PIC17C75X
12.3.2 WRITING A 16-BIT VALUE TO TMR0
12.3
Read/Write Consideration for 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.
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 12-2: 16-BIT WRITE
Example 12-1 shows a 16-bit read. To ensure a proper
read, interrupts must be disabled during this routine.
BSF
CPUSTA, GLINTD ; Disable interrupts
MOVFP
MOVFP
BCF
RAM_L, TMR0L
RAM_H, TMR0H
CPUSTA, GLINTD ; Done, enable
interrupts
;
;
EXAMPLE 12-1: 16-BIT READ
MOVPF
MOVPF
MOVFP
TMR0L, TMPLO
TMR0H, TMPHI
TMPLO, WREG
;read low tmr0
;read high tmr0
;tmplo −> wreg
;tmr0l < wreg?
;no then return
;read low tmr0
;read high tmr0
;return
;
CPFSLT TMR0L
RETURN
12.4
Prescaler Assignments
MOVPF
MOVPF
RETURN
TMR0L, TMPLO
TMR0H, TMPHI
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.
FIGURE 12-4: TMR0 TIMING: WRITE HIGH OR LOW BYTE
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
AD15:AD0
ALE
PC+1
PC+2
PC+3
PC+4
PC
T0
T0+1
New T0 (NT0)
New T0+1
TMR0L
Fetch
MOVFP W,TMR0L MOVFP TMR0L,W MOVFP TMR0L,W MOVFP TMR0L,W
Instruction
executed
Write to TMR0L
Read TMR0L
(Value = NT0)
Read TMR0L
(Value = NT0)
Read TMR0L
(Value = NT0 +1)
TMR0H
1997 Microchip Technology Inc.
Preliminary
DS30264A-page 89
MICROCHIP [ MICROCHIP ]
