Microchip Technology GPIODM-KPLCD Data Sheet
© 2009 Microchip Technology Inc.
DS39632E-page 23
PIC18F2455/2550/4455/4550
2.0
OSCILLATOR
CONFIGURATIONS
CONFIGURATIONS
2.1
Overview
Devices in the PIC18F2455/2550/4455/4550 family
incorporate a different oscillator and microcontroller
clock system than previous PIC18F devices. The addi-
tion of the USB module, with its unique requirements
for a stable clock source, make it necessary to provide
a separate clock source that is compliant with both
USB low-speed and full-speed specifications.
To accommodate these requirements, PIC18F2455/
2550/4455/4550 devices include a new clock branch to
provide a 48 MHz clock for full-speed USB operation.
Since it is driven from the primary clock source, an
additional system of prescalers and postscalers has
been added to accommodate a wide range of oscillator
frequencies. An overview of the oscillator structure is
shown in Figure 2-1.
Other oscillator features used in PIC18 enhanced
microcontrollers, such as the internal oscillator block
and clock switching, remain the same. They are
discussed later in this chapter.
incorporate a different oscillator and microcontroller
clock system than previous PIC18F devices. The addi-
tion of the USB module, with its unique requirements
for a stable clock source, make it necessary to provide
a separate clock source that is compliant with both
USB low-speed and full-speed specifications.
To accommodate these requirements, PIC18F2455/
2550/4455/4550 devices include a new clock branch to
provide a 48 MHz clock for full-speed USB operation.
Since it is driven from the primary clock source, an
additional system of prescalers and postscalers has
been added to accommodate a wide range of oscillator
frequencies. An overview of the oscillator structure is
shown in Figure 2-1.
Other oscillator features used in PIC18 enhanced
microcontrollers, such as the internal oscillator block
and clock switching, remain the same. They are
discussed later in this chapter.
2.1.1
OSCILLATOR CONTROL
The operation of the oscillator in PIC18F2455/2550/
4455/4550 devices is controlled through two Configu-
ration registers and two control registers. Configuration
registers, CONFIG1L and CONFIG1H, select the
oscillator mode and USB prescaler/postscaler options.
As Configuration bits, these are set when the device is
programmed and left in that configuration until the
device is reprogrammed.
The OSCCON register (Register 2-2) selects the Active
Clock mode; it is primarily used in controlling clock
switching in power-managed modes. Its use is
discussed in Section 2.4.1 “Oscillator Control
Register”.
The OSCTUNE register (Register 2-1) is used to trim
the INTRC frequency source, as well as select the
low-frequency clock source that drives several special
features. Its use is described in Section 2.2.5.2
“OSCTUNE Register”.
4455/4550 devices is controlled through two Configu-
ration registers and two control registers. Configuration
registers, CONFIG1L and CONFIG1H, select the
oscillator mode and USB prescaler/postscaler options.
As Configuration bits, these are set when the device is
programmed and left in that configuration until the
device is reprogrammed.
The OSCCON register (Register 2-2) selects the Active
Clock mode; it is primarily used in controlling clock
switching in power-managed modes. Its use is
discussed in Section 2.4.1 “Oscillator Control
Register”.
The OSCTUNE register (Register 2-1) is used to trim
the INTRC frequency source, as well as select the
low-frequency clock source that drives several special
features. Its use is described in Section 2.2.5.2
“OSCTUNE Register”.
2.2
Oscillator Types
PIC18F2455/2550/4455/4550 devices can be operated
in twelve distinct oscillator modes. In contrast with pre-
vious PIC18 enhanced microcontrollers, four of these
modes involve the use of two oscillator types at once.
Users can program the FOSC3:FOSC0 Configuration
bits to select one of these modes:
1.
in twelve distinct oscillator modes. In contrast with pre-
vious PIC18 enhanced microcontrollers, four of these
modes involve the use of two oscillator types at once.
Users can program the FOSC3:FOSC0 Configuration
bits to select one of these modes:
1.
XT
Crystal/Resonator
2.
HS
High-Speed Crystal/Resonator
3.
HSPLL High-Speed Crystal/Resonator
with PLL Enabled
4.
EC
External Clock with F
OSC
/4 Output
5.
ECIO
External Clock with I/O on RA6
6.
ECPLL External Clock with PLL Enabled
and F
OSC
/4 Output on RA6
7.
ECPIO External Clock with PLL Enabled,
I/O on RA6
8.
INTHS Internal Oscillator used as
Microcontroller Clock Source, HS
Oscillator used as USB Clock Source
Oscillator used as USB Clock Source
9.
INTIO
Internal Oscillator used as
Microcontroller Clock Source, EC
Oscillator used as USB Clock Source,
Digital I/O on RA6
Microcontroller Clock Source, EC
Oscillator used as USB Clock Source,
Digital I/O on RA6
10. INTCKO Internal Oscillator used as
Microcontroller Clock Source, EC
Oscillator used as USB Clock Source,
F
Oscillator used as USB Clock Source,
F
OSC
/4 Output on RA6
2.2.1
OSCILLATOR MODES AND
USB OPERATION
USB OPERATION
Because of the unique requirements of the USB module,
a different approach to clock operation is necessary. In
previous PIC
a different approach to clock operation is necessary. In
previous PIC
®
devices, all core and peripheral clocks
were driven by a single oscillator source; the usual
sources were primary, secondary or the internal oscilla-
tor. With PIC18F2455/2550/4455/4550 devices, the pri-
mary oscillator becomes part of the USB module and
cannot be associated to any other clock source. Thus,
the USB module must be clocked from the primary clock
source; however, the microcontroller core and other
peripherals can be separately clocked from the
secondary or internal oscillators as before.
Because of the timing requirements imposed by USB,
an internal clock of either 6 MHz or 48 MHz is required
while the USB module is enabled. Fortunately, the
microcontroller and other peripherals are not required
to run at this clock speed when using the primary
oscillator. There are numerous options to achieve the
USB module clock requirement and still provide flexibil-
ity for clocking the rest of the device from the primary
oscillator source. These are detailed in Section 2.3
“Oscillator Settings for USB”.
sources were primary, secondary or the internal oscilla-
tor. With PIC18F2455/2550/4455/4550 devices, the pri-
mary oscillator becomes part of the USB module and
cannot be associated to any other clock source. Thus,
the USB module must be clocked from the primary clock
source; however, the microcontroller core and other
peripherals can be separately clocked from the
secondary or internal oscillators as before.
Because of the timing requirements imposed by USB,
an internal clock of either 6 MHz or 48 MHz is required
while the USB module is enabled. Fortunately, the
microcontroller and other peripherals are not required
to run at this clock speed when using the primary
oscillator. There are numerous options to achieve the
USB module clock requirement and still provide flexibil-
ity for clocking the rest of the device from the primary
oscillator source. These are detailed in Section 2.3
“Oscillator Settings for USB”.