PIC18F452-E/PT Microchip Technology Inc., PIC18F452-E/PT Datasheet - Page 31

• Manufacturer Part Number: PIC18F452-E/PT
• Description: 44 PIN, 32 KB FLASH, 1536 RAM, 34 I/O
• Manufacturer: Microchip Technology Inc.

Specifications of PIC18F452-E/PT

• A/d Inputs: 8-Channel, 10-Bit
• Comparators: 2
• Cpu Speed: 10 MIPS
• Eeprom Memory: 256 Bytes
• Input Output: 36
• Interface: I2C/SPI/USART
• Memory Type: Flash
• Number Of Bits: 8
• Package Type: 44-pin TQFP
• Programmable Memory: 32K Bytes
• Ram Size: 1.5K Bytes
• Speed: 40 MHz
• Timers: 1-8-bit, 3-16-bit
• Voltage, Range: 2-5.5 V
• Lead Free Status / Rohs Status: RoHS Compliant part Electrostatic Device

2.7.1
The OSCCON register (Register 2-2) controls several
aspects of the device clock’s operation, both in full
power operation and in power managed modes.
The System Clock Select bits, SCS1:SCS0, select the
clock source. The available clock sources are the
primary clock (defined by the FOSC3:FOSC0 configu-
ration bits), the secondary clock (Timer1 oscillator) and
the internal oscillator block. The clock source changes
immediately after one or more of the bits is written to,
following a brief clock transition interval. The SCS bits
are cleared on all forms of Reset.
The
(IRCF2:IRCF0) select the frequency output of the
internal oscillator block to drive the device clock. The
choices are the INTRC source, the INTOSC source
(8 MHz) or one of the frequencies derived from the
INTOSC postscaler (31.25 kHz to 4 MHz). If the
internal oscillator block is supplying the device clock,
changing the states of these bits will have an immedi-
ate change on the internal oscillator’s output. On
device Resets, the default output frequency of the
internal oscillator block is set at 1 MHz.
When a nominal output frequency of 31 kHz is selected
(IRCF2:IRCF0 = 000), users may choose which inter-
nal oscillator acts as the source. This is done with the
INTSRC bit in the OSCTUNE register (OSCTUNE<7>).
Setting this bit selects INTOSC as a 31.25 kHz clock
source by enabling the divide-by-256 output of the
INTOSC postscaler. Clearing INTSRC selects INTRC
(nominally 31 kHz) as the clock source.
This option allows users to select the tunable and more
precise INTOSC as a clock source, while maintaining
power savings with a very low clock speed. Regardless
of the setting of INTSRC, INTRC always remains the
clock source for features such as the Watchdog Timer
and the Fail-Safe Clock Monitor.
The OSTS, IOFS and T1RUN bits indicate which clock
source is currently providing the device clock. The
OSTS bit indicates that the Oscillator Start-up Timer
has timed out and the primary clock is providing the
device clock in primary clock modes. The IOFS bit
indicates when the internal oscillator block has stabi-
lized and is providing the device clock in RC Clock
modes. The T1RUN bit (T1CON<6>) indicates when
the Timer1 oscillator is providing the device clock in
secondary clock modes. In power managed modes,
only one of these three bits will be set at any time. If
none of these bits are set, the INTRC is providing the
clock or the internal oscillator block has just started and
is not yet stable.
 2004 Microchip Technology Inc.
Internal
OSCILLATOR CONTROL REGISTER
Oscillator
Frequency
Select
PIC18F2420/2520/4420/4520
Preliminary
bits
The IDLEN bit determines if the device goes into Sleep
mode or one of the Idle modes when the SLEEP
instruction is executed.
The use of the flag and control bits in the OSCCON
register is discussed in more detail in Section 3.0
“Power Managed Modes”.
2.7.2
PIC18F2420/2520/4420/4520 devices contain circuitry
to prevent clock “glitches” when switching between
clock sources. A short pause in the device clock occurs
during the clock switch. The length of this pause is the
sum of two cycles of the old clock source and three to
four cycles of the new clock source. This formula
assumes that the new clock source is stable.
Clock transitions are discussed in greater detail in
Section 3.1.2 “Entering Power Managed Modes”.
Note 1: The Timer1 oscillator must be enabled to
2: It is recommended that the Timer1
OSCILLATOR TRANSITIONS
select the secondary clock source. The
Timer1 oscillator is enabled by setting the
T1OSCEN bit in the Timer1 Control regis-
ter (T1CON<3>). If the Timer1 oscillator
is not enabled, then any attempt to select
a secondary clock source will be ignored.
oscillator be operating and stable before
selecting the secondary clock source or a
very long delay may occur while the
Timer1 oscillator starts.
DS39631A-page 29