PIC18F2455_07 Datasheet, PDF (31/430 Pages) Microchip Technology – 28/40/44-Pin, High Performance, Enhanced Flash, USB Microcontrollers with nanoWatt Technology

PIC18F2455/2550/4455/4550
2.3 Oscillator Settings for USB
When the PIC18F4550 is used for USB connectivity, it
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:
Note 1:
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).
Only valid when the USBDIV Configuration bit is cleared.
© 2007 Microchip Technology Inc.
Preliminary
DS39632D-page 29