PIC18F2331 Datasheet, PDF (45/396 Pages) Microchip Technology – 28/40/44-Pin Enhanced Flash Microcontrollers with nanoWatt Technology, High Performance PWM and A/D

PIC18F2331/2431/4331/4431
3.6.1 EXAMPLE – USART
An adjustment may be indicated when the USART
begins to generate framing errors, or receives data with
errors while in Asynchronous mode. Framing errors
indicate that the system clock frequency is too high –
try decrementing the value in the OSCTUNE register to
reduce the system clock frequency. Errors in data
may suggest that the system clock speed is too low –
increment OSCTUNE.
3.6.2 EXAMPLE – TIMERS
This technique compares system 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 – decrement OSCTUNE.
3.6.3
EXAMPLE – CCP IN CAPTURE
MODE
A CCP module can use free running Timer1, 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 – decrement OSCTUNE. If the measured time
is much less than the calculated time, the internal
oscillator block is running too slow – increment
OSCTUNE.
 2003 Microchip Technology Inc.
Preliminary
DS39616B-page 43