PIC17C752 datasheet(89/320 Pages) MICROCHIP

89 / 320 page

Preliminary

DS30264A-page 89

PIC17C75X

12.3 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.

12.3.1

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 12-1 shows a 16-bit read. To ensure a proper

read, interrupts must be disabled during this routine.

EXAMPLE 12-1: 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

12.3.2

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 ﬁrst and

TMR0H second in two consecutive instructions, as

shown in Example 12-2. The interrupt must be dis-

abled. Any write to either TMR0L or TMR0H clears the

prescaler.

EXAMPLE 12-2: 16-BIT WRITE

12.4 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 ﬂy” 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-

ﬁcult to take this unknown time into account.

BSF

CPUSTA, GLINTD ; Disable interrupts

MOVFP

RAM_L, TMR0L

;

MOVFP

RAM_H, TMR0H

;

BCF

CPUSTA, GLINTD ; Done, enable

;

interrupts

FIGURE 12-4: 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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