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PIC16C6X
DS30234D-page 72
1997 Microchip Technology Inc.
8.1
Timer1 Operation in Timer Mode
Timer mode is selected by clearing bit TMR1CS
(T1CON<1>). In this mode, the input clock to the timer
is Fosc/4. The synchronize control bit T1SYNC
(T1CON<2>) has no effect since the internal clock is
always in sync.
8.2
Timer1 Operation in Synchronized
Counter Mode
Counter mode is selected by setting bit TMR1CS. In
this mode the timer increments on every rising edge of
clock input on T1OSI when enable bit T1OSCEN is set
or pin with T1CKI 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 pres-
caler stage is an asynchronous ripple counter.
In this configuration, during SLEEP mode, Timer1 will
not increment even if an external clock is present, since
the synchronization circuit is shut off. The prescaler,
however, will continue to increment.
Applicable Devices
61 62 62A R62 63 R63 64 64A R64 65 65A R65 66 67
Applicable Devices
61 62 62A R62 63 R63 64 64A R64 65 65A R65 66 67
Note:
The T1OSI function is multiplexed to differ-
ent pins, depending on the device. See the
pinout descriptions to see which pin has
the T1OSI function.
8.2.1
EXTERNAL CLOCK INPUT TIMING FOR
SYNCHRONIZED COUNTER MODE
When an external clock input is used for Timer1 in syn-
chronized counter mode, it must meet certain require-
ments. The external clock requirement is due to
internal phase clock (Tosc) synchronization. Also, there
is a delay in the actual incrementing of TMR1 after syn-
chronization.
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 2Tosc (and
a small RC delay of 20 ns) and low for at least 2Tosc
(and a small RC delay of 20 ns). Refer to appropriate
electrical specification section, 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
symmetrical. 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 4Tosc (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 applicable electrical specification sec-
tion, parameters 40, 42, 45, 46, and 47.
FIGURE 8-2:
TIMER1 BLOCK DIAGRAM
TMR1H
TMR1L
T1OSC
T1SYNC
TMR1CS
T1CKPS1:T1CKPS0
SLEEP input
T1OSCEN
Enable
Oscillator
(1)
TMR1IF
Overflow
Interrupt
flag bit
Fosc/4
Internal
Clock
TMR1ON
on/off
Prescaler
1, 2, 4, 8
Synchronize
det
1
0
0
1
Synchronized
clock input
2
T1OSO
(2)
T1OSI
(2)
TMR1
(3)
Note 1:
When enable bit T1OSCEN is cleared, the inverter and feedback resistor are turned off. This eliminates power drain.
See pinouts for pins with T1OSO and T1OSI functions.
For the PIC16C62/64/65, the Schmitt Trigger is not implemented in external clock mode.
2:
3: