11278

Topic 2.2.3 – BCD Counter

Topic 2.2.4 – Decade Counter

Learning Objectives:

2.2.3 BCD Counter

At the end of this topic you will be able to;

 explain the function of BCD counters;

 complete a truth table to show the signals needed to display a given character on a common cathode 7-segment display;

 realise that dedicated decoder/driver ICs are available in a number of formats to drive common cathode / common anode 7-segment displays;

 recognise and analyse the block diagram for a single digit decimal counting system.

2.2.4 Decade Counter

At the end of this topic you will be able to;

 describe the operation and use of a decade counter;

 draw simple timing diagrams for a decade counter.

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GCSE Electronics.

Unit E2 : Applications of Electronics

2.2.3 BCD Counters

The previous topic covered binary counters, and their use linked to logic systems, in generating a sequence of output states.

Sometimes we need to view the count as it takes place, for example when counting cars entering a car park.

Whilst computers count in binary, for humans it is difficult to translate a binary number such as 101011010

2

into the equivalent decimal number (316) at a glance.

To improve the readability of a counter output for humans, we need: i.

a counter that follows a decimal count sequence. i.e. 0-9 ii.

a decimal display device that is easy to read.

(i) A counter that follows a decimal count system.

Binary Coded Decimal

Look at the following table.

Decimal Binary BCD

4

5

6

7

0

1

2

3

8

9

10

0000

0001

0010

0011

0100

0101

0110

0111

0000

0001

0010

0011

0100

0101

0110

0111

1000

1001

1000

1001

1010 1 0000

2



The third column in this table is a modified version of binary, called Binary

Coded Decimal (or BCD for short). In this the maximum count is restricted to

1001, i.e. 9 in decimal. After that, resetting, occurs when the count reaches

1010. The binary counter IC can be made into a BCD counter as shown below.

When the reset is active-low, a NAND gate is used:

D C

R

B A

C out

When the reset is active-high, an AND gate is used:

3



(ii) 7-segment displays

The most common display for counting systems is the 7-segment display, a single package that has 7 LED’s arranged as shown below. a f g b e d c

Each LED (or segment) can be controlled independently and are used to create our familiar numbers as shown below.

4



When the seven segment display is manufactured with all seven anodes connected together then the display is referred to as a common anode display. Common anode displays are controlled by ICs that sink current.

When the display is manufactured with all seven cathodes connected together then the display is referred to as a common cathode display.

Common cathode displays are controlled by ICs that source current. The circuit symbol for a seven segment display is shown in the following diagram.

Current limiting resistors

The best method of limiting the current through a seven segment display is to use a current limiting resistor in series with each of the seven LEDs.

The brightness of a particular LED does not depend on the state of the other six LEDs.

If you were not particularly concerned with a constant brightness, then a single limiting resistor could be used.

The brightness now depends on how many segments are illuminated.

5



Decoder/Driver ICs

Next, we need an interface to link the two parts together as shown below.

Interface

Unit f a g b e c d

D C

R

B A

C out

The purpose of the interface unit is to convert the BCD output from the counter into appropriate logic signals to light the correct segment of the display. We can show this in a truth table as shown below.

BCD Output 7-segment display inputs

D

0

C

0

B

0

A

0 a

1 b

1 c

1 d

1 e

1 f

1

0

0

0

0

0

1

1

0

0

1

1

1

1

0

0

1

0

1

0

0

0

0

0

0

1

1

1

0

0

1

0

1

1

0

1

1

1

0

1

1

1

1

0

1

0

0

0

0

1

1

0

0

1

1

1

1

0

0

1

1

0

0

0

1

0

1

0

1

1

1

0

1

1

1

1

1

1

1

1

1

1

0

1

1

1

0

1

1

1

1 g

0

0

1

1

1

1

1

1

1

1

6



The interface unit is a complex logic system that has 4 inputs and 7 outputs.

We do not need to know the exact logic function required for each output since a dedicated IC, as a decoder/driver IC, is available. The device performs two functions for us: i.

it decodes the BCD counter outputs

A

,

B

,

C

and

D

into the 7 input signals a

, b

, c

, d

, e

, f

, g

required for the 7-segment display. (this is the decoder part) ii.

It provides a current boost to the output signals to provide enough current to light the display segments directly. (this is the driver part)

Decoder /

Driver f a g b e c d

D C

R

B A

C out

The completed system to produce numbers 0 – 9, making it easy for humans to interface with a binary counting unit.

7



Dedicated BCD Counters

IC manufacturers have produced a special version of the binary counter, which has the reset internally wired to reset after a count of nine has been reached. Using this device, called a BCD counter, removes the need for the external NAND gate to reset the counter.

Using a BCD counter, the system will look like this.

D C

BCD Counter

R

B A

C out

Decoder /

Driver f e a g d b c

The completed system using a BCD counter

8



Activity 1: BCD Counter/Decoder/Display System

1a. Set up the following counter which contains a CMOS 4510 BCD counter and a 4511 decoder driver

1b. Press and release switch SW1 about twenty times and comment on what you observe.

…………………………………………………………………………………………………………………………………

1c. Investigate the effect of connecting the U/D pin of the 4510 IC to 0V rather than 9V.

…………………………………………………………………………………………………………………………………

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Extending the BCD Count

A 0 to 9 counter would have one 4-bit BCD counter whilst a 00 to 99 counter would have two 4-bit BCD counters.

Look at the table below:

Decimal Binary BCD

4

5

6

7

8

9

0

1

2

3

10

11

12

13

14

15



50



99

0000 0000 0000

0001 0000 0001

0010 0000 0010

0011 0000 0011

0100 0000 0100

0101 0000 0101

0110 0000 0110

0111 0000 0111

1000 0000 1000

1001 0000 1001

1010

1011

1100

1101

0001 0000

0001 0001

0001 0010

0001 0011

1110

1111



0001 0100

0001 0101



110010 0101 0000

 

1100011 1111 1111

Each group of 4 BCD digits is connected to its own BCD counter with the

C

OUT terminal of the first counter connected to the clock of the second one as shown on the next page.

10



Clock in

D C

BCD Counter

R

B A

C out

D C

R

B

BCD Counter

A

C out

A 2 digit counter to count from 00 – 99

Useful circuit for project work

To build the 0-99 counter shown above is quite complicated. A better solution would be to use a CMOS 4026 IC which has a BCD counter and Decoder driver on the same chip. If you have time set up and test the following circuit. You can add current limiting resistors to the display if needed.

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Creating other characters

The 7-segment display can also be used to display other characters as shown below:

L H C A n

y E u P F

When we want to create these different characters we cannot use a decoder/driver IC, as this is pre-programmed to convert the output of a BCD

Counter into the numbers 0,1,2,3,4,5,6,7,8,9.

To create special characters we would have to design the logic decoder required ourselves.

Now here are a few examples for you to try.

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Student Exercise 1:

1. The following circuit shows a binary counter, connected to a decoder/driver and display. Write down the sequence of numbers displayed as the clock input is pulsed 13 times.

Decoder /

Driver f a g b e c d

D C B A

C out

R

Clock Pulse Display Shows

4

5

6

7

0

1

2

3

8

9

10

11

12

13

0

13



2. The following circuit contains a BCD counter, connected to a 7-segment display decoder/driver and display. Add to the circuit any required logic gate and connections required to ensure that the largest number displayed is ‘7’

3. The following diagram shows a 7-segment display, showing the number 3. a f b g a) e d c

What segments will be on to display the number 1?

................................................................................................................. b) What segments will be on to display the number 4?

..................................................................................................................

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Topic 2.2.4 – Decade Counter c) What segments will be off to display the number 2?

.................................................................................................................. d) What segments will be off to display the number 7?

.................................................................................................................. e) Describe the operation of a BCD to 7-Segment Display decoder chip.

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................

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2.2.4 Decade Counters

A decade counter as it name suggests counts in decades or tens, however it is not a binary counter. It has one clock input and ten outputs. Each output is activated in turn when the clock pulses arrive. This is illustrated in the following circuit diagram.

The output of the Logic Analyser shows what happens at the output pins after the reset switch SW1 is momentarily closed:

Ch. 0 represents the clock pulse, Ch. 1 to 10 show the 10 outputs of the

Decade counter. You should be able to see that only one output at a time is at logic 1 and that changes occur on the rising edge of the clock pulse.

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Activity 2: Light Chaser

2a. Set up the following circuit using the CMOS 4017 Decade counter

2b. Adjust VR1 on the 555 astable to vary the speed at which the sequence changes.

2c. Connect the Q6 output of the counter to the R input and comment on the effect this has on the sequence produced.

……………………………………………………………………………………………………………………………

2d. Investigate the effect of connecting different outputs to the R input and use your result to complete the following sentence:

“To shorten the sequence, connect the R input to the Q output which is

……………… the last output that you want in the sequence”

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Activity 3: Electronic Dice

Note: You will not be examined on the use of the enable pin EN. When EN is high the display freezes and when low it allows the outputs to go high in turn.

3a. Set up the following circuit:

3b. Press and release the Roll switch to test the electronic coin.

3c. Modify the circuit to make an electronic dice. You will need to change the position of the wire which goes from the reset pin. You will also need to connect 6 LEDs to the output pins.

3d. Comment on how well your dice works:

……………………………………………………………………………………………………………………………

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Solutions to Student Exercises

Student Exercise 1:

1.

Clock Pulse Display Shows

8

9

10

5

6

7

11

12

13

0

1

2

3

4

8

9

0

5

6

7

1

2

3

0

1

2

3

4

2.

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3. a) e and f. b) b, c, f and g. c) c and f. d) d, e, f and g. e) The BCD to 7-segment display decoder chip takes the output of a

BCD counter and decodes this to provide the 7 outputs required to light the segments of the display unit so that the decimal numbers

0 – 9 are displayed correctly.

Now for some examination style questions

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Examination Style Questions

1. A seven segment display can be used with a BCD counter and decoder to display numbers.

(a) What do the letters BCD stand for?

………………………………………………………………………………………………..

[1]

(b) A seven segment display can also be used with a suitable decoder to display letters.

A pupil has designed a simple light meter which uses a seven segment display.

The letter b is displayed when the light level is too bright.

The letter d is displayed when it is too dark.

If the light level is satisfactory the letter S is displayed.

The arrangement of the seven LED segments that make up the display is shown on the following diagram.

Complete the following table to show which segments are lit when each of the letters b , d and S are displayed.

[3]

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2. The block diagram of a counting system is shown below.

(a) What is the name of the block labelled A in the above diagram?

………………………………………………………………………………………………

[2]

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(b) The arrangement of the seven LED segments that make up the display is shown on the following diagram.

Complete the following table to show the numbers displayed when different segments are lit.

[4]

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3. (a) The diagram shows the arrangement of the LED’s in a seven segment display.

A segment lights up when it receives a logic 1 signal.

Complete the following table.

(b) Here is the block diagram for a single digit decimal counting system.

[2]

Which one of the following statements, A , B , C or D , is true for a BCD counter?

A : The BCD counter counts faster than a binary counter.

B : The BCD counter resets on every tenth pulse.

C : The BCD counter delivers more current to the decoder/driver.

D : The BCD counter displays letters as well as numbers.

Answer

…………………………..

[1]

24

4.

5.



The two circuits below show a 5Hz clock connected to a decade counter and bargraph LED display.

(a) How long will it take for the sequence to cycle through all 10 outputs in circuit A ……....

[1]

(b) Which LEDs will come on in circuit B ...…………………………………………..………

[1]

(c)

How long will it take to cycle through the sequence displayed in circuit B …………

[1]

The circuit below shows a 20 Hz clock connected to a decade counter, four OR gates and 6 LEDs.

(a) Complete the table below to show which LED comes on for each step of the sequence produced by the decade counter.

STEP

Q0

Q1

Q2

Q3

Q4

Q5

Q6

Q7

Q8

Q9

LED ON

D1

(b)

How long will it take for the sequence to cycle through all 10 outputs …………

[1]

[3]

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6.

(a) The diagram shows the arrangement of the LEDs in a seven segment display.

Complete the following table.

(b) Here is the block diagram for a counting system. The system shows how many times the switch has been pressed.

(i) What does BCD stand for?

………………………………………………………………………………………………..

[1]

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(ii) The table shows the output of the BCD counter after the switch has been pressed a number of times.

Complete the table

[2]

27



7. A fairground ride contains a counting system which shows the operator how many people have passed through a turnstile.

(a) The block diagram of the counting system is given below.

(i) What do the letters BCD stand for?

………………………………………………………………………………………………

[1]

(ii) What does block X contain? Choose your answer from the following list. pulse generator decoder multiplexer comparator

.................................................................................................................................................

[1]

(b) A 4-bit binary counter is used for the BCD counter. The counter must reset when the ninth person passes through the turnstile. The counter is reset by taking the reset pin to logic 1.

Complete the diagram below to show how this can be done.

Bit A of the counter is the least significant bit.

[3]

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(c) The arrangement of the seven LED segments that make up the display is shown on the following diagram.

Complete the table to show the numbers displayed when different segments are lit.

[4]

29

Op6

Op7

Op4

Op5

Op2

Op3

Clk

Op1

Op8

Op9

Op10

8.



The block diagram gives the main components involved in a counting system which drives a bar graph display unit.

Clock X

(a) What is the name of the counter in Block X?

Bargraph

Display

.......................................................................................................

(b) Complete the output graphs Op1 – Op10 below to show the output from Block X in response to the clock pulses provided. Initially all outputs are at logic 0.

[1]

[5]

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Self Evaluation Review

Learning Objectives

My personal review of these objectives:

  

2.2.3 BCD Counter explain the function of BCD counters; complete a truth table to show the signals needed to display a given character on a common cathode 7-segment display; realise that dedicated decoder/driver ICs are available in a number of formats to drive common cathode / common anode 7segment displays; recognise and analyse the block diagram for a single digit decimal counting system.

2.2.4 Decade Counter describe the operation and use of a decade counter; draw simple timing diagrams for a

decade counter.

Targets: 1. ………………………………………………………………………………………………………………

………………………………………………………………………………………………………………

2.

………………………………………………………………………………………………………………

………………………………………………………………………………………………………………

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11278

Learning Objectives:

2.2.3 BCD Counters

2.2.4 Decade Counters

Examination Style Questions

Learning Objectives

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11278

Learning Objectives:

2.2.3 BCD Counters

2.2.4 Decade Counters

Examination Style Questions

Learning Objectives

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