# Using 2 Sevent Segment to display only 0-15 decimal

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,855
thank you sir, you are very kind.
i will discuss that with my friends and try it. so in this case, we need add "ic logic gate" before "Input" 7447. am i right?
Yes, you are right, but not just one "ic logic gate" will suffice, you will need several gates... unless you happen to have on-hand an EPROM (Electrically Programmable Read Only Memory) and the means to program it using the values in your truth table as the "memory data". IF you happened to have one of those, your 4-bit input would be used as the EPROM address input and five of the EPROM outputs would be programmed to produce the Decoder function you require. But I am going to bet you aren't going to do that. Too simple, and you won't learn anything about combinatorial logic, DeMorgan's Theorems, Karnaugh Maps, and Boolean Algebra... all of which I am sure you are eager to learn and apply in your digital designs.

EPROMs are now "old school" technology, having been replaced by EEPROMs (Electrically Erasable Programmable Read Only Memories) and FLASH EPROM memory, but I am not suggesting you use either EPROM or EEPROM or FLASH to implement your "black box" Decoder solution. That would be like using a sledge hammer to kill a house fly. Still, if you would like more information for later use in your studies, visit this web site for an overview of the technology as it was then and as it is today. The company also sells a US\$300 EPROM+ programming system that your school would perhaps be interested in purchasing for a more advanced course of study involving mircroprocessor design.

For the task at hand, building a combinatorial logic circuit that will translate your 4-bit binary input into two BCD (Binary Coded Decimal) outputs, you will need some multiple-input AND gates and some INVERTERS. You can design and build any combinatorial logic circuit using just those two components! That doesn't mean you should. There are many ICs that combine digital function blocks, for example an AND gate with a built-in INVERTER connected to its output becomes a NAND (Not AND) gate. You can also purchase ICs that implement OR gates, NOR (Not OR) gates, Exclusive-OR (XOR) gates and many other functions with two (or more) inputs. These components can often be used to simplify a more complex implementation that would otherwise use just AND gates and INVERTERS.

As it turns out, you can build any combinatorial logic circuit with just NAND gates or just NOR gates because if you connect all the inputs of either device together to form just one input, each device becomes an INVERTER. Add an INVERTER to the output of a NAND gate, for example, and it becomes an AND gate. Add an INVERTER to the output of a NOR gate and it becomes an OR gate.

So if all you have to work with is a large box full of quad 2-input NAND gates (SN7400 in TTL), you could implement your Decoder using just those parts. But first things first: show us your truth table.

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