Intel mcs-48 manual As an Event Counter, As a Timer, Clock and Timing Circuits, Oscillator

SINGLE COMPONENT SYSTEM

PRESCALER

XTAL-c 15

	LOAD OR READ
CLEARED ON START TIMER
	1	JUMP ON
		TIMER FLAG
.....---- ... , START
COUNTER	8 BIT TIMER/
EDGE
DETECTOR	EVENT COUNTER
o	NOT CLEARED ON RESET
STOP T

INT

ENABLE ----a_~

TIMER/EVENT COUNTER

serviced and immediately be recognized upon return from the service routine. The pending timer interrupt is reset by the Call to location 7 or may be removed by executing a DIS TCNTI instruction.

As an Event Counter

Execution of a START CNT instruction con- nects the T1 input pin to the counter input and enables the counter. Subsequent high to low transitions on T1 will cause the counter to increment. The maximum rate at which the counter may be incremented is once per three instruction cycles (every 7.5/-tsec when using a 6MHz crystal)-there is no minimum frequency. T1 input must remain high for at least 500ns (at 6MHz) after each transition.

As a Timer

Execution of a START T instruction connects an internal clock to the counter input and enables the counter. The internal clock is derived by passing the basic 400 KHz machine cycle clock ALE through a -7-32 prescaler. The prescaler is reset during the START T instruction. The resulting 12.5 KHz clock increments the counter every 80 /-tsec (assuming 6 MHz XTAL). Various delays between 80 /-tsec and 20 msec (256 counts) can be obtained by presetting the counter and detecting overflow. limes longer than 20

msec may be achieved by accumulating mUl- tiple overflows in a register under software control. For time resolution less than 80 /-tsec an external clock can be applied to the T1 input and the counter operated in the event counter mode. ALE divided by 3 or more can serve as this external clock. Very small delays or "fine tuning" of larger delays can be easily accomplished by software delay loops.

2.1.11Clock and Timing Circuits

liming generation for the 8048 is completely self-contained with the exception of a fre- quency reference which can be XTAL, inductor, or external clock source. The Clock and Timing circuitry can be divided into the following functional blocks:

Oscillator

The on-board oscillator is a high gain parallel resonant circuit with a frequency range of 1 to 6MHz. The X1 external pin is the input to the amplifier stage while X2 is the output. A crystal or inductor connected between X1 and X2 provides the feedback and phase shift required for oscillation. A 5.9904 MHz crystal provides for easy derivation of all standard communications frequencies. If an accurate frequency reference and maximum processor speed are not required, an induc-

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