PIC16C923-04I/CL Microchip Technology, PIC16C923-04I/CL Datasheet - Page 53

• Manufacturer Part Number: PIC16C923-04I/CL
• Description: 8-Bit CMOS Microcontroller with LCD Driver
• Manufacturer: Microchip Technology

8.3
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-from or write-to the Timer1
register pair (TMR1H:TMR1L) (Section 8.3.2).
In asynchronous counter mode, Timer1 cannot be used
as a time-base for capture or compare operations.
8.3.1
If control bit T1SYNC is set, the timer will increment
completely asynchronously. The input clock must meet
certain minimum high time and low time requirements,
as specified in timing parameters 45, 46, and 47.
8.3.2
Reading TMR1H or TMR1L while the timer is running,
from an external asynchronous clock, will ensure a
valid read (taken care of in hardware). However, the
user should keep in mind that reading the 16-bit timer
in two 8-bit values itself poses certain problems 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
register is incrementing. This may produce an unpre-
dictable value in the timer register.
Reading the 16-bit value requires some care.
Example 8-1 is an example routine to read the 16-bit
timer value. This is useful if the timer cannot be
stopped.
1997 Microchip Technology Inc.
Timer1 Operation in Asynchronous
Counter Mode
EXTERNAL CLOCK INPUT TIMING WITH
UNSYNCHRONIZED CLOCK
READING AND WRITING TMR1 IN
ASYNCHRONOUS COUNTER MODE
EXAMPLE 8-1:
; All interrupts are disabled
;
; 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.
;
; Re-enable the Interrupt (if required)
CONTINUE
8.4
A crystal oscillator circuit is built in between pins T1OSI
(input) and T1OSO (amplifier output). It is enabled by
setting control bit T1OSCEN (T1CON<3>). The oscilla-
tor is a low power oscillator rated up to 200 kHz. It will
continue to run during SLEEP. It is primarily intended
for a 32 kHz crystal. Table 8-1 shows the capacitor
selection for the Timer1 oscillator.
The Timer1 oscillator is identical to the LP oscillator.
The user must provide a software time delay to ensure
proper oscillator start-up.
TABLE 8-1: CAPACITOR SELECTION FOR
Crystals Tested:
32.768 kHz
100 kHz
200 kHz
Note 1: Higher capacitance increases the stability
Osc Type
MOVF
MOVWF
MOVF
MOVWF
MOVF
SUBWF
BTFSC
GOTO
MOVF
MOVWF
MOVF
MOVWF
LP
These values are for design guidance only.
2: Since each resonator/crystal has its own
Timer1 Oscillator
of oscillator but also increases the start-up
time.
characteristics, the user should consult the
resonator/crystal manufacturer for appropri-
ate values of external components.
TMR1H, W
TMPH
TMR1L, W
TMPL
TMR1H, W
TMPH,
STATUS,Z
CONTINUE
TMR1H, W
TMPH
TMR1L, W
TMPL
Epson C-001R32.768K-A
Epson C-2 100.00 KC-P
STD XTL 200.000 kHz
THE TIMER1 OSCILLATOR
100 kHz
200 kHz
32 kHz
Freq
READING A 16-BIT
FREE-RUNNING TIMER
W
;Read high byte
;
;Read low byte
;
;Read high byte
;Sub 1st read
; with 2nd read
;Is result = 0
;Good 16-bit read
;Read high byte
;
;Read low byte
;
;Continue with your code
PIC16C9XX
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