PIC18C858T-I/PT【PDF 23/246 页】IC MCU OTP 16KX16 CAN 80TQFP

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2000 Microchip Technology Inc.

Advanced Information

DS30475A-page 119

PIC18CXX8

11.2

Timer1 Oscillator

A crystal oscillator circuit is built-in between pins T1OSI

(input) and T1OSO (amplifier output). It is enabled by

setting control bit T1OSCEN (T1CON register). The

oscillator is a low power oscillator rated up to 200 kHz.

It will continue to run during SLEEP. It is primarily

intended for a 32 kHz crystal. Table 11-1 shows the

capacitor selection for the Timer1 oscillator.

The user must provide a software time delay to ensure

proper start-up of the Timer1 oscillator.

11.3

Timer1 Interrupt

The TMR1 Register pair (TMR1H:TMR1L) increments

from 0000h to FFFFh and rolls over to 0000h. The

TMR1 Interrupt, if enabled, is generated on overflow,

which is latched in interrupt flag bit TMR1IF (PIR regis-

ters). This interrupt can be enabled/disabled by set-

ting/clearing TMR1 interrupt enable bit TMR1IE (PIE

registers).

11.4

Resetting Timer1 using a CCP Trigger

Output

If the CCP module is configured in Compare mode

generate

“special

event

trigger"

(CCP1M3:CCP1M0 = 1011), this signal will reset

Timer1 and start an A/D conversion (if the A/D module

is enabled).

Timer1 must be configured for either timer or Synchro-

nized Counter mode to take advantage of this feature.

If Timer1 is running in Asynchronous Counter mode,

this RESET operation may not work.

In the event that a write to Timer1 coincides with a spe-

cial event trigger from CCP1, the write will take prece-

dence.

In this mode of operation, the CCPR1H:CCPR1L regis-

ters pair, effectively becomes the period register for

Timer1.

11.5

Timer1 16-Bit Read/Write Mode

Timer1 can be configured for 16-bit reads and writes

(see Figure 11-2). When the RD16 control bit (T1CON

buffer register for the high byte of Timer1. A read from

TMR1L will load the contents of the high byte of Timer1

into the Timer1 high byte buffer. This provides the user

with the ability to accurately read all 16 bits of Timer1,

without having to determine whether a read of the high

byte followed by a read of the low byte is valid, due to

a rollover between reads.

A write to the high byte of Timer1 must also take place

through the TMR1H buffer register. Timer1 high byte is

updated with the contents of TMR1H when a write

occurs to TMR1L. This allows a user to write all 16 bits

to both the high and low bytes of Timer1 at once.

The high byte of Timer1 is not directly readable or writ-

able in this mode. All reads and writes must take place

through the Timer1 high byte buffer register. Writes to

TMR1H do not clear the Timer1 prescaler. The

prescaler is only cleared on writes to TMR1L.

TABLE 11-1: CAPACITOR SELECTION FOR

THE ALTERNATE OSCILLATOR

Osc Type

Freq

32 kHz

TBD(1)

Crystal to be Tested:

32.768 kHz

Epson C-001R32.768K-A

± 20 PPM

Note 1: Microchip suggests 33 pF as a starting

point in validating the oscillator circuit.

2: Higher capacitance increases the stability

of the oscillator, but also increases the

start-up time.

3: Since each resonator/crystal has its own

characteristics, the user should consult the

resonator/crystal manufacturer for appropri-

ate values of external components.

4: Capacitor values are for design guidance

only.

Note:

The special event triggers from the CCP1

module will not set interrupt flag bit

TMR1IF (PIR registers).

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