EEE515J1 ASICs and DIGITAL DESIGN EGBCDCNT.pdf

1. EEE515J1ASICs and DIGITAL DESIGN EGBCDCNT.pdf An example of a synchronous sequential machine by the generalised method Ian McCrum Room 5D03B Tel: 90 366364 voice mail on 6th ring Email: IJ.McCrum@Ulster.ac.uk Web site: http://www.eej.ulst.ac.uk

2. In general designing FSMs depends on • defining a STATE DIAGRAM and then • a CRUDE STATE TABLE. • Flipflop types are chosen, • their characteristic equations and tables identified. The next stage is to decide • what binary patterns will be used for each state, this is the STATE ASSIGNMENT. The rest of the stages are straightforward , if a bit tedious: • a FULL STATE TABLE is drawn. From this, KMAPS of the circuits that feed the flipflop inputs are drawn and minimised. • The resulting circuit implements the FSM. In some cases • STATE REDUCTION may be possible.

3. It is important you realise that shortcuts are possible: these save time but may obscure your understanding of the complete process. We will skip many of the steps above in this particular example. There are a number of reasons for this. Counters with no inputs are very simple. • In counter design the states of our circuit are usually chosen to be the same as our desired outputs, this saves gates. This is possible only if all outputs in our sequence are unique. Every state must have a unique binary code. The second simplificatioin is that because we are implementing using a FPGA we are forced to use D-Types and Logic Minimisation is not necessary. In fact designing with D-Types is also particularly easy though I will use the longwinded approach to D-types below, you may observe I have added columns that are not needed. This is to show the general purpose full state table which we will use for JK and other implementations.

4. Example A BCD counter that sequences from 0 to 9 as the clock pulses. • A state diagram for a simple counter is trivial.... merely 10 state circles looped in a circle • In general there are two ways of drawing State Diagrams, with outputs associated with the states alone or associated with both a state and an input condition. ( Moore type and Mealy type). A 0000 B 0001 C 0010 D 0011 F 0101 G 0110 H 0111 I 1000 J 1001 E 0100

5. PRESENT STATE NEXT STATE PRESENT OUTPUT A B 0000 B C 0001 C D 0010 D E 0011 E F 0100 F G 0101 G H 0110 H I 0111 I J 1000 J A 1001 The crude state table merely lists the present states, coded as a letter or symbol and the next states. It also lists the desired present outputs for each present state. Normally this table must take into account all possible input patterns, though here we have no inputs.

6. State Code State Code A 0000 F 0101 B 0001 G 0110 C 0010 H 0111 D 0011 I 1000 E 0100 J 1001 In general we choose flipflop types and the STATE ASSIGNMENT For fairly obvious reasons we will choose the following state codes

7. D Input Action 0 RESET 1 SET Consider the D-Type flipflop. We use verbs of Action to denote changes of state. These changes do not occur until the clock changes. Thus the verb of RESET is initiated by putting D low but the output of the flipflop will not change until the active transition of the clock, either the appropriate edge or the appropriate level Using this information we can draw up a FULL STATE TABLE.

8. STATE PQRS NEXT STATE PQRS NEXT STATE BITS. We look at each bit of the next state in turn and state what ACTION is required of our flipflops to cause the next bit to become the correct value. We also write down what INPUT to our flipflops will cause this action. P DP Q DQ R DR S DS A 0000 B 0001 RESET 0 RESET 0 RESET 0 SET 1 B 0001 C 0010 RESET 0 RESET 0 SET 1 RESET 0 C 0010 D 0011 RESET 0 RESET 0 SET 1 SET 1 D 0011 E 0100 RESET 0 SET 1 RESET 0 RESET 0 E 0100 F 0101 RESET 0 SET 1 RESET 0 SET 1 F 0101 G 0110 RESET 0 SET 1 SET 1 RESET 0 G 0110 H 0111 RESET 0 SET 1 SET 1 SET 1 H 0111 I 1000 SET 1 RESET 0 RESET 0 RESET 0 I 1000 J 1001 SET 1 RESET 0 RESET 0 SET 1 J 1001 A 0000 RESET 0 RESET 0 RESET 0 RESET 0 REST - xxxx - x - x - x - x PRESENT

9. From the table above we see that we want to generate DP = 1 when we are in a present state of H or I. In other words if the four flipflops are presently outputting { 0111 or 1000}. We have a combinational network, called the NEXT STATE FORMING network, or the NEXT STATE DECODER that generates DP= /P* Q* R* S + P*/Q*/R*/S DQ= /P*/Q*R* S + /P*Q*/R*/S +/P* Q* /R* S +/P* Q* R* /S DR= /P*/Q*/R* S + /P*/Q* R* /S +/P* Q* /R* S +/P* Q* R* /S DS= /P*/Q*/R*/S + /P*/Q* R* /S +/P* Q* /R*/S +/P* Q* R* /S + P*/Q*/R*/S

10. The steps in designing a counter or any clocked sequential machine that has inputs are as follows • 1)Produce a formal specification for the design; usually a state diagram, sometimes a timing diagram • 2)Produce a crude state table, this has letters representing states and is just a text version of the state diagram. • 3)If the state diagram is complex it may be that superfluous states exist. The technique of state reduction is simple but tedious, it finds what states can be considered equivalent and allows a simpler state diagram to be drawn. Equivalent states have identical outputs for all possible inputs and also have identical next states for all inputs. (or the next states must subsequently proven to be equivalent) • 4)Decide what binary code should be used for each state, if you have many outputs it may be sensible to use the actual output code as the state code. The only rule is that each state must have a unique code. This problem is called the state assignment problem; two common assignments are straight binary where A=00, B=01, C=10 and D=11 for a 4-state machine or the one hot code where we use one bit per state and have only one bit "hot", the code for a 4 bit problem would be (0001,0010,0100 and 1000). One hot codes are very inefficient but very easy to design with. If implementing designs in a register rich device it may be worth using it. The third option is to use the desired outputs, possibly with the addition of an extra bit or two to ensure every state is unique. A fourth option is to apply state assignment guidelines to find a pattern that minimises the cost of the final combinational network.

11. Continued • 5)Once the code is known, the flipflop type can be chosen, D-Types are the easiest while JK or T-Types offer more minimal logic, the combinational next state decoder will have fewer gates. Such flipflops are more expensive so it is not always worth the added expense (I use a cost model of 6p and 9p for Ds and Others) • 6)The full state table can now be drawn up. It must show the present and next state bits. The change from one to the other must be analysed and expressed in a verb that the flipflop is capable of supplying: D-types can reset or set the next state, T-types can hold or toggle a bit and JKs can Hold, Reset, Set or toggle a bit (the verbs of action for JKs are often expressed as stay at zero, stay at one, goto 1 and goto 0) • 7)Once we know what actions we require of our flipflops we can write down what pattern of 0s and 1s we must apply to the flipflop inputs to cause this to occur, you will need 2n patterns to control n JK flipflops. We write these into the full state table using the characteristic table for the relevant flipflop. • 8)We can now design the combinational network that takes the present state of the circuit, the present output of the flipflops and the present inputs and produces the required signals to the flipflops so that they will click to the correct next state when the clock pulse arrives, the output circuit can also be drawn. The combinational circuits can be minimised with KMAPS. This gives the solution as the specification for the Next State Decoder and the Output Decoder

12. Exercise:Tut 1 • (a) Design a counter that counts from 0 to 15 but misses out 13 (a superstitious counter) • (b) Design an excess-3 counter that counts from binary 0011 to 1100 by • (b)(i) Designing a counter that uses the desired outputs as the actual state codes. • (b)(ii) Designing an output combinational circuit that has a 4 bit BCD input and a 4 bit Excess-3 output • (c) Design an Up/Down Counter that counts 0 to 7, the circuit has one input labeled DIRN.