Creating a 16-1 multiplexer from four 4-1 multiplexers and one 4-2 decoder

[alexy97]

Does anyone know how to implement this?

 

[Harald Kapp]

Looks suspiciously like homework or an assignment. I'll move the thread therefore to another section of thsi forum.

As a hint:

• four 4-1 multiplexers leave you with 4-out-of-16 signals
• what is the function table of the 4-2 decoder? How would you create a single output from the 4 signals above using this decoder?

 

[Electrical Engineer David]

My bad, I missed the point where you wanted to use a 4 to 2 decoder. To do that you will need your 4:1 multiplexers to have open collector outputs with enable inputs. Open collector outputs are outputs that when they are low connect to ground (a logic 0) but when they are high (logic 1) they just disconnect completely from the output rather than tying the output directly to a logic high.

Also when the chip is disabled it will disconnect from the output. When a chip disconnects from the output you will sometimes hear it called "floating the output" meaning it will allow the output to float to whatever value some other chip drives it to. If the enable input is off then no matter the inputs the output is disconnected (floating). This allows you to connect the outputs of all 4 stage 1 multiplexers together and still avoid the electrical contention which would occur if 1 chip wanted to drive the output high while another chip wanted to drive that same line low. As long as only one of the 4 multiplexer chips is enabled at a time then they won't conflict.

But wait, when the selected multiplexer outputs a logic high it will disconnect from the output which will leave the output floating rather than connecting it directly to Vcc (logic 1). To solve this last issue you place one pull-up resistor on the 4 common (tied together) outputs. When a chip is enabled and drives the output low it is able to pull enough current to pull the output all the way down near zero volts. Think of it as a voltage divider. The output will have a logic 0 resistance to ground of only a few ohms. So a 1 KOhm pullup on the output will result in an output voltage near 0 volts. When the chip wants to output a logic high it floats the output and the result is the 1 KOhm pullup resistor is what drives the output to 5 volts, not really the chip itself.

Now for the enables. Use the 2 to 4 decoder and the select input bits A and B to select which of the 4 multiplexer chips you want enabled. Whallah you have what you wanted.

 

[(*steve*)]

Did you read the bit about us not giving the answers?