WEEK 8.1 Registers and Counters. ECE124 Digital Circuits and Systems Page 1


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1 WEEK 8.1 egisters and Counters ECE124 igital Circuits and Systems Page 1
2 Additional schematic FF symbols Active low set and reset signals. S Active high set and reset signals. S ECE124 igital Circuits and Systems Page 2
3 Characteristic tables and equations for FFs We can describe the behavior of a flipflop via a characteristic table. The characteristic table shows what the next flipflop output value will be given the current flipflop input value after the clock makes its active edge transition. We can also write this as a characteristic equation: ECE124 igital Circuits and Systems Page 3
4 Toggle flipflops (TFF) Another type of flipflop that has a different behavior when compared to a FF. Symbol for a positive edgetriggered TFF: T Symbol for a negative edgetriggered TFF: T ECE124 igital Circuits and Systems Page 4
5 Characteristic tables and equations for TFFs The characteristic table for the TFF: The characteristic equation for the TFF: So with a TFF, the output toggles (or flips) its value if the input is T=1, otherwise it remains the same. ECE124 igital Circuits and Systems Page 5
6 Making a TFF from a FF We can actually build a TFF using a FF and a 2input XO gate. T T CLOCK ECE124 igital Circuits and Systems Page 6
7 JK flipflops (JKFF) Another type of flipflop that has different behavior compared to a FF or to a TFF. Positive edgetriggered JKFF: J K Negative edgetriggered JKFF: ECE124 igital Circuits and Systems Page 7
8 Characteristic tables and equations for JKFFs The characteristic table for the JKFF: We can derive the characteristic equation for the JKFF (I find it easy to explain via a K Map): JK (t) ECE124 igital Circuits and Systems Page 8
9 Making a JKFF from a FF We can actually build a JKFF using a FF and some other gates. J J K CLOCK K ECE124 igital Circuits and Systems Page 9
10 Timing analysis with flipflops There are some important things to understand when we go to actually make and implement a circuit with flipflops. In reality, it takes time for gates to change their output values according to the input values i.e., there are propagation delays due to resistance, capacitance, etc. Changes in flipflop outputs occur at the active clock edge. There are three timing parameters that are especially important: Setup Time (TSU). Hold Time (TH). ClockToOutput Time (TCO). ECE124 igital Circuits and Systems Page 10
11 efinitions for timing analysis of flipflops Setup Time (TSU): The setup time of a flipflop is the amount of time that the data inputs need to be held stable (not changing) PIO to the arrival of the active clock edge. Hold Time (TH): The hold time of a flipflop is the amount of time that the data inputs need to be held stable (not changing) AFTE the arrival of the active clock edge. ClockToOutput (TCO): The clocktooutput time of a flipflop is the amount of time it takes for the output to become stable (at its new value) AFTE the arrival of the active clock edge. ECE124 igital Circuits and Systems Page 11
12 Comments If these timing specifications are not met, then it is possible that the flipflop will not behave as expected. That is, if we don t observe setup and hold times at the data inputs, then our output might not change as expected. That is, if we don t wait long enough (clocktooutput time) for the output to change, then we might use an incorrect value. If we violate any of these timing parameters, then we have a timing violation. These timing parameters (as we will see later) have an influence on how fast we can clock a circuit. ECE124 igital Circuits and Systems Page 12
13 Illustration of timing parameters (for a FF) TSU TH TCO CLOCK should not change in this interval not stable (trustworthy) until this interval ends ECE124 igital Circuits and Systems Page 13
14 egisters A single FF stores one bit A group of n FFs stores nbits and is called an nbit register. I0 A0 Illustration: When clear=0, all flipflop outputs are forced to zero (active low reset). I1 A1 When clear=1, the rising edge of the clock (the active clock edge), results in the 4bit input transferred to register output. I2 A2 I3 A3 clock clear ECE124 igital Circuits and Systems Page 14
15 Parallel loads (1) We might want to prevent the transfer of data from input to output even though the active clock edge arrives we want the register to HOL ITS CUENT VALUE. We can do this by feeding the register outputs back to the inputs and adding some additional logic to control the register operation. ECE124 igital Circuits and Systems Page 15
16 Parallel loads (2) When load=1, the data inputs reach the input of the flipflop. When the active clock edge arrives, the data gets transferred, or loaded, to the register output. When load=0, the data output of each flipflop is fed back to its input. I0 I1 A0 A1 When the active clock edge arrives, the data input gets transferred to the register output, but since the values are the same for all flipflops, the register holds its current value. I2 I3 A2 A3 load clock clear ECE124 igital Circuits and Systems Page 16
17 Shift registers Might want a register that can shift data serially in a direction This type of register is called a shift register. Illustration of a 4bit shift register. serial in serial out clock clear As active clock edges arrive, the data present at the serial input gets transferred towards the serial output so, data gets shifted to the right one bit at a time as clock edges arrive. ECE124 igital Circuits and Systems Page 17
18 Comment Whenever we would like to add an operation to our register, we are simply placing a MUX in from of each FF in order to direct the correct information to the FF inputs in order to obtain the correct operation. ECE124 igital Circuits and Systems Page 18
19 Universal shift registers (1) Perhaps we want a more general circuit e.g., we want to be able to clear the register, load the register, and perform both a shift right and a shift left operation. ECE124 igital Circuits and Systems Page 19
20 Universal shift registers (2) 4bit register capable of multiple operations (use multiplexers instead of AN/O gates at FF inputs): data in (rshift) I A0 I A1 I A2 I A3 data in (lshift) c1 c0 clock clear ECE124 igital Circuits and Systems Page 20
21 Universal shift registers (3) Has an asynchronous clear signal Has a clock signal Has data inputs for parallel load Has data inputs for both left and right shifts Has two control inputs that determine behavior: ECE124 igital Circuits and Systems Page 21
22 Counters A counter is a register whose outputs go through a prescribed sequence of states upon the arrival of the active edge of some triggering signal (e.g., like a clock). The prescribed sequence of states, or register outputs, can be anything. Counters can come in two varieties asynchronous or synchronous. ECE124 igital Circuits and Systems Page 22
23 ipple counters ipple counters consist of a series of flipflops where the output of one flipflop is used (somehow) as the clock for the next flipflop. It is the lack of a common clock signal for each flipflop that makes the counter asynchronous. ECE124 igital Circuits and Systems Page 23
24 Binary ripple counter (1) A type of ripple counter that has nbits, and can count in binary from 0 to 2 n 1 and repeat. E.g., count sequence for 4bits: ECE124 igital Circuits and Systems Page 24
25 Binary ripple counter (2) We can make a binary ripple counter easily via observation 1 count T A0 A 0 always flips A 1 flips with A 0 goes 1 > 0 T A1 A 2 flips with A 1 goes 1 > 0 Etc So, the ith bit complements the (i+1)th bit when it goes 1 > 0. T A2 Binary ripple counter easily done with TFFs. T A3 clear ECE124 igital Circuits and Systems Page 25
26 Up and down binary ripple counters Up counters count in the sequence 0, 1, 2,, 2 n 1 and repeats. own counters count in the sequence 2 n 1,, 2, 1, 0 and repeats. Should be able to take our previous example and construct a binary ripple down counter ECE124 igital Circuits and Systems Page 26
27 ipple counter delays ecall: Flipflops have clocktooutput times (it takes time for the output to change once the clock signal arrives on the flipflop). Because of this, it can take a lot of time for the higher order bits of a ripple counter to change (remember that the ith bit output is used as the clock for the (i+1)th bit). count A0 A1 A2 A3 transition time due to clocktooutput times So, the FF outputs do not change at the same time due to the ripple effect in the clock inputs. We would like FF outputs to change simultaneously. This is why we prefer to design synchronous counters. ECE124 igital Circuits and Systems Page 27
28 Synchronous counters ifferent from ripple counters in that (the same) clock pulses are applied to the clock inputs of all flipflops simultaneously. Flipflop outputs then change simultaneously. ECE124 igital Circuits and Systems Page 28
29 Example: Synchronous Binary Up Counter Can design a 4bit synchronous binary up counter with enable: If enable=0 circuit should not count; If enable=1 the circuit should count. Observation: A 0 is always toggling, A 1 toggles if A 0 is 1, A 2 toggles if A 0, A 1 are 1, A 3 toggles if A 0, A 1, A 2 are 1 Observations motivate the use of TFF ECE124 igital Circuits and Systems Page 29
30 Example: Synchronous Binary Up Counter Synchronous 4bit binary up counter with enable using TFF: enable T A0 T A1 T A2 T A3 clock clear next stage ECE124 igital Circuits and Systems Page 30
31 Example: Synchronous Binary own Counter Can design a 4bit synchronous binary up counter with enable: If enable=0 circuit should not count; If enable=1 the circuit should count. Observation: A 0 is always toggling, A 1 toggles if A 0 is 0 A 2 toggles if A 0, A 1 are 0, A 3 toggles if A 0, A 1, A 2 are 0 Observations motivate the use of TFF ECE124 igital Circuits and Systems Page 31
32 Example: Synchronous Binary own Counter Synchronous 4bit binary down counter with enable using TFF: down T A0 T A1 T A2 T A3 clock clear next stage ECE124 igital Circuits and Systems Page 32
33 Example: Synchronous Binary Up/own Counter Can combine the up and down counter to get an up/down counter (reuse previously built circuits!) Trick is getting correct input to the FF in order to perform correct operation. Consider the following control inputs which determine circuit behaviour: ECE124 igital Circuits and Systems Page 33
34 Example: Synchronous Binary Up/own Counter Have both logic for up count and down count; up has priority (it disables down). ECE124 igital Circuits and Systems Page 34
35 Example: Synchronous Binary Up/own Counter With Parallel Load Often useful to have a counter that we can load with a starting count. This requires adding another control line (load) and data inputs: ECE124 igital Circuits and Systems Page 35
36 Example: Synchronous Binary Up/own Counter With Parallel Load load up down L U I0 T A0 I1 T A1 I2 T A2 I3 T A3 clock clear ECE124 igital Circuits and Systems Page 36
37 Comments All of the previous counters could have been built using FF or JKFF. esigning counters that perform multiple operations involves: Making sure that the correct inputs get to the FF inputs in order to perform the specific function; and The logic feeding the FF input will cause the FF output to change appropriately. If the control signals and operations have priority, we need to make sure that the control signal priority disables the operation due to other control lines (e.g., parallel load turns off up/down count signals. ECE124 igital Circuits and Systems Page 37
38 Symbols We might have different symbols to represent counters: E.g., I0 I1 I2 I3 CL UP A0 A1 A2 A3 ECE124 igital Circuits and Systems Page 38
39 Modulo counters Sometimes we don t want to count through the entire sequence of binary numbers. E.g., we might want to count 0, 1,,2,, 10 and repeat. (This is effectively a modulo 10 counter). We could design such a counter using synchronous circuit design principles, but we could also attempt something a bit trickier We can use additional circuitry to detect our maximum count number and use a parallel load to restart the counting. ECE124 igital Circuits and Systems Page 39
40 Modulo counter build from a binary counter building block E.g., design a counter that counts 0, 1,,2,, 10 and repeats. CLK I0 A0 I1 A1 I2 A2 I3 A3 CL UP LOA ECE124 igital Circuits and Systems Page 40
41 Some Other Types of Counters: ing counters An nbit counter in which only one output is high at any given time. E.g., Can illustrate with a timing diagram for 4bits: CLK A0 A1 A2 A3 Outputs like this useful to generate timing signals that indicate an order of operation in another circuit block. ECE124 igital Circuits and Systems Page 41
42 Some Other Types of Counters: ing counters Can make an nbit ring counter with a shift register. E.g., 4bit ring counter: A0 A1 A2 A3 S clear clk To generate n nonoverlapping timing signals, we need a shift register with n flipflops. ECE124 igital Circuits and Systems Page 42
43 Some Other Types of Counters: ing counters Can also use a combination of a counter and a decoder. E.g., 4bit ring counter with 2bit counter and decoder. 1 CL A0 A1 x y 0 1 A0 A1 CLK 1 COUNT 2 3 A2 A3 2bit counter 2to4 decoder ECE124 igital Circuits and Systems Page 43
44 Some Other Types of Counters: Switchtail ring counters Consider the following 4bit circuit: A0 A1 A2 A3 clear clk esembles a 4bit ring counter, but complement of MSB fed back to LSB. ECE124 igital Circuits and Systems Page 44
45 Some Other Types of Counters: Switchtail ring counters Count pattern can be determined: So, with n bits we get a count pattern of 2 n output states. ECE124 igital Circuits and Systems Page 45
46 Some Other Types of Counters: Johnson counters We can add additional detection circuitry in order to generate the disjoint timing signals like a ring counter. This results in a Johnson Counter. With a Johnson Counter, we can get 2 n nonoverlapping timing signals using n flipflops and some extra 2input AN gates. ECE124 igital Circuits and Systems Page 46
47 Closing emarks Synchronous counters are nice because the counter outputs all change at the same time (according to the clock). We have only talked about some types of counters; e.g., Binary up/down counters; modulo counters. In general, counters are simply an example of a clocked sequential circuit. We can design any type of counter (e.g., any sequence of values we want) once we learn about clocked sequential circuit design (coming up in the course). E.g., we will see how to design a counter that counts 0, 3, 5, 4, 1, and repeats (if we wanted to). ECE124 igital Circuits and Systems Page 47
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