Datasheet

PIC24FJ256GB110 FAMILY
DS39897C-page 128  2009 Microchip Technology Inc.
8.5 Oscillator Modes and USB
Operation
Because of the timing requirements imposed by USB,
an internal clock of 48 MHz is required at all times while
the USB module is enabled. Since this is well beyond
the maximum CPU clock speed, a method is provided
to internally generate both the USB and system clocks
from a single oscillator source. PIC24FJ256GB110
family devices use the same clock structure as other
PIC24FJ devices, but include a two-branch PLL system
to generate the two clock signals.
The USB PLL block is shown in Figure 8-2. In this
system, the input from the Primary Oscillator is divided
down by a PLL prescaler to generate a 4 MHz output.
This is used to drive an on-chip 96 MHz PLL frequency
multiplier to drive the two clock branches. One branch
uses a fixed divide-by-2 frequency divider to generate
the 48 MHz USB clock. The other branch uses a fixed
divide-by-3 frequency divider and configurable PLL
prescaler/divider to generate a range of system clock
frequencies. The CPDIV bits select the system clock
speed; available clock options are listed in Table 8-2.
The USB PLL prescaler does not automatically sense
the incoming oscillator frequency. The user must man-
ually configure the PLL divider to generate the required
4 MHz output, using the PLLDIV<2:0> Configuration
bits. This limits the choices for Primary Oscillator fre-
quency to a total of 8 possibilities, shown in Table 8-3.
TABLE 8-2: SYSTEM CLOCK OPTIONS
DURING USB OPERATION
TABLE 8-3: VALID PRIMARY OSCILLATOR
CONFIGURATIONS FOR USB
OPERATIONS
FIGURE 8-2: USB PLL BLOCK
MCU Clock Division
(CPDIV<1:0>)
Microcontroller
Clock Frequency
None (00)32MHz
2 (01)16MHz
4 (10)8MHz
8 (11)4MHz
Input Oscillator
Frequency
Clock Mode
PLL Division
(PLLDIV<2:0>)
48 MHz ECPLL 12 (111)
40 MHz ECPLL 10 (110)
24 MHz HSPLL, ECPLL 6 (101)
20 MHz HSPLL, ECPLL 5 (100)
16 MHz HSPLL, ECPLL 4 (011)
12 MHz HSPLL, ECPLL 3 (010)
8 MHz ECPLL, XTPLL 2 (001)
4 MHz ECPLL, XTPLL 1 (000)
PLL
96 MHz
PLL
 3
 2
Prescaler
4 MHz
PLL
Prescaler
48 MHz Clock
for USB Module
PLL Output
for System Clock
CPDIV<1:0>
PLLDIV<2:0>
Input from
POSC
Input from
FRC
FNOSC<2:0>
(4 MHz or
8 MHz)
00
01
10
11
32 MHz
111
110
101
100
011
010
001
000
 12
 8
 10
 6
 5
 4
 3
 2
 1
 4
 2
 1
PLLDIS