PIC18F2450/4450
however, the microcontroller core and other peripherals
can be separately clocked from the secondary or internal
oscillators as before.
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 PICmicro® devices, 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;
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
flexibility 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
PLLDIV
USB Clock Source
÷ 12
÷ 10
111
110
USBDIV
÷ 6
101
(4 MHz Input Only)
0
1
÷ 5
÷ 4
÷ 3
Primary Oscillator
100
011
010
96 MHz
PLL
÷ 2
OSC2
OSC1
Sleep
÷ 2
÷ 1
FSEN
001
000
1
HSPLL, ECPLL,
XTPLL, ECPIO
USB
Peripheral
CPUDIV
0
÷ 4
÷ 6
÷ 4
÷ 3
÷ 2
11
10
CPUDIV
01
00
÷ 4
÷ 3
÷ 2
÷ 1
11
10
01
00
CPU
XT, HS, EC, ECIO
1
0
Primary
Clock
IDLEN
Peripherals
FOSC3:FOSC0
Secondary Oscillator
T1OSO
T1OSI
T1OSC
T1OSCEN
Enable
Oscillator
OSCCON<6:4>
Internal Oscillator
Clock
Control
Internal RC Oscillator
31.25 kHz
FOSC3:FOSC0
OSCCON<1:0>
Clock Source Option
for other Modules
WDT, PWRT, FSCM
and Two-Speed Start-up
DS39760A-page 24
Advance Information
© 2006 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
