PIC17C75X
13.2.4 EXTERNAL CLOCK INPUT FOR TIMER3
13.2.5 READING/WRITING TIMER3
When TMR3CS is set, the 16-bit TMR3 increments on
the falling edge of clock input TCLK3. The input on the
RB5/TCLK3 pin is sampled and synchronized by the
internal phase clocks twice every instruction cycle.This
causes a delay from the time a falling edge appears on
TCLK3 to the time TMR3 is actually incremented. For
the external clock input timing requirements, see the
Electrical Specification section. Figure 13-10 shows
the timing diagram when operating from an external
clock.
Since Timer3 is a 16-bit timer and only 8-bits at a time
can be read or written, care should be taken when
reading or writing while the timer is running. The best
method is to stop the timer, perform any read or write
operation, and then restart Timer3 (using the TMR3ON
bit). However, if it is necessary to keep Timer3 free-run-
ning, care must be taken. For writing to the 16-bit
TMR3, Example 13-2 may be used. For reading the
16-bit TMR3, Example 13-3 may be used. Interrupts
must be disabled during this routine.
EXAMPLE 13-2: WRITING TO TMR3
BSF
CPUSTA, GLINTD
RAM_L, TMR3L
RAM_H, TMR3H
CPUSTA, GLINTD
; Disable interrupts
;
;
MOVFP
MOVFP
BCF
; Done, enable interrupts
EXAMPLE 13-3: READING FROM TMR3
MOVPF
MOVPF
MOVFP
CPFSLT TMR3L, WREG
RETURN
MOVPF
MOVPF
RETURN
TMR3L, TMPLO
TMR3H, TMPHI
TMPLO, WREG
; read low TMR3
; read high TMR3
; tmplo −> wreg
; TMR3L < wreg?
; no then return
; read low TMR3
; read high TMR3
; return
TMR3L, TMPLO
TMR3H, TMPHI
FIGURE 13-10: TIMER1, TIMER2, AND TIMER3 OPERATION (IN COUNTER MODE)
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
TCLK12
or TCLK3
34h
35h
A8h
A9h
00h
TMR1, TMR2, or TMR3
PR1, PR2, or PR3H:PR3L
WR_TMR
'A9h'
'A9h'
RD_TMR
TMRxIF
MOVWF
TMRx
MOVFP
MOVFP
TMRx,W
TMRx,W
Instruction
executed
Write to TMRx
Read TMRx
Read TMRx
Note 1: TCLK12 is sampled in Q2 and Q4.
2: ↓ indicates a sampling point.
3: The latency from TCLK12 ↓ to timer increment is between 2Tosc and 6Tosc.
DS30264A-page 104
Preliminary
1997 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
