Datasheet

© 2009 Microchip Technology Inc. DS40044G-page 51
PIC16F627A/628A/648A
7.1 Timer1 Operation in Timer Mode
Timer mode is selected by clearing the TMR1CS
(T1CON<1>) bit. In this mode, the input clock to the
timer is F
OSC/4. The synchronize control bit T1SYNC
(T1CON<2>) has no effect since the internal clock is
always in sync.
7.2 Timer1 Operation in Synchronized
Counter Mode
Counter mode is selected by setting bit TMR1CS. In
this mode, the TMR1 register pair value increments on
every rising edge of clock input on pin RB7/T1OSI/PGD
when bit T1OSCEN is set or pin RB6/T1OSO/T1CKI/
PGC when bit T1OSCEN is cleared.
If T1SYNC
is cleared, then the external clock input is
synchronized with internal phase clocks. The synchro-
nization is done after the prescaler stage. The
prescaler stage is an asynchronous ripple-counter.
In this configuration, during Sleep mode, the TMR1
register pair value will not increment even if the
external clock is present, since the synchronization
circuit is shut off. The prescaler however will continue
to increment.
7.2.1 EXTERNAL CLOCK INPUT TIMING
FOR SYNCHRONIZED COUNTER
MODE
When an external clock input is used for Timer1 in
Synchronized Counter mode, it must meet certain
requirements. The external clock requirement is due to
internal phase clock (T
OSC) synchronization. Also,
there is a delay in the actual incrementing of the TMR1
register pair value after synchronization.
When the prescaler is 1:1, the external clock input is
the same as the prescaler output. The synchronization
of T1CKI with the internal phase clocks is accom-
plished by sampling the prescaler output on the Q2 and
Q4 cycles of the internal phase clocks. Therefore, it is
necessary for T1CKI to be high for at least 2 T
OSC (and
a small RC delay of 20 ns) and low for at least 2 T
OSC
(and a small RC delay of 20 ns). Refer to Table 17-8 in
the Electrical Specifications Section, timing parameters
45, 46 and 47.
When a prescaler other than 1:1 is used, the external
clock input is divided by the asynchronous ripple-counter
type prescaler so that the prescaler output is symmetri-
cal. In order for the external clock to meet the sampling
requirement, the ripple-counter must be taken into
account. Therefore, it is necessary for T1CKI to have a
period of at least 4 T
OSC (and a small RC delay of 40 ns)
divided by the prescaler value. The only requirement on
T1CKI high and low time is that they do not violate the
minimum pulse width requirements of 10 ns). Refer to
the appropriate electrical specifications in Table 17-8,
parameters 45, 46 and 47.
FIGURE 7-1: TIMER1 BLOCK DIAGRAM
TMR1H
TMR1L
T1OSC
T1SYNC
TMR1CS
T1CKPS<1:0>
Sleep Input
T1OSCEN
Enable
Oscillator
(1)
FOSC/4
Internal
Clock
TMR1ON
Prescaler
1, 2, 4, 8
Synchronize
det
1
0
0
1
Synchronized
Clock Input
2
RB6/T1OSO/T1CKI/PGC
RB7/T1OSI/PGD
Note 1: When the T1OSCEN bit is cleared, the inverter and feedback resistor are turned off. This eliminates power drain.
Set flag bit
TMR1IF on
Overflow
TMR1