PIC18LF4520ESLASHSPQTP datasheet(131/390 Pages) MICROCHIP

131 / 390 page

 2004 Microchip Technology Inc.

Preliminary

DS39631A-page 129

PIC18F2420/2520/4420/4520

12.2

Timer1 16-Bit Read/Write Mode

Timer1 can be configured for 16-bit reads and writes

(see

Figure 12-2).

When

the

RD16

control

bit

(T1CON<7>) is set, the address for TMR1H is mapped

to a 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, has

become invalid due to a rollover between reads.

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

through the TMR1H Buffer register. The 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

writable 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.

12.3

Timer1 Oscillator

An on-chip crystal oscillator circuit is incorporated

between pins T1OSI (input) and T1OSO (amplifier out-

put). It is enabled by setting the Timer1 Oscillator Enable

bit, T1OSCEN (T1CON<3>). The oscillator is a low-

power circuit rated for 32 kHz crystals. It will continue to

run during all power managed modes. The circuit for a

typical LP oscillator is shown in Figure 12-3. Table 12-1

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.

FIGURE 12-3:

EXTERNAL

COMPONENTS FOR THE

TIMER1 LP OSCILLATOR

TABLE 12-1:

CAPACITOR SELECTION FOR

THE TIMER OSCILLATOR

12.3.1

USING TIMER1 AS A

CLOCK SOURCE

The Timer1 oscillator is also available as a clock source

in power managed modes. By setting the clock select

bits, SCS1:SCS0 (OSCCON<1:0>), to ‘01’, the device

switches to SEC_RUN mode; both the CPU and

peripherals are clocked from the Timer1 oscillator. If the

IDLEN bit (OSCCON<7>) is cleared and a SLEEP

instruction is executed, the device enters SEC_IDLE

mode. Additional details are available in Section 3.0

“Power Managed Modes”.

Whenever the Timer1 oscillator is providing the clock

source, the Timer1 system clock status flag, T1RUN

(T1CON<6>), is set. This can be used to determine the

controller’s current clocking mode. It can also indicate

the clock source being currently used by the Fail-Safe

Clock Monitor. If the Clock Monitor is enabled and the

Timer1 oscillator fails while providing the clock, polling

the T1RUN bit will indicate whether the clock is being

provided by the Timer1 oscillator or another source.

12.3.2

LOW-POWER TIMER1 OPTION

The Timer1 oscillator can operate at two distinct levels

of power consumption based on device configuration.

When the LPT1OSC configuration bit is set, the Timer1

oscillator operates in a low-power mode. When

LPT1OSC is not set, Timer1 operates at a higher power

level. Power consumption for a particular mode is rela-

tively constant, regardless of the device’s operating

mode. The default Timer1 configuration is the higher

power mode.

As the low-power Timer1 mode tends to be more sen-

sitive to interference, high noise environments may

cause some oscillator instability. The low-power option

is, therefore, best suited for low noise applications

where power conservation is an important design

consideration.

Note:

See the Notes with Table 12-1 for additional

information about capacitor selection.

XTAL

PIC18FXXXX

T1OSI

T1OSO

32.768 kHz

27 pF

Osc Type

Freq

32 kHz

27 pF(1)

Note 1: Microchip suggests these values 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

appropriate

values

external

components.

4: Capacitor values are for design guidance

only.

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