PIC16F628-20SLASHP datasheet(47/160 Pages) MICROCHIP

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Preliminary

DS40300B-page 47

PIC16F62X

6.2

Using Timer0 with External Clock

When an external clock input is used for Timer0, it must

meet

certain

requirements.

The

external

clock

requirement is due to internal phase clock (TOSC)

synchronization. Also, there is a delay in the actual

incrementing of Timer0 after synchronization.

6.2.1

EXTERNAL CLOCK SYNCHRONIZATION

When no prescaler is used, the external clock input is

the same as the prescaler output. The synchronization

T0CKI

with

the

internal

phase

clocks

accomplished by sampling the prescaler output on the

Q2 and Q4 cycles of the internal phase clocks

(Figure 6-5). Therefore, it is necessary for T0CKI 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 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

meet

the

sampling

requirement, the ripple-counter must be taken into

account. Therefore, it is necessary for T0CKI 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

T0CKI high and low time is that they do not violate the

minimum pulse width requirement of 10 ns. Refer to

parameters 40, 41 and 42 in the electrical specification

of the desired device.

6.2.2

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 TMR0 is

actually incremented. Figure 6-5 shows the delay from

the external clock edge to the timer incrementing.

FIGURE 6-5:

TIMER0 TIMING WITH EXTERNAL CLOCK

Q3 Q4

External Clock Input or

Prescaler output (2)

External Clock/Prescaler

Output after sampling

Increment Timer0 (Q4)

Timer0

T0 + 1

T0 + 2

Small pulse

misses sampling

(3)

(1)

Note 1:

Delay from clock input change to Timer0 increment is 3Tosc to 7Tosc. (Duration of Q = Tosc). Therefore, the error in

measuring the interval between two edges on Timer0 input =

±4Tosc max.

External clock if no prescaler selected, Prescaler output otherwise.

The arrows indicate the points in time where sampling occurs.

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