# HELP designing a logic circuit.

Discussion in 'Homework Help' started by djkobra81, May 5, 2009.

1. ### djkobra81 Thread Starter New Member

May 5, 2009
1
0
design a circuit whose output will be high if the 4-bit data input value is greater than 9.

I have attempted to create a truth table and a Kmap, but will no luck.

Last edited: May 5, 2009
2. ### StayatHomeElectronics Well-Known Member

Sep 25, 2008
864
40
What ideas do you have to begin?

3. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
If we label the bits B3, B2, B1, B0 it should be manifestly clear that there are six values greater than 9, namely 10, 11, 12, 13, 14, and 15

Each of them has B3 = 1. Equally clearly the value of B0 is a don't care -- that is it has no effect on the result. What we are left with is that either B2 or B1 or both must be equal to 1.

Thus:

GREATER_THAN_9 = B3 & (B2 | B1) ;

or in words B3 .AND. (B2 .OR. B1) ;

Can ya draw the K-map now?

4. ### mik3 Senior Member

Feb 4, 2008
4,846
63
All the 4 bit combinations above 9 should give a one on the truth table. Use SOP or POS to derive the boolean equation and then the logic circuit.

If you want you can use a simple DAC with 4 resistors and a comparator to sense the desired value.