OVERVIEW
Devices in the PIC18F2455/2550/4455/4550
family incorporate a different oscillator and microcontrollerclock system than
previous 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, PIC18F2455/ 2550/4455/4550
devices include a new clock branch toprovide 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 below.Other oscillator features used in PIC18 enhanced microcontrollers, such as
the internal oscillator block and clock switching.
PIC18F2455/2550/4455/4550 CLOCK DIAGRAM
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