Code: The Hidden Language of Computer Hardware and Software, 2nd Edition by Charles Petzold

Author:Charles Petzold [Charles Petzold]
Language: eng
Format: epub
Publisher: Microsoft Press
Published: 2022-08-19T00:00:00+00:00

The next-most significant BCD bit, inverted

The next-most significant BCD bit

The most significant (leftmost) BCD bit, inverted

These four inputs are all 1 only for the BCD number 0101.

A more common approach to displaying decimal numbers is a seven-segment display. This consists of seven elongated lights arranged in a simple pattern:

When these displays are manufactured, generally there are seven pins on the back, one for each of the seven segments. An eighth pin is for the ground. Voltages applied to combinations of those seven pins light up the appropriate segments to show a particular decimal digit:

For the ease of wiring up the seven-segment display, each of the segments is assigned an identifying letter:

The following table shows what segments must be lit for each decimal digit 0 through 9:

Signals corresponding to the decimal digits are already available. They are the outputs of the AND gates in the BCD decoder that was just used to light up the Nixie tube.

Let’s look at the “a” segment first. This segment must be lit when the decimal number is 0, 2, 3, 5, 6, 7, 8, or 9. This means that the outputs of the eight AND gates corresponding to those numbers can be inputs to an eight-input OR gate:

You can do something similar for the segments “b” through “g.”

Or you might seek something a little simpler.

The “a” segment is lit up for numbers 0, 2, 3, 5, 6, 7, 8, or 9, which means that it’s not lit up for numbers 1 and 4. Therefore, you can use the outputs of the two AND gates corresponding to 1 and 4 as inputs to a normal two-input NOR gate:

The output of this NOR gate is 1 except when the inputs are the 1 or 4 signals. For these two numbers, the top segment is not illuminated.

These two approaches aren’t exactly the same. There might be times when you want the seven-segment display to be entirely blank rather than displaying a decimal digit. This can be achieved if none of the ten signals from the AND gates in a BCD decoder are 1. The eight-input OR gate would work correctly in that situation, but the two-input NOR gate would continue to illuminate the top segment.

If your seven-segment display will always be displaying a digit, then a seven-segment decoder can be built from a BCD decoder as shown here:

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.