Today we’re going to build a circuit to control a seven segment display.
The display has seven segments, as follows:
aaa
b f
b f
b f
ggg
c e
c e
c e
ddd
The segments each light up if a signal is applied, and we can use that to make any digit 0-9. For example, to make a 5, we apply signal to segments a, b, d, e, and g:
aaa
b
b
b
ggg
e
e
e
ddd
Let’s display a digital number on our display.
In order to distinguish all 10 possible decimal digits, we need 4 input bits.
To convert from 4 bits to 7 segments, we’ll need seven truth tables.
| a | |
|---|---|
| 0000 | 1 |
| 0001 | 0 |
| 0010 | 1 |
| 0011 | 1 |
| 0100 | 0 |
| 0101 | 1 |
| 0110 | 1 |
| 0111 | 1 |
| 1000 | 1 |
| 1001 | 1 |
| 1010 | x |
| 1011 | x |
| 1100 | x |
| 1101 | x |
| 1111 | x |
- x is don’t care
- That leaves mostly 1’s.
- So we can pull out product-of-sums form: (w+x+y+z’)(w+x’+y+z)
So let’s start building this mess.
