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1996 Microchip Technology Inc.

DS30412C-page 69

PIC17C4X

11.3

Read/Write Consideration for TMR0

Although TMR0 is a 16-bit timer/counter, only 8-bits at

a time can be read or written during a single instruction

cycle. Care must be taken during any read or write.

11.3.1

READING 16-BIT VALUE

The problem in reading the entire 16-bit value is that

after reading the low (or high) byte, its value may

change from FFh to 00h.

Example 11-1 shows a 16-bit read. To ensure a proper

read, interrupts must be disabled during this routine.

EXAMPLE 11-1: 16-BIT READ

MOVPF

TMR0L, TMPLO

;read low tmr0

MOVPF

TMR0H, TMPHI

;read high tmr0

MOVFP

TMPLO, WREG

;tmplo

> wreg

CPFSLT

TMR0L

;tmr0l < wreg?

RETURN

;no then return

MOVPF

TMR0L, TMPLO

;read low tmr0

MOVPF

TMR0H, TMPHI

;read high tmr0

RETURN

;return

11.3.2

WRITING A 16-BIT VALUE TO TMR0

Since writing to either TMR0L or TMR0H will effectively

inhibit increment of that half of the TMR0 in the next

cycle (following write), but not inhibit increment of the

other half, the user must write to TMR0L rst and

TMR0H next in two consecutive instructions, as shown

in Example 11-2. The interrupt must be disabled. Any

write to either TMR0L or TMR0H clears the prescaler.

EXAMPLE 11-2: 16-BIT WRITE

BSF

CPUSTA, GLINTD ; Disable interrupt

MOVFP RAM_L, TMR0L

;

MOVFP RAM_H, TMR0H

;

BCF

CPUSTA, GLINTD ; Done, enable interrupt

11.4

Prescaler Assignments

Timer0 has an 8-bit prescaler. The prescaler assign-

ment is fully under software control; i.e., it can be

changed “on the y” during program execution. When

changing the prescaler assignment, clearing the pres-

caler is recommended before changing assignment.

The value of the prescaler is “unknown,” and assigning

a value that is less then the present value makes it dif-

cult to take this unknown time into account.

FIGURE 11-4: TMR0 TIMING: WRITE HIGH OR LOW BYTE

Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4

Q1 Q2 Q3 Q4

AD15:AD0

ALE

TMR0L

TMR0H

MOVFP W,TMR0L

Write to TMR0L

MOVFP TMR0L,W

Read TMR0L

(Value = NT0)

MOVFP TMR0L,W

Read TMR0L

(Value = NT0)

MOVFP TMR0L,W

Read TMR0L

(Value = NT0 +1)

T0+1

New T0 (NT0)

New T0+1

PC+1

PC+2

PC+3

PC+4

Fetch

Instruction

executed

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