TLH10837 – 9

FIGURE 6 Low Power Operation of Cascaded Timers

Monostable Operation

Precision Timing

In precision timing applications the LM2240 is used in its

monostable or self-resetting mode The generalized circuit

connection for this application is shown in

Figure 3

(S1 closed) The output is normally HIGH and goes LOW

following a trigger input It remains LOW for the time dura-

of timing components at RC pin 13 (see Performance

Counter Output Programming

are open collector type stages and can be shorted together

to a common pull-up resistor to form a wired-OR connec-

tion the combined output will be LOW as long as any one of

the outputs is LOW The time delays associated with each

counter output can thus be added together This is done by

simply shorting the outputs together to form a common out-

put bus as shown in

Figure 3 For example if only pin 6 is

connected to the output and the rest left open the total

and 6 are shorted to the output bus the total time delay is

choice of counter terminals connected to the output bus

s

255T

Ultra Long Time Delay Application

Two LM2240 units can be cascaded as shown in

Figure 7 to

generate extremely long time delays Total timing cycle of

shorting one or more of the counter outputs from Unit 2 to

the output bus In this application the reset and the trigger

terminals of both units are tied together and the Unit 2 time

base is disabled Normally the output is HIGH when the

system is reset On triggering the output goes LOW where it

remains for a total of 256 (255) or 65280 cycles of the time-

base oscillator

In cascaded operation the time-base section of Unit 2 can

be powered down to reduce power consumption by using

the circuit connection of

minal (pin 16) of Unit 2 is left open and the second unit is

powered from the regulator output of Unit 1 by connecting

Astable Operation

The LM2240 can be operated in its astable or free running

mode by disconnecting the reset terminal (pin 10) from the

counter outputs Two typical circuits are shown in

Figures 8

and

9 The circuit in Figure 8 operates in its free running

mode with external trigger and reset signals It starts count-

ing and timing following a trigger input until an external reset

pulse is applied Upon application of a positive going reset

signal to pin 10 the circuit reverts back to its reset state

This circuit is essentially the same as that of

Figure 3 with

the feedback switch S1 open

The circuit of

Figure 9 is designed for continuous operation

It self triggers automatically when the power supply is

turned on and continues to operate in its free running mode

indefinitely In astable or free running operation each of the

counter outputs can be used individually as synchronized

oscillators or they can be interconnected to generate com-

plex pulse patterns

7