9-29

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

debounce filtering. Referring to Figure 4, only the pulses with

a frequency of less than 40Hz are valid and will be counted.

Input pulses with a frequency of 50Hz to 120kHz are not

counted individually, but each burst of input pulses will be

counted as one pulse if it lasts at least 12.5ms. Driving S/S

with a frequency between 40Hz to 50Hz has an indetermi-

nate result and should be avoided.

In counter modes 5, 7, 9 and 11 the S/S input is not

subjected to any debouncing action and input pulses will be

counted up to a frequency of 120kHz.

Wink Segment

The wink segment is provided as a annunciator to indicate the

ICM7249 is working. It can be connected to any kind of annun-

ciator on an LCD, like the flashing colons in a clock type LCD.

In the timer modes, the wink segment flashes while timing is

taking place. The wink segment waveform is shown on

Figure 3 for timer modes. On the positive transition of S/S,

the wink output turns off. It remains off for 16 BP cycles and

turns back on for another 16 cycles. If timing is still active,

this will be repeated, giving a wink flash rate of 1Hz; other-

wise, the wink segment remains on while timing is not active.

In the counter modes, the wink segment stays on until a

pulse occurs on S/S input, then it winks off indicating a pulse

is counted. This will happen regardless of whether the

display is incremented. Figure 5 shows the wink waveform

for counter modes. When a count occurs, the wink segment

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

ICM7249