PIC24FJ256GB210 Datasheet, PDF (147/386 Pages) Microchip Technology – 64/100-Pin, 16-Bit Flash Microcontrollers with USB On-The-Go (OTG)

PIC24FJ256GB210 FAMILY
8.5.3
CONSIDERATIONS FOR USB
OPERATION
When using the USB On-The-Go module in
PIC24FJ256GB210 family devices, users must always
observe these rules in configuring the system clock:
• For USB operation, the selected clock source
(EC, HS or XT) must meet the USB clock
tolerance requirements.
• The Primary Oscillator/PLL modes are the only
oscillator configurations that permit USB opera-
tion. There is no provision to provide a separate
external clock source to the USB module.
• While the FRCPLL Oscillator mode is used for
USB applications, users must always ensure that
the FRC source is configured to provide a
frequency of 4 MHz or 8 MHz (RCDIV<2:0> = 001
or 000) and that the USB PLL prescaler is
configured appropriately.
All other oscillator modes are available; however, USB
operation is not possible when these modes are
selected. They may still be useful in cases where other
power levels of operation are desirable and the USB
module is not needed (e.g., the application is sleeping
and waiting for a bus attachment).
8.6 Reference Clock Output
In addition to the CLKO output (FOSC/2) available in
certain oscillator modes, the device clock in the
PIC24FJ256GB210 family devices can also be config-
ured to provide a reference clock output signal to a port
pin. This feature is available in all oscillator configurations
and allows the user to select a greater range of clock
submultiples to drive external devices in the application.
This reference clock output is controlled by the
REFOCON register (Register 8-4). Setting the ROEN bit
(REFOCON<15>) makes the clock signal available on
the REFO pin. The RODIV bits (REFOCON<11:8>)
enable the selection of 16 different clock divider options.
The ROSSLP and ROSEL bits (REFOCON<13:12>)
control the availability of the reference output during
Sleep mode. The ROSEL bit determines if the oscillator
on OSCI and OSCO, or the current system clock
source, is used for the reference clock output. The
ROSSLP bit determines if the reference source is
available on REFO when the device is in Sleep mode.
To use the reference clock output in Sleep mode, both
the ROSSLP and ROSEL bits must be set. The device
clock must also be configured for one of the primary
modes (EC, HS or XT); otherwise, if the POSCEN bit is
not also set, the oscillator on OSCI and OSCO will be
powered down when the device enters Sleep mode.
Clearing the ROSEL bit allows the reference output
frequency to change as the system clock changes
during any clock switches.
 2010 Microchip Technology Inc.
DS39975A-page 147