PIC16F88-ISLASHSS datasheet(76/214 Pages) MICROCHIP

76 / 214 page

PIC16F87/88

DS30487B-page 74

Preliminary

 2003 Microchip Technology Inc.

7.5

Timer1 Operation in

Asynchronous Counter Mode

If control bit T1SYNC (T1CON<2>) is set, the external

clock input is not synchronized. The timer continues to

increment asynchronous to the internal phase clocks.

The timer will continue to run during SLEEP and can

generate an interrupt on overflow that will wake-up the

processor. However, special precautions in software

are needed to read/write the timer (Section 7.5.1

“Reading and Writing Timer1 in Asynchronous

Counter Mode”).

In Asynchronous Counter mode, Timer1 cannot be

used as a time base for capture or compare operations.

7.5.1

READING AND WRITING TIMER1 IN

ASYNCHRONOUS COUNTER

MODE

Reading TMR1H or TMR1L, while the timer is running

from an external asynchronous clock, will ensure a

valid read (taken care of in hardware). However, the

user should keep in mind that reading the 16-bit timer

in two 8-bit values itself, poses certain problems, since

the timer may overflow between the reads.

For writes, it is recommended that the user simply stop

the timer and write the desired values. A write conten-

tion may occur by writing to the timer registers while the

unpredictable value in the timer register.

Reading the 16-bit value requires some care. The

example

codes

provided

Example 7-1

and

Example 7-2 demonstrate how to write to and read

Timer1 while it is running in Asynchronous mode.

EXAMPLE 7-1:

WRITING A 16-BIT FREE-RUNNING TIMER

EXAMPLE 7-2:

READING A 16-BIT FREE-RUNNING TIMER

; All interrupts are disabled

CLRF

TMR1L

; Clear Low byte, Ensures no rollover into TMR1H

MOVLW

HI_BYTE

; Value to load into TMR1H

MOVWF

TMR1H, F

; Write High byte

MOVLW

LO_BYTE

; Value to load into TMR1L

MOVWF

TMR1H, F

; Write Low byte

; Re-enable the Interrupt (if required)

CONTINUE

; Continue with your code

; All interrupts are disabled

MOVF

TMR1H, W

; Read high byte

MOVWF

TMPH

MOVF

TMR1L, W

; Read low byte

MOVWF

TMPL

MOVF

TMR1H, W

; Read high byte

SUBWF

TMPH,

; Sub 1st read with 2nd read

BTFSC

STATUS,Z

; Is result = 0

GOTO

CONTINUE

; Good 16-bit read

; TMR1L may have rolled over between the read of the high and low bytes.

; Reading the high and low bytes now will read a good value.

MOVF

TMR1H, W

; Read high byte

MOVWF

TMPH

MOVF

TMR1L, W

; Read low byte

MOVWF

TMPL

; Re-enable the Interrupt (if required)

CONTINUE

; Continue with your code

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