digital ic counters

Prof Jess Role @UEAB 2008

DIGITAL IC COUNTERS

Lecture 7


Asynchronous and Synchronous Circuits

Valid Output

Clock

Q0

Q1

Q2

Q3

• Consider the timing diagram for the 4-bit counter circuit.

• There is a delay or ripple effect on the output transitions, each output being delayed from the previous due to the propagation delay through the flip-flop.

• When all the outputs do not become valid “at once”, the circuit is ASYNCHRONOUS.

• These circuits are generally identified by the different flip-flops being clocked by different signals.

• This counter circuit is often called a RIPPLE COUNTER.


Synchronous Circuits

• In many circuits it is necessary to ensure that all outputs change simultaneously or SYNCHRONOUSLY. This can be achieved by clocking all the flip-flops with the same clock signal.

• Look at a Synchronous 4-bit counter circuit

• This circuit must produce synchronous outputs because all the flip-flops are clocked by the same signal.

D Q

D Q

D Q

D Q

C0

Q0C1

Q1C2

Q2C3

Q3

Clock

Q0

Q1

Q2

Q3


Synchronous Counter: A Closer Look

To look at how this might work, we will first simplify it to a 2-bit counter, ie: 00->01->10->11->00->…

At startup the outputs are both 0.These feed back into the comb. cct to putvalues of 1 for Y0 and 0 for Y1, ready forthe next clock pulse.The clock “strikes”: the 1 and 0 flowthrough to X0 and X1, but again feedback to the comb. cct which places 0 ontoY0 & 1 onto Y1 (ready for the next clock).The clock “strikes”: the 0 and 1 flowthrough to X0 and X1, but again feedback to the combinatorial circuit which places 1 onto Y0 & 1 onto Y1.The clock “strikes”: the 1 and 1 flow through to X0 and X1, but againfeed back to the combinatorial cct which places 0 onto Y0 & 0 onto Y1.Clock: The X’s are both 0 & the Y’s are 1 & 0, we’re back to the start.

X0

X1

Clock

Combinatorial Circuit

Y0

Y1


Synchronous Counter as a State Machine

How is this counter working?

The outputs are part of the inputs for the next Y0 and Y1 pair, ie the circuit is remembering its past output.

That is, it’s behaving like a state machine.

Sometimes it is better to model thebehaviour of a machine ratherthan what it actually looks like.

The PALASM system allows us tospecify a machine as a state machineand it will program the circuits for us.

So that is the direction we must now take:

State Machines and The PALASM System Section 3.

S0

00

S1

01

S2

10

S3

11

X1 X0


What's A 7-Segment Display?

A 7-segment display is a package with 7 bar-shaped LEDs arranged to allow the display of many useful digits and some letters.

Each segment (labeled A-G) contains an LED which may be individually controlled. DP is an eighth LED, the decimal point.


Common cathode means that each segment's cathode is connected to common pins – 3 & 8, allowing the anode of each to be connected to the controller.


BCD to 7 Segment


Limiting resistor Computation

RS = mA

V

10

3.2 = 230


BCD to 7 Segment Crystal Display


74ALS193


74HC193 Two Stage Arrangement


Basic Integrated Circuit Counter7490 Decade Counter

Clock

R9(1)

CLK A

7490

QA

QB

QC

QD

CLK B

R9(2)

R0(1)

R0(2)

14

7

6

3

2

VCC

GNDInput

5

10

1

12

9

8

11

+5V


Modifying the count sequence

R9(1)

CLK A

7490

QA

QB

QC

QD

CLK B

R9(2)

R0(1)

R0(2)

14

7

6

3

2

VCC

GNDInput

5

10

1

12

9

8

11

+5V


Cascading Stages

R9(1)

CLK A

7490

QA

QB

QC

QD

CLK B

R9(2)

R0(1)

R0(2)

14

7

6

3

2

VCC

GNDInput

5

10

1

12

9

8

11

+5V

R9(1)

CLK A

7490

QA

QB

QC

QD

CLK B

R9(2)

R0(1)

R0(2)

14

7

6

3

2

VCC

GNDInput

5

10

1

12

9

8

11

+5V

SECONDS(One’s Digit)

SECONDS(Ten’s Digit)


Cascading Stages

R9(1)

CLK A

7490

QA

QB

QC

QD

CLK B

R9(2)

R0(1)

R0(2)

14

7

6

3

2

VCC

GNDInput

5

10

1

12

9

8

11

+5V

R9(1)

CLK A

7490

QA

QB

QC

QD

CLK B

R9(2)

R0(1)

R0(2)

14

7

6

3

2

VCC

GNDInput

5

10

1

12

9

8

11

+5V

SECONDS(One’s Digit)

SECONDS(Ten’s Digit)

??


Modulo Counter


Asynchronous counter


Presettable Parallel Counter with Asynchronous Preset


Up/Down Counter

digital ic counters

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