Datasheet

© 2008 Microchip Technology Inc. DS39626E-page 27
PIC18F2525/2620/4525/4620
2.6.5.1 Compensating with the EUSART
An adjustment may be required when the EUSART
begins to generate framing errors or receives data with
errors while in Asynchronous mode. Framing errors
indicate that the device clock frequency is too high; to
adjust for this, decrement the value in OSCTUNE to
reduce the clock frequency. On the other hand, errors
in data may suggest that the clock speed is too low; to
compensate, increment OSCTUNE to increase the
clock frequency.
2.6.5.2 Compensating with the Timers
This technique compares device clock speed to some
reference clock. Two timers may be used; one timer is
clocked by the peripheral clock, while the other is
clocked by a fixed reference source, such as the
Timer1 oscillator.
Both timers are cleared, but the timer clocked by the
reference generates interrupts. When an interrupt
occurs, the internally clocked timer is read and both
timers are cleared. If the internally clocked timer value
is greater than expected, then the internal oscillator
block is running too fast. To adjust for this, decrement
the OSCTUNE register.
2.6.5.3 Compensating with the CCP Module
in Capture Mode
A CCP module can use free-running Timer1 (or
Timer3), clocked by the internal oscillator block and an
external event with a known period (i.e., AC power
frequency). The time of the first event is captured in the
CCPRxH:CCPRxL registers and is recorded for use
later. When the second event causes a capture, the
time of the first event is subtracted from the time of the
second event. Since the period of the external event is
known, the time difference between events can be
calculated.
If the measured time is much greater than the
calculated time, the internal oscillator block is running
too fast; to compensate, decrement the OSCTUNE
register. If the measured time is much less than the
calculated time, the internal oscillator block is running
too slow; to compensate, increment the OSCTUNE
register.
REGISTER 2-1: OSCTUNE: OSCILLATOR TUNING REGISTER
R/W-0 R/W-0
(1)
U-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0
INTSRC PLLEN
(1)
— TUN4 TUN3 TUN2 TUN1 TUN0
bit 7 bit 0
Legend:
R = Readable bit W = Writable bit U = Unimplemented bit, read as ‘0’
-n = Value at POR ‘1’ = Bit is set ‘0’ = Bit is cleared x = Bit is unknown
bit 7 INTSRC: Internal Oscillator Low-Frequency Source Select bit
1 = 31.25 kHz device clock derived from 8 MHz INTOSC source (divide-by-256 enabled)
0 = 31 kHz device clock derived directly from INTRC internal oscillator
bit 6 PLLEN: Frequency Multiplier PLL for INTOSC Enable bit
(1)
1 = PLL enabled for INTOSC (4 MHz and 8 MHz only)
0 = PLL disabled
bit 5 Unimplemented: Read as ‘0’
bit 4-0 TUN4:TUN0: Frequency Tuning bits
011111 = Maximum frequency
• •
• •
000001
000000 = Center frequency. Oscillator module is running at the calibrated frequency.
111111
• •
• •
100000 = Minimum frequency
Note 1: Available only in certain oscillator configurations; otherwise, this bit is unavailable and reads as ‘0’. See
Section 2.6.4 “PLL in INTOSC Modes” for details.