PIC18F4550ESPQTP datasheet(31/438 Pages) MICROCHIP

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DS39632E-page 29

PIC18F2455/2550/4455/4550

2.2.5.4

Compensating for INTOSC Drift

It is possible to adjust the INTOSC frequency by

modifying the value in the OSCTUNE register. This has

no effect on the INTRC clock source frequency.

Tuning the INTOSC source requires knowing when to

make the adjustment, in which direction it should be

made and in some cases, how large a change is

needed. When using the EUSART, for example, an

adjustment may be required when it 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 sug-

gest that the clock speed is too low; to compensate,

increment OSCTUNE to increase the clock frequency.

It is also possible to verify device clock speed against

a 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.

Finally, 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.

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