PIC16C925T-SSLASHCL datasheet(51/182 Pages) MICROCHIP

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 2001 Microchip Technology Inc.

Preliminary

DS39544A-page 49

PIC16C925/926

6.3

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 which will wake-up

the processor. However, special precautions in soft-

ware are needed to read from, or write to the Timer1

In Asynchronous Counter mode, Timer1 cannot be

used as a time-base for capture or compare opera-

tions.

6.3.1

EXTERNAL CLOCK INPUT TIMING

WITH UNSYNCHRONIZED CLOCK

If control bit T1SYNC is set, the timer will increment

completely asynchronously. The input clock must meet

certain minimum high time and low time requirements,

as specified in timing parameters 45, 46, and 47.

6.3.2

READING AND WRITING TMR1 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

dictable value in the timer register.

Reading

the

16-bit

value

requires

some

care.

Example 6-1 is an example routine to read the 16-bit

timer value. This is useful if the timer cannot be

stopped.

EXAMPLE 6-1:

READING A 16-BIT FREE-RUNNING TIMER

; 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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