Digital Fundamentals

Transcription

1 Digital Fundamentals Tenth Edition Floyd Chapter 9 Sections 9-1 thru Pearson Education, Upper 2008 Pearson Saddle River, Education NJ All Rights Reserved

2 ET285 Agenda Week 2 Quiz 0: Covered materials and reading assignments from Unit 0. Lecture: Chapter 9, Counters, pp Lab 1 continuation: Experiment 17 in Buchla Section of Supplemental Text to accompany Digital Fundamentals. Assignment: See Instructotr. Prepare for Unit 1 Quiz. (Chapter 9, Section 1-5)

3 Counting in Binary As you know, the binary count sequence follows a familiar pattern of 0 s and 1 s as described in Section 2-2 of the text. The next bit changes on every fourth number LSB changes on every number. The next bit changes on every other number.

4 Counting in Binary A counter can form the same pattern of 0 s and 1 s with logic levels. The first stage in the counter represents the least significant bit notice that these waveforms follow the same pattern as counting in binary. LSB MSB

5 J-K Flip Flop Truth Table Inputs Outputs J K CLK Q Q Comments

6 Three bit Asynchronous Counter In an asynchronous counter, the clock is applied only to the first stage. Subsequent stages derive the clock from the previous stage. The three-bit asynchronous counter shown is typical. It uses J-K flip-flops in the toggle mode. Called asynchronous because the flip-flops are never simultaneously triggered. HIGH J 0 Q 0 J Q 1 1 J Q 2 2 CLK C C C K 0 Q 0 Q 1 K 1 K 2 Waveforms are on the following slide

7 Three bit Asynchronous Counter Notice that the Q 0 output is triggered on the leading edge of the clock signal. The following stage is triggered from Q0. The leading edge of Q0 is equivalent to the trailing edge of Q0. The resulting sequence is that of an 3-bit binary up counter. CLK Q Q Q

8 Flip-Flop Operating Characteristics See next slides

9 Flip-Flop Operating Characteristics

10 Flip-Flop Operating Characteristics

11 Propagation Delay Asynchronous counters are sometimes called ripple counters, because the stages do not all change together. For certain applications requiring high clock rates, this is a major disadvantage. Notice how delays are cumulative as each stage in a counter is clocked later than the previous stage. CLK Q 0 Q 1 Q Q 0 is delayed by 1 propagation delay, Q 2 by 2 delays and Q 3 by 3 delays.

12 Total Propagation Delay Tp (tot) = Tp1 + Tp2 + Tp3 + Tp4 Maximum Frequency of counter = 1/Tp (tot ) For previous slide example assuming Tp = 10 ns for all F/F Tp (tot) = 4 x 10ns = 40 ns F max = 1/40ns = 25 MHz

13 Example 9-1 (page 463) 4 bit asynchronous binary counter example

14 Asynchronous Decade Counters The modulus of a counter is the number of unique states that a counter will sequence through. The maximum modulus of a counter is 2**n (2 n ), where n is the number of flip flops in the counter. MOD(N) A counter can be designed to have less than its maximum modulus and the resulting sequence is called a truncated sequence. A decade counter has ten states (0 thru 9, or 0000 thru 1001) and is considered a truncated counter because it requires 4 flip flops to build which normally would have a maximum of 16 states. MOD10 Useful for digital readouts.

15 Asynchronous Decade Counter This counter uses partial decoding to recycle the count sequence to zero after the 1001 state. The flip-flops are trailing-edge triggered, so clocks are derived from the Q outputs. Other truncated sequences can be obtained using a similar technique. HIGH CLR J 0 Q 0 Q 1 Q 2 J 1 J 2 J 3 Q 3 CLK C C C C K 0 K 1 K 2 K 3 Waveforms are on the following slide

16 Asynchronous Decade Counter When Q 1 and Q 3 are HIGH together, the counter is cleared by a glitch on the CLR line. CLK Q 0 Q 1 Glitch Glitch Q 2 Q 3 CLR Glitch

17 Example of Asynchronous MOD12 binary counter Cycles on 1100

18 Asynchronous Counter Using D Flip-flops D flip-flops can be set to toggle and used as asynchronous counters by connecting Q back to D. The counter in this slide is a Multisim simulation of one described in the lab manual. Can you figure out the sequence? LSB MSB Q to D puts D flip-flop in toggle mode The next slide shows the scope

19 CLK LSB MSB CLR Note that it is momentarily in state 3 which causes it to clear. The sequence is (CLR) (repeat)

20 The 74LS93A Asynchronous Counter The 74LS93A has one independent toggle J-K flip-flop driven by CLK A and three toggle J-K flip-flops that form an asynchronous counter driven by CLK B. The counter can be extended to form a 4-bit counter by connecting Q 0 to the CLK B input. Two inputs are provided that clear the count. CLK B (1) CLK A (14) J J J C C C J 3 C RO (1) RO (2) All J and K inputs are connected internally HIGH (2) (3) K 0 (12) (9) (8) (11) Q 0 K 1 K 2 Q 1 Q 2 K 3 Q 3

21 74LS93A connected as a modulus-12 counter

22 Synchronous Counters In a synchronous counter all flip-flops are clocked together with a common clock pulse. Synchronous counters overcome the disadvantage of accumulated propagation delays, but generally they require more circuitry to control states changes. This 3-bit binary synchronous counter has the same count sequence as the 3-bit asynchronous counter shown previously. Note to Instructor: draw the below circuit on board CLK HIGH J 0 K 0 J 1 Q 0 Q 0 Q 1 Q 0 Q 1 Q J 2 C C C K 1 K 2 The next slide shows how to analyze this counter by writing the logic equations for each input. Notice the inputs to each flip-flop 2

23 Analysis of Synchronous Counters A tabular technique for analysis is illustrated for the counter on the previous slide. Start by setting up the outputs as shown, then write the logic equation for each input. This has been done for the counter. 1. Put the counter in an arbitrary state; then determine the inputs for this state. Outputs 2. Use the new inputs to determine the next state: Q 2 and Q 1 will latch and Q 0 will toggle. Logic for inputs 3. Set up the next group of inputs from the current output. Q 2 Q 1 Q 0 J 2 = Q 0 Q 1 K 2 = Q 0 Q 1 J 1 = Q 0 K 1 = Q 0 J 0 = 1 K 0 = Q 2 will latch again but both Q 1 and Q 0 will toggle. Continue like this, to complete the table. The next slide shows the completed table

24 Analysis of Synchronous Counters Outputs Logic for inputs Q 2 Q 1 Q 0 J 2 = Q 0 Q 1 K 2 = Q 0 Q 1 J 1 = Q 0 K 1 = Q 0 J 0 = 1 K 0 = At this points all states have been accounted for and the counter is ready to recycle

25 A 4-bit Synchronous Binary Counter Q 1 Q 0 Q 2 Q 1 Q 0 FF0 FF1 G1 G 2 FF2 FF3 HIGH J 0 Q 0 J 1 Q 1 J 2 Q 2 J 3 Q 3 CLK C C C C K 0 Q 0 K 1 Q 1 K 2 Q 2 K 3 Q 3 The 4-bit binary counter has one more AND gate than the 3-bit counter just described. The shaded areas show where the AND gate outputs are HIGH causing the next FF to toggle. Q 0 Q 1 Q 2 Q 3

26 BCD Decade Counter With some additional logic, a binary counter can be converted to a BCD synchronous decade counter. After reaching the count 1001, the counter recycles to The gate detects 1001, and causes FF3 to toggle on the next clock pulse. FF0 toggles on every clock pulse. Thus, the count starts over at HIGH FF0 FF1 FF2 FF3 Q 3 Q 0 J 0 Q 0 J 1 Q 1 J 2 Q 2 J 3 Q 3 C C C C K 0 Q 0 K 1 Q 1 K 2 Q 2 K 3 Q 3 Q 3 CLK

27 BCD Decade Counter Waveforms for the decade counter: CLK Q Q Q Q These same waveforms can be obtained with an asynchronous counter in IC form the 74LS90. It is available in a dual version the 74LS390, which can be cascaded. It is slower than synchronous counters (max count frequency is 35 MHz), but is simpler. 0

28 A 4-bit Synchronous Binary Counter The 74LS163 is a 4-bit IC synchronous counter with additional features over a basic counter. It has parallel load, a CLR input, two chip enables, and a ripple count output that signals when the count has reached the terminal count. Data inputs D 0 D 1 D 2 D 3 (3) (4) (5) (6) CLR LOAD ENT ENP CLK (1) (9) (10) (7) (2) C CTR DIV 16 TC = 15 (15) RCO (14) (13) (12) (11) Q 0 Q 1 Q 2 Q 3 Data outputs Example waveforms are on the next slide

29 CLR LOAD D 0 Data inputs D 1 D 2 D 3 CLK ENP ENT Q 0 Data outputs Q 1 Q 2 Q 3 RCO Clear Preset Count Inhibit

30 Up/Down Synchronous Counters An up/down counter is capable of progressing in either direction depending on a control input. UP/DOWN HIGH FF0 FF1 FF2 J J 1 J 2 Q 0 Q 1 0 UP Q 0. UP C C C Q 2 K 0 Q 0 Q 1 Q 2 K 1 K 2 CLK DOWN Q 0. DOWN Example waveforms from Multisim are on the next slide

31 Up/Down Synchronous Counters Q 0 Q 1 Q 2 UP/DOWN Count up Count down

32 Up/Down Synchronous Counters The 74HC190 is a high speed CMOS synchronous up/down decade counter with parallel load capability. It also has a active LOW ripple clock output (RCO) and a MAX/MIN output when the terminal count is reached. The 74HC191 has the same inputs and outputs but is a synchronous up/down binary counter. CTEN D/U LOAD CLK CTEN D/U LOAD CLK (4) (5) (11) (14) (4) (5) (11) (14) 74HC190 C 74HC191 C D 0 D 1 D 2 D 3 (15) (1) (10) (9) CTR DIV 10 (3) (2) (6) (7) Q 0 Q 1 Q 2 Q 3 D 0 D 1 D 2 D 3 (15) (1) (10) (9) CTR DIV 16 (3) (2) (6) (7) Q 0 Q 1 Q 2 Q 3 Data inputs (12) (13) MAX/MIN RCO Data outputs Data inputs (12) (13) MAX/MIN RCO Data outputs

33 Synchronous Counter Design FF0 FF1 FF2 Q 2 J 0 J 1 J 2 Q 0 Q 1 C C C K 0 Q 0 Q 1 Q 2 K 1 K 2 CLK Omitting the details of a Gray Code Counter design, we get the above (state machine) circuit. The circuit can be checked with Multisim before constructing it. The next slide shows the Multisim result

34 Q 0 Q 1 Q 2

35 Cascaded counters Cascading is a method of achieving higher-modulus counters. For synchronous IC counters, the next counter is enabled only when the terminal count of the previous stage is reached. HIGH Counter 1 Counter 2 16 CTEN TC CTEN CLK C Q 0 Q 1 Q 2 f in CTR DIV 16 CTR DIV 16 ƒ in Q C 3 Q 0 Q 1 Q 2 Q 3 TC f out ƒ in 256 a) What is the modulus of the cascaded DIV 16 counters? b) If f in =100 khz, what is f out? a) Each counter divides the frequency by 16. Thus the modulus is 16 2 = 256 b) The output frequency is 100 khz/256 = 391 Hz

36 Modulus 100 counter using 2 cascaded MOD 10 counters

37 Three MOD 10 counters cascaded to divide F (in) by 1000

38 Example 9-8 (page 491)

39 Cascade Counters with Truncated Sequences Normally the cascaded circuit above would be MOD 16 = = 2**16 To change the circuit to count to preload the circuit so it starts counting from = 25536

40 Selected Key Terms Asynchronous Modulus Synchronous Terminal count State machine Cascade Not occurring at the same time. The number of unique states through which a counter will sequence. Occurring at the same time. The final state in a counter s sequence. A logic system exhibiting a sequence of states or values. To connect end-to-end as when several counters are connected from the terminal count output of one to the enable input of the next counter.

41 ET285 Agenda - Week 2 Quiz 0: Covered materials and reading assignments from Unit 0. Lecture: Chapter 9, Counters, pp Lab 1 continuation: Experiment 17 in Buchla Section of Supplemental Text to accompany Digital Fundamentals. Assignment: See Instructor. Prepare for Unit 1 Quiz. (Chapter 9, Section 1-5)