ECE 171: Introduction to Digital Circuits

Fall 1999

Rev: 11.23.99

Lecture Notes 15

Last Time

• Latches & Flip Flops
  • SR Latch
  • S'R' Latch
  • D Latch
  • D Flip Flop
  • Enables
  • Presets & Clears
• Exam 2 Topics & Review

This Time

• Flip flops
  • JK Flip Flops
  • T Flip Flops
• Ripple Counters/Frequency Dividers
  • Example Application
  • Glitches & Delays
• Synchronous Counters

JK Flip Flops

• Draw symbol
• Can add preset and clear
• Book uses a symbol that is inconsistent and confusing
• Truth Table
  

  CLK	J	K	Q	Q'
  	0	0	Q*	Q'*
  	0	1	0	1
  	1	0	1	0
  	1	1	Q'*	Q*
  0	X	X	Q*	Q'*
  1	X	X	Q*	Q'*

T Flip Flop

• Toggles every clock
• Draw symbol
• Draw waveform
• Is also called a frequency divider because the output waveform has half the frequency of the input waveform
• Can be combined with an enable

Ripple Counters & Frequency Dividers

• Draw three bit ripple counter with T Flip Flops (falling edge triggered)
• Draw waveforms and show that it counts from 0 to 7 and then starts over at 0 again
• This is also thought of as a frequency divider because the output waveforms have a frequency that is some power of 2 times less than the clock signal driving the counter
• The ripple counters can only divide by powers of 2
• You can build a ripple counter that is of any modulo (number of different output states) by resetting the flip flops when you reach the final state.
• Show how a three bit modulo-8 ripple counter can be converted into a modulo-6 ripple counter.

Example Application

• Recall Gate's Room Illumination System that we discussed earlier in the quarter
• There were two doors that people could enter from, door A and door B, and one exit, door E. Each of these is connected to a sensor that produces a 1 when the door is open and a 0 when the door is closed.
• Earlier in the quarter we were only able to turn the lights on when either A or B was high, which was acheived with a simple OR gate.
• Now we can turn the lights off as well. Suppose the output of our system, L, controls whether the lights are on (1) or off (0).
• There are many ways to solve theis problem. The one we will use is to use two counters: one to count the number of entries and one to count the number of exits. If they're both the same, then the room is empty and we can turn the lights out.
• Draw two counters.
• How big should the counters be? What is the room capacity? If 30, then we need a modulo-32 counter, which has 5 outputs.
• We need some method of determining whether the outputs are the same. We can do this bit by bit with an exclusive NOR.
• We need some method of combining all of the comparators. This can be done by a single AND gate. But we want the output L to be low when the counters are equal, so we should use a 5-input NAND gate.
• The input to the exits counter should be the door E signal.
• But we have two signals for the entries: A and B. How should we combine them to produce a single clock signal?
• Draw possible input waveforms with a set of overlapping pulses on the A input and B input.
• If we use an OR gate, the clock signal will miss it when door A and B are opened at the same time.
• If we use an XOR gate, it will count all of the entries, even when the pulses are overlapping or of different sizes.
• However, this won't catch the problem if the pulses are at exactly the same time. But this would be unlikely to occur, especially since they would have to be within nanoseconds of one another for our logic to miss it.
• The book describes other applications of counters:
  • Rhythm Generators
  • Electric Organ
  • Decimal Counters
  • Digital Clocks

Ripple Counter Glitches & Delays

• Draw first 5 or 6 clock pulse outputs of a three bit ripple counter.
• Show that the delays increase as the counter becomes larger (delay).
• Show that there are some intermediate states that are completely wrong (glitch).
• These are the problems of using ripple counters.

Synchronous Counters

• Synchronous counters avoid the delays and glitches of ripple counters because all of the flip flops change at the same time.
• Draw the architecture of a three-bit synchronous counter using D flip flops.
• When the clock edge occurs, all three flip flops change simultaneously.
• The propagation delay is the same regardless of the size of the counter.
• The propagation delay is also relatively small: the same as for a single flip flop.
• This eliminates glitches.
• Combinational logic is used to decode the next state before the next clock edge occurs.

Synchronous Counter Design w/ D Flip Flops

• Fill out the truth table for a 3-bit synchronous counter with D flip flops.
• Most of the expressions can be obtained by observation directly from the truth table.
  • A+ = A'
  • B+ = A XOR B
  • C is more difficult and can be obtained from a Karnaugh map.
    C+ = C'AB + CB' + CA'
    = C'(AB) + C(A' + B')
    = C'(AB) + C(AB)'
    = C XOR (AB)
• Draw logic diagram

Synchronous Counters w/T Flip Flops

• This type of design is much easier
• Draw truth table
• Each variable only toggles when all of the less significant bits are 1.
• Draw logic diagram.
• For each bit you can use the outputs of the preceding AND gates, so you only need a 2-input AND gate for each additional stage.