DRIVE A 7-SEGMENT LED

It might not be something that you’ll need to understand how a microprocessor works – indeed, this would undoubtedly be something that would be done in software – but a good exercise to help consolidate what we’ve seen so far is to build a 7-segment LED decoder in Logisim.

Logisim has a 7-segment display as part of its library of components but, for this exercise, it’s too clever. What we mean by this is that it inputs a 4-bit signal and illuminates the appropriate segments to display the hexadecimal digit corresponding to the binary number on its inputs. In other words, it already includes the decoder that we’re suggesting you design. Instead, therefore, you’ll have to use seven separate output pins or LEDS to represent the seven segments. To make it easier to see whether your circuit is working correctly, we suggest that you arrange them on screen so that they’re in the same position, with respect to each other, that they’d be in a 7-segment LED.

Now you need to add four input pins, and between these and the seven suitably positioned output pins, you need to devise and add some circuitry to drive those output pins correctly. There are several ways to do this, but in general you’ll find the approach involves a layer of AND gates, the outputs of which connect to a layer of OR gates, and a few inverters. All in, you should be able to do it with about 16 gates in total, and four inverters.