EEE515J1 Combinational Logic: Truth tables to equations

1. EEE515J1Combinational Logic: Truth tables to equations 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 www.eej.ulst.ac.uk/~ian

2. Two example circuits www.eej.ulst.ac.uk/~ian

3. To analyse the bottom circuit • Create a table with columns for the 8 possible input patterns. • There are 3 inputs so there are 2^3=8 unique input patterns • Add columns and labels for intermediate signals as well as the output www.eej.ulst.ac.uk/~ian

4. To come up with a circuit from a truth table, concentrate on each output at a’1’ that is needed We need to detect four particular input patterns, {010,011,110,111} This could be done by using a three input AND gate to detect each ‘1’ and then ORing each of the “on-term” detectors. www.eej.ulst.ac.uk/~ian

5. “On-term” detectors also called “Product Term detectors I.E go high when the input is /AB/C This type of circuit is called AND-OR And directly generates the SUM of PRODUCTS (SOP) form Y=/ABC + /ABC + AB/C + ABC e.g. to detect the product term {110} (sometimes called m6) we use an invertor on C, so the AND gate will go high when the input is AB/C www.eej.ulst.ac.uk/~ian

6. A B C AND OR i.e. Generate a ‘1’ when inputs are 010 or 011. Also generate a ‘1’ when the inputs are 110 or 111. but for an input pattern of 010 or 011 you only need to detect 01 on the A and B inputs. (/AB) Likewise detect 11 on the A or B inputs, C can be either a ‘0’ or a ‘1’ – it doesn’t matter. Hence use the term AB. Now Y=/AB+AB ; again A can be ‘0’ or ‘1’ so the answer is just B Note again in the truth table, the bold terms are when we want to o/p to be a ‘1’. www.eej.ulst.ac.uk/~ian

7. With practice you can spot these minimisations by inspection. They are examples of the “logic adjacency theorem” – if two product terms are absolutely identical except they differ in having one variable in a normal form in one term and in the complementary form in the other term then you can remove that term. Taking the first pair of ones… /A B /C + /A B C = /A B www.eej.ulst.ac.uk/~ian

8. What you should know • How to write down a SOP equation from a truthtable • Save ink if possible and be quick (try and apply the adjacency theorem by inspection) Don’t worry if you don’t/can’t • If you really need to minimise – use a computer! See the package McBoole or let Quartus do it for you. www.eej.ulst.ac.uk/~ian

9. Points so far • A product term or minterm or “on-term” generates a ‘1’ output in a truth table • A canonical product term contains every variable • The “Sum of Product form or AND-OR circuit is a useful way of generating an o/p • Two product terms can be combined – and a variable is removed, by using the adjacency theorem • We can cost circuits – according to a “Cost model” www.eej.ulst.ac.uk/~ian

10. Cost models • What costs? • Silicon area • Gate count • Power consumption • Speed • Number of soldered joints • Number of packages • Number of unusual packages • Stores inventory • Etc…!!! www.eej.ulst.ac.uk/~ian

11. “McCrum’s Cost Model” • The simplest I could come up with and still allows you to show you have thought about costing. • One penny per gate input, with free invertors! • Later on we will add 6p per D-type flop-flop and 9p for any other flip-flop type. www.eej.ulst.ac.uk/~ian

12. Example 3p 2p+2p+3p+3p = 10p A canonical solution will cost 3p+3p+3p+3p + 4p = 16p Since the truth table had 4 on-terms in it – 4 product term detectors each of which was a 3 i/p AND gate. www.eej.ulst.ac.uk/~ian

13. Tutorials (verify by quartus!) ABCD P Q R S This is taken from the file super13.doc. It is 4 separate circuits – one for P, one for Q, one for R and one for the S output. There are 4 inputs A,B,C and D. Generate the schematics and simulate to prove the truthtable/schematic is correct. [Tut L2_1, L2_2, L2_3 and L2_4] We could also specify this problem by numbering the input patterns, m0 to m15 Thus P = f(ABCD) = ∑(m7-m12, m14) Some software will allow the use of don’t care terms, using a ‘d’ or ‘x’ term. See the file McBoole.txt in the files section of the website for an example. www.eej.ulst.ac.uk/~ian

14. More costings [Tutorials] ABCD P Q R S Each product term will require a 4 input AND gate, ignoring m13 we need 15 such gates or 60p P needs a 7 i/p OR Q needs a 7 i/p OR R needs a 8 i/p OR S needs a 7 i/p OR Total cost = 89p (cost of P is 35p) This solution costs 4p+3p+3p+3p for AND gates and 4p for the output gate for P (cost of P is 13p) TUT L2_5; what is a more minimal cost of Q,R and S? www.eej.ulst.ac.uk/~ian

15. Conclusion • A product term or minterm or “on-term” generates a ‘1’ output in a truth table • A canonical product term contains every variable • The “Sum of Product form or AND-OR circuit is a useful way of generating an o/p • Two product terms can be combined – and a variable is removed, by using the adjacency theorem • We can cost circuits – according to a “Cost model” • Be able to move from truth tables to AND-OR equations and circuits • Be able to do a little minimisation be inspection • Be able to “cost” a circuit • You now have 5 tutorials to try! [Tut L2_1 to L2_5] www.eej.ulst.ac.uk/~ian