
Clock
(4 MHz Input Only)
Internal RC Oscillator
31.25
kHz