PIC18F2X1X Datasheet, PDF (29/380 Pages) Microchip Technology – 28/40/44-PIN FLASH MICROCONTROLLERS WITH 10-BIT A/D AND NANO WATT TECHNOLOGY

PIC18F2X1X/4X1X
REGISTER 2-1:
OSCTUNE: OSCILLATOR TUNING REGISTER
R/W-0 R/W-0⁽¹⁾
U-0
INTSRC PLLEN⁽¹⁾
—
R/W-0
TUN4
R/W-0
TUN3
bit 7
R/W-0
TUN2
R/W-0
TUN1
R/W-0
TUN0
bit 0
bit 7
bit 6
bit 5
bit 4-0
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
PLLEN: Frequency Multiplier PLL for INTOSC Enable bit⁽¹⁾
1 = PLL enabled for INTOSC (4 MHz and 8 MHz only)
0 = PLL disabled
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.
Unimplemented: Read as ‘0’
TUN4:TUN0: Frequency Tuning bits
01111 = Maximum frequency
•
•
•
•
00001
00000 = Center frequency. Oscillator module is running at the calibrated frequency.
11111
•
•
•
•
10000 = Minimum frequency
Legend:
R = Readable bit
-n = Value at POR
W = Writable bit
‘1’ = Bit is set
U = Unimplemented bit, read as ‘0’
‘0’ = Bit is cleared x = Bit is unknown
2.6.5.1 Compensating with the USART
An adjustment may be required when the USART
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 calcu-
lated 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.
 2004 Microchip Technology Inc.
Preliminary
DS39636A-page 27