Datasheet

© 2008 Microchip Technology Inc. DS41319B-page 33
PIC12F519
7.1 Using Timer0 with an External
Clock
When an external clock input is used for Timer0, it must
meet certain requirements. The external clock require-
ment is due to internal phase clock (TOSC) synchroniza-
tion. Also, there is a delay in the actual incrementing of
Timer0 after synchronization.
7.1.1 EXTERNAL CLOCK
SYNCHRONIZATION
When no prescaler is used, the external clock input is
the same as the prescaler output. The synchronization
of T0CKI with the internal phase clocks is accom-
plished by sampling the prescaler output on the Q2 and
Q4 cycles of the internal phase clocks (Figure 7-4).
Therefore, it is necessary for T0CKI to be high for at
least 2 T
OSC (and a small RC delay of 2 Tt0H) and low
for at least 2 T
OSC (and a small RC delay of 2 Tt0H).
Refer to the electrical specification of the desired
device.
When a prescaler is used, the external clock input is
divided by the asynchronous ripple counter-type
prescaler, so that the prescaler output is symmetrical.
For the external clock to meet the sampling require-
ment, the ripple counter must be taken into account.
Therefore, it is necessary for T0CKI to have a period of
at least 4 T
OSC (and a small RC delay of 4 Tt0H) divided
by the prescaler value. The only requirement on T0CKI
high and low time is that they do not violate the
minimum pulse width requirement of Tt0H. Refer to
parameters 40, 41 and 42 in the electrical specification
of the desired device.
7.1.2 TIMER0 INCREMENT DELAY
Since the prescaler output is synchronized with the
internal clocks, there is a small delay from the time the
external clock edge occurs to the time the Timer0
module is actually incremented. Figure 7-4 shows the
delay from the external clock edge to the timer
incrementing.
FIGURE 7-4: TIMER0 TIMING WITH EXTERNAL CLOCK
Increment Timer0 (Q4)
External Clock Input or
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
Timer0
T0 T0 + 1 T0 + 2
Small pulse
misses sampling
External Clock/Prescaler
Output After Sampling
(3)
Prescaler Output
(2)
(1)
Note 1: Delay from clock input change to Timer0 increment is 3 T
OSC to 7 TOSC. (Duration of Q = TOSC). Therefore, the error
in measuring the interval between two edges on Timer0 input = ±4 T
OSC max.
2: External clock if no prescaler selected; prescaler output otherwise.
3: The arrows indicate the times at which sampling occurs.