16F628 datasheet(52/168 Pages) MICROCHIP

52 / 168 page

PIC16F627A/628A/648A

DS40044A-page 50

Preliminary

 2002 Microchip Technology Inc.

7.3

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/write the timer (Section 7.3.2).

7.3.1

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 and low time requirements. Refer

to Table 17-9 in the Electrical Specifications Section,

timing parameters 45, 46, and 47.

7.3.2

READING AND WRITING TIMER1 IN

ASYNCHRONOUS COUNTER

MODE

Reading the TMR1H or TMR1L register while the timer

is running, from an external asynchronous clock, will

produce a valid read (taken care of in hardware). How-

ever, the user should keep in mind that reading the 16-

bit timer in two 8-bit values itself poses certain prob-

lems 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 7-1 is an example routine to read the 16-bit

timer value. This is useful if the timer cannot be

stopped.

EXAMPLE 7-1:

READING A 16-BIT FREE-

RUNNING TIMER

Note:

In Asynchronous Counter mode, Timer1

cannot be used as a time-base for capture

or compare operations.

; 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, W

;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 Interrupts (if required)

CONTINUE

;Continue with your

;code

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