Microchip Technology MCP3421DM-WS Data Sheet
PIC18F2455/2550/4455/4550
DS39632E-page 30
© 2009 Microchip Technology Inc.
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.
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
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.
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
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.
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).
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.