MC14553B

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5

OPERATING CHARACTERISTICS

The MC14553B three−digit counter, shown in Figure 4,

consists of three negative edge−triggered BCD counters

which are cascaded in a synchronous fashion. A quad latch

at the output of each of the three BCD counters permits

storage of any given count. The three sets of BCD outputs

(active high), after going through the latches, are time

division multiplexed, providing one BCD number or digit at

a time. Digit select outputs (active low) are provided for

display control. All outputs are TTL compatible.

An on−chip oscillator provides the low frequency

scanning clock which drives the multiplexer output selector.

The frequency of the oscillator can be controlled externally

by a capacitor between pins 3 and 4, or it can be overridden

and driven with an external clock at pin 4. Multiple devices

can be cascaded using the overflow output, which provides

one pulse for every 1000 counts.

The Master Reset input, when taken high, initializes the

three BCD counters and the multiplexer scanning circuit.

While Master Reset is high the digit scanner is set to digit

one; but all three−digit select outputs are disabled to prolong

display life, and the scan oscillator is inhibited. The Disable

input, when high, prevents the input clock from reaching the

counters, while still retaining the last count. A pulse shaping

circuit at the clock input permits the counters to continue

operating on input pulses with very slow rise times.

Information present in the counters when the latch input

goes high, will be stored in the latches and will be retained

while the latch input is high, independent of other inputs.

Information can be recovered from the latches after the

counters have been reset if Latch Enable remains high

during the entire reset cycle.

Figure 4. Expanded Block Diagram

PULSE

SHAPER

CLOCK

12

11

DISABLE

(ACTIVE

HIGH)

C

R

Q0

Q1

Q2

Q3

÷10

UNITS

C

R

Q0

Q1

Q2

Q3

÷10

TENS

C

R

Q0

Q1

Q2

Q3

÷10

HUNDREDS

10

LATCH ENABLE

QUAD

LATCH

QUAD

LATCH

QUAD

LATCH

R

R

SCAN

OSCILLATOR

SCANNER

PULSE

GENERATOR

C1

4

3

MULTIPLEXER

9

7

6

5

Q0

Q1

Q2

Q3

BCD

OUTPUTS

(ACTIVE

HIGH)

13

14

21

15

MR

(ACTIVE HIGH)

OVERFLOW

DS1DS2DS3

(LSD) DIGIT SELECT (MSD)

(ACTIVE LOW)