29

Detailed Description

As the Functional Diagram shows the device consists of the

following building blocks:

• A 32.768kHz crystal oscillator with the associated dividers

to generate timebase signals for periods of 1s (frequency

of 1Hz), 6s (1/10 min) and 36s (1/100 hour), and 32Hz

signal for LCD drivers.

• A debounce/high-pass detect circuit for the S/S (Start/Stop)

input.

• A chain of cascaded decade counters, 3 decade counters

driving.

• Display control circuitry and BCD to 7-segment decoder/

drivers.

• A control decoder to select different modes of operation.

This is done by routing different signals to the different

points in the chain of decade counters.

The control decoder has 4 inputs for selecting 16 possible

modes of operation, numbered 0 to 15. The 16 modes are

selected by placing the binary equivalent of the mode num-

ber on inputs C0 to C3. Table 2 shows the control inputs and

the modes of operation.

to become active and for oscillation to begin, as shown in

Figure 1. The BP (backplane) output changes state once

every 512 cycles of the crystal oscillator, resulting in a

square wave of 32Hz. The display segments drive signal has

the same level and frequency as BP. Segments are off when

in phase with BP and are on when out of phase with BP.

A non-multiplexed LCD display is used because it is more

stable over temperature and allows many standard LCD

displays to be used.

Timer Mode of Operation

In modes 0 to 3 the device functions as an interval timer. In

this mode, one of the timebase signals will be routed to the

decade counters at a proper point in the chain. Depending

on the selected mode the display will be incremented at 0.1

min, 0.01 hour, 0.1 hour or 1 hour rates.

Control of timing function is handled by the S/S input. There

is a high-pass filtering effect on the S/S input in timer modes.

Referring to Figure 2, timing is active when either S/S is held

high for more than 12.5ms, or if input frequency is 50Hz to

120kHz. Driving S/S with a frequency between 40Hz to 50Hz

has an indeterminate effect on timing and should be

also applied to the intervals when the S/S input is low.

Counter Mode of Operation

In modes 4 to 11 the device functions as an event counter or

totalizer. In this mode the S/S input will be routed to the decade

counters at a proper point in the chain. Each positive transition

of the S/S will be registered as one count. Depending on the

selected mode, the display will be incremented by each pulse,

every 10 pulses, every 100 pulses or every 1000 pulses.

In counter modes 4, 6, 8 and 10 the S/S input is subjected to

31

91

Seven-Segment LCD Output.

32

e1

Seven-Segment LCD Output.

33

d1

Seven-Segment LCD Output.

34

c1

Seven-Segment LCD Output.

35

b1

Seven-Segment LCD Output.

36

a1

Seven-Segment LCD Output.

37

W

Wink-Segment Output.

38

BP

Backplane for LCD Reference.

39

Positive Supply Voltage.

40

OSC IN

Quartz Crystal Connection.

41

OSC OUT

Quartz Crystal Connection.

42

GND

Supply GRouND.

43

C0

Mode-select Control Input.

44

C1

Mode-select Control Input.

45

C2

Mode-select Control Input.

46

C3

Mode-select Control Input.

47

S/S

Start/Stop Input.

48

DT

Display Test Input.

TABLE 1. MODE SELECT TABLE

MODE

CONTROL PIN INPUTS

FUNCTION

C3

C2

C1

C0

0

0000

1 Hour Interval Timer

1

0001

0.1 Hour Interval Timer

2

0010

0.01 Hour Interval Timer

3

0011

0.1 Minute Interval Timer

4

0100

1’s Counter with Debounce

5

0101

1’s Counter

6

0110

10’s Counter with Debounce

7

0111

10’s Counter

8

1000

100’s Counter with Debounce

9

1001

100’s Counter

10

1010

1000’s Counter with Debounce

11

1011

1000’s counter

12

1100

Test Display Digits

13

1101

Internal Test

14

1110

Internal Test

15

1111

Reset

PIN

NAME

DESCRIPTION

ICM7249