PIC12C67X datasheet(41/129 Pages) MICROCHIP

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DS30561B-page 41

PIC12C67X

7.2

Using Timer0 with an External Clock

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

meet certain requirements. The requirements ensure

the external clock can be synchronized with the internal

phase clock (TOSC). Also, there is a delay in the actual

incrementing of Timer0 after synchronization.

7.2.1

EXTERNAL CLOCK SYNCHRONIZATION

When no prescaler is used, the external clock input is

used as the clock source. The synchronization of

T0CKI with the internal phase clocks is accomplished

by sampling the prescaler output on the Q2 and Q4

cycles of the internal phase clocks (Figure 7-5). There-

fore, 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 pres-

caler, so that the prescaler output is symmetrical. For

the external clock to meet the sampling requirement,

the ripple-counter must be taken into account. There-

fore, 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 mini-

mum pulse width requirement of 10 ns. Refer to param-

eters 40, 41 and 42 in the electrical specification of the

desired device.

7.2.2

TMR0 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 mod-

ule is actually incremented. Figure 7-5 shows the delay

from the external clock edge to the timer incrementing.

FIGURE 7-5:

TIMER0 TIMING WITH EXTERNAL CLOCK

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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