PIC18F4550ESPQTP datasheet(32/438 Pages) MICROCHIP

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PIC18F2455/2550/4455/4550

DS39632E-page 30

2.3

Oscillator Settings for USB

When these devices are used for USB connectivity,

they must have either a 6 MHz or 48 MHz clock for

USB operation, depending on whether Low-Speed or

Full-Speed mode is being used. This may require some

forethought in selecting an oscillator frequency and

programming the device.

The full range of possible oscillator configurations

compatible with USB operation is shown in Table 2-3.

2.3.1

LOW-SPEED OPERATION

The USB clock for Low-Speed mode is derived from the

primary oscillator chain and not directly from the PLL. It

is divided by 4 to produce the actual 6 MHz clock.

Because of this, the microcontroller can only use a

clock frequency of 24 MHz when the USB module is

active and the controller clock source is one of the

primary oscillator modes (XT, HS or EC, with or without

the PLL).

This restriction does not apply if the microcontroller

clock source is the secondary oscillator or internal

oscillator block.

2.3.2

RUNNING DIFFERENT USB AND

MICROCONTROLLER CLOCKS

The USB module, in either mode, can run asynchro-

nously with respect to the microcontroller core and

other peripherals. This means that applications can use

the primary oscillator for the USB clock while the micro-

controller runs from a separate clock source at a lower

speed. If it is necessary to run the entire application

from only one clock source, full-speed operation

provides a greater selection of microcontroller clock

frequencies.

TABLE 2-3:

OSCILLATOR CONFIGURATION OPTIONS FOR USB OPERATION

Input Oscillator

Frequency

PLL Division

(PLLDIV2:PLLDIV0)

Clock Mode

(FOSC3:FOSC0)

MCU Clock Division

(CPUDIV1:CPUDIV0)

Microcontroller

Clock Frequency

48 MHz

N/A(1)

EC, ECIO

None (00)48 MHz

÷2 (01)

24 MHz

÷3 (10)16 MHz

÷4 (11)12 MHz

48 MHz

÷12 (111)

EC, ECIO

None (00)48 MHz

÷2 (01)

24 MHz

÷3 (10)16 MHz

÷4 (11)12 MHz

ECPLL, ECPIO

÷2 (00)48 MHz

÷3 (01)32 MHz

÷4 (10)

24 MHz

÷6 (11)16 MHz

40 MHz

÷10 (110)

EC, ECIO

None (00)40 MHz

÷2 (01)20 MHz

÷3 (10)

13.33 MHz

÷4 (11)10 MHz

ECPLL, ECPIO

÷2 (00)48 MHz

÷3 (01)32 MHz

÷4 (10)

24 MHz

÷6 (11)16 MHz

24 MHz

÷6 (101)

HS, EC, ECIO

None (00)

24 MHz

÷2 (01)12 MHz

÷3 (10)8 MHz

÷4 (11)6 MHz

HSPLL, ECPLL, ECPIO

÷2 (00)48 MHz

÷3 (01)32 MHz

÷4 (10)

24 MHz

÷6 (11)16 MHz

Legend:

All clock frequencies, except 24 MHz, are exclusively associated with full-speed USB operation (USB clock of 48 MHz).

Bold is used to highlight clock selections that are compatible with low-speed USB operation (system clock of 24 MHz,

USB clock of 6 MHz).

Note 1:

Only valid when the USBDIV Configuration bit is cleared.

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