PIC18F4550ESOSQTP datasheet(25/438 Pages) MICROCHIP

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DS39632E-page 23

PIC18F2455/2550/4455/4550

2.0

OSCILLATOR

CONFIGURATIONS

2.1

Overview

Devices in the PIC18F2455/2550/4455/4550 family

incorporate a different oscillator and microcontroller

clock system than previous PIC18F devices. The addi-

tion 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 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 oscillator block

and clock switching, remain the same. They are

discussed later in this chapter.

2.1.1

OSCILLATOR CONTROL

The operation of the oscillator in PIC18F2455/2550/

4455/4550 devices is controlled through two Configu-

ration 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 2-2) 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

The OSCTUNE register (Register 2-1) is used to trim

the INTRC frequency source, as well as select the

low-frequency clock source that drives several special

features. Its use is described in Section 2.2.5.2

“OSCTUNE Register”.

2.2

Oscillator Types

PIC18F2455/2550/4455/4550 devices can be operated

in twelve distinct oscillator modes. In contrast with pre-

vious 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:

Crystal/Resonator

High-Speed Crystal/Resonator

HSPLL High-Speed Crystal/Resonator

with PLL Enabled

External Clock with FOSC/4 Output

ECIO

External Clock with I/O on RA6

ECPLL External Clock with PLL Enabled

and FOSC/4 Output on RA6

ECPIO External Clock with PLL Enabled,

I/O on RA6

INTHS Internal Oscillator used as

Microcontroller Clock Source, HS

Oscillator used as USB Clock Source

INTIO

Internal Oscillator used as

Microcontroller Clock Source, EC

Oscillator used as USB Clock Source,

Digital I/O on RA6

10. INTCKO Internal Oscillator used as

Microcontroller Clock Source, EC

Oscillator used as USB Clock Source,

FOSC/4 Output on RA6

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® devices, all core and peripheral clocks

were driven by a single oscillator source; the usual

sources were primary, secondary or the internal oscilla-

tor. With PIC18F2455/2550/4455/4550 devices, the pri-

mary 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 flexibil-

ity for clocking the rest of the device from the primary

oscillator source. These are detailed in Section 2.3

“Oscillator Settings for USB”.

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