# Studying for a test help appreciated

#### TimTimBoBim

Joined Nov 23, 2008
6
hi

i have a test coming up and my teacher was nice enough to provide us some example questions. there are two that i don't understand. i have a really hard time with these types of problems and i think i need a step by step. i would really appreciate a walkthrough on them. thank you

1. design a combinational circuit that compares two 4-bit numbers to check if they are equal. the circuit output is equal to 1 when the two numbers are equal to 0 otherwise. assume two numbers are A = A3 A2 A1 A0, B = B3 B2 B1 B0. (those are A sub 3 A sub 2 etc)

i think i just need a boolean function for number 1

2. design and implement a sequential circuit for even parity check, using t flip flops, received bit by bit in serial. the circuit output is 0 when the number of 1s in binary code is even; otherwise, output is 1.

for number 2 i need the boolean function and i need to draw the circuit.

#### beenthere

Joined Apr 20, 2004
15,819
Get the data sheet for a 7485 magnitude comparator to see how that is done.

The parity generator is very easy to imagine if you spend a bit thinking about it (what makes a number even?). Post up some result if you get somewhere.

#### TimTimBoBim

Joined Nov 23, 2008
6
I found the data sheet you mentioned and i don't really know what i'm looking at. it looks far more complicated than what i'm doing.

For the second one, i've spent a lot of time thinking about it over the pas few days but i still don't understand the concept of it. this is an intro to digital logic class and my teacher doesnt explain very well.

#### beenthere

Joined Apr 20, 2004
15,819
If you want to post up what you're doing, then there is something to talk about.

A larger hint: If bit 0 is clear, then the value must be even.

#### TimTimBoBim

Joined Nov 23, 2008
6
That's the thing. i don't really know where to start on either of these problems. the professor gave us 12 example questions on what to expect. i've completed 10 of them and these two quite frankly i don't understand at all.

#### TimTimBoBim

Joined Nov 23, 2008
6
Okay so for the first problem i think i know how to draw it. i have 4 xor gates taking 2 inputs each. then 2 and gates taking two inputs each and then 1 and gate taking 2 inputs.

in my head that makes sense. but i'm not supposed to draw it, i'm supposed to get the boolean function. so i don't know how to get that.

#### TimTimBoBim

Joined Nov 23, 2008
6
also, for the second problem i'm starting to understand t flip flops but how many inputs will i have? what does bit by bit in serial mean? any hints as to how many t flip flops i'll need?

#### beenthere

Joined Apr 20, 2004
15,819
For that kind of serial data accumulation, think shift register. I've already told you how to test for even. The length of the register is not related to the odd/even property.

I am rusty with Boolean. I just put it together with gates, as I am generally building the function anyway.

#### TimTimBoBim

Joined Nov 23, 2008
6
shift registers have never been discussed in this class. i don't think thats what he's looking for. he said use t flip flops.

i know your tryign to get me to think about it myself but i honestly dont know what to do .

#### beenthere

Joined Apr 20, 2004
15,819
Shift registers are composed of toggle flip flops.