# Multiplexer

Discussion in 'Homework Help' started by garcon, Jan 3, 2014.

1. ### garcon Thread Starter Member

Jan 3, 2014
33
0
Hi, can any one help on how to resolve this problem please?

I add english version as follow:
A racing car has a protection system to prevent the pilot from
forcing the machine to go beyond the limits of the engine .
In addition , the car has two modes of operation : run mode, and training mode. In training mode, the system
is more permissive with the driver and allows him to force the engine a bit more.
This protection is implemented with a combinational system that takes as input the
power demanded by the driver, which is encoded on values between 0 and 7.
The system outputs the input value divided by 2. In trainig mode, the outputs are rounded
up , and training mode these outputs are rounded down . For example , if the
pilot requested a power of 5 in training mode, the system will enable power
of 3 (5/2 = 2.5) , but in training mode will allow the system 2 .
You need to design combinatorial system that receives inputs the desired power by
the pilot and the mode of operation, and outputs the allowable power . To do this:
a) Define the inputs and outputs of your system. (names and coding) .
b) Write the truth table .

2. ### tshuck Well-Known Member

Oct 18, 2012
3,527
675

Start by:
a) Define the inputs and outputs of your system. (names and coding) .
b) Write the truth table .

Also, in your description, you have not indicated what happens when the car is in run mode, though what you meant is discernable.

3. ### garcon Thread Starter Member

Jan 3, 2014
33
0
Hi tshuck, thanks for the hint.
But my question is that as I am completely new, I have can say I am almost lost in this exercise where to start. I would greatly appreciate if you can help me on how to put things on paper. And also is it a Multiplexer case or decoder, demultiplexer?
I have learnt all these recently and I have a vague and uncleared knowlledge of these.
for your last question, there is no such cases.
much appreciate

dan

4. ### tshuck Well-Known Member

Oct 18, 2012
3,527
675
If there are no run cases, why is it even mentioned? And, how does this make any sense?
Okay, so break it down. How many inputs do you have? How many bits do you need to select one of two options? How many bits do you need to represent a number between 0 and 7?

How many outputs are needed to represent a maximum count of 7/2 rounded up?

5. ### garcon Thread Starter Member

Jan 3, 2014
33
0
Thanks.

Ok, let me try,
How many inputs do you have?
=> I think I have 8 inputs (0,1,2,3,4,5,6,7)

How many bits do you need to select one of two options?
=> 3 selectors S0,S1,S2
How many bits do you need to represent a number between 0 and 7?
=>3 bits.
Is that correct? and what should I do next?

Jan 3, 2014
33
0
7. ### tshuck Well-Known Member

Oct 18, 2012
3,527
675
Almost. you only need one bit to represent two options (e.g. 1 = run mode and 0 = training mode)

Sorry, I should have phrased it to say represent two options...

You are getting ahead of yourself. This is just to identify inputs and outputs. The output is a number between 0 and (7/2) rounded up. The input from the driver is a number between 0 and 7, which, as you surmised, requires 3 bits.

Inputs:
-Driver input (0 to 7, requiring 3 bits)
-mode selection (run or training mode, requiring1 bit)

Output:
-allowable power (0 to 7/2 rounded up, requiring x bits)

Once you've identified them, name them. Then you are done with part A.

Oct 18, 2012
3,527
675

9. ### garcon Thread Starter Member

Jan 3, 2014
33
0
Input from driver : 0,1,2,3,4,5,6,7 requires 3 bits 000,001,010,011,100,101,110,111 is this what you mean by 3 bits?

What does this indicate this ? Run/training=1,0 what I do with this bits?
Is there a difference between allowable power and output? As both seems between 0 and 7/2 round up

10. ### garcon Thread Starter Member

Jan 3, 2014
33
0
I saw few mistake of translation and I recorrect the sentece above,....

A racing car has a protection system to prevent the pilot from
forcing the machine to go beyond the limits of the engine .
In addition , the car has two modes of operation : run mode, and training mode. In running mode, the system is more permissive with the driver and allows him to force the engine a bit more.
This protection is implemented with a combinational system that takes as input the
power demanded by the driver, which is encoded on values between 0 and 7.

The system outputs the input value divided by 2. In run mode, the outputs are rounded
up , and training mode these outputs are rounded down . For example , if the
pilot requested a power of 5 in run mode, the system will enable power
of 3 (5/2 = 2.5) , but in training mode will allow the system 2 .
You need to design combinatorial system that receives inputs the desired power by
the pilot and the mode of operation, and outputs the allowable power . To do this:
a) Define the inputs and outputs of your system. (names and coding) .
b) Write the truth table .

11. ### tshuck Well-Known Member

Oct 18, 2012
3,527
675
Yes, the 8 possible input values from the driver can be represented with 3 bits.

Per your initial post, there are two running modes that determine the rounding direction(up or down).
This is what the question from post # 1 references as the output...

12. ### garcon Thread Starter Member

Jan 3, 2014
33
0
Ok, now:

Inputs: (0,1,2,3,4,5,6,7)
selects: s2,s1,s0
.------------------------
output: for the rounded up..run (from 0 to 7/2 rounded up.i.e. 0,1,1,2,2,3,3,4)...this means 0/2=0; 1/2=0.5 and when rounded up =1; 2/2=1, 3/2=2, 4/2=2, 5/2=3, 6/2=3, 7/2=4
Out put:for the rounded down training: 0,0,1,1,2,2,3,3
How can I use both ?

Last edited by a moderator: Jan 3, 2014
13. ### tshuck Well-Known Member

Oct 18, 2012
3,527
675
Again, the input for the mode is only one bit - there are only two choices. You can use three, but them you will ignore two of them (unless you do one hot encoding, or some other kind, but I think that's a little further in your courses).
Okay, so how many bits must you have to represent all possible output values?
You need to start by giving them names as part A requires...

14. ### garcon Thread Starter Member

Jan 3, 2014
33
0
I am thinking this way now,
=>driver input: (0 to 7): 8
=>control bits (select for the driver inoputs 3 bits namely s2,s1,s0)
=>mode (running or training needs 1 bit of selection), I mean to choose either run or training.
First is this correct? if not plelase correct me with the correct values, thanks.

15. ### tshuck Well-Known Member

Oct 18, 2012
3,527
675
You need to clarify what your signals are...

What are the control bits and what are they coming from?

Also, is there a specific approach that should be used here (multiplexers, adders, decoders, basic gates, etc.)?

16. ### garcon Thread Starter Member

Jan 3, 2014
33
0
The question seems that I should identify what to use:
multiplexers, adders, decoders, basic gates, etc.
I think an experimented person knows by reading the problem what to use but as it is my first exercise, I have no idea.
I am just looking for help what to do and where to start.

17. ### tshuck Well-Known Member

Oct 18, 2012
3,527
675
I am trying to show you where to start: identify and list the input and outputs as part A requires, but you don't seem to want to...

18. ### garcon Thread Starter Member

Jan 3, 2014
33
0
sorry, it is not that I don't want but I think I am a bit lost.

19. ### tshuck Well-Known Member

Oct 18, 2012
3,527
675
No worries, it takes a little while to get used to.

Now, we've established that there are 3 inputs from the driver, another from the mode, and three bits for the output.

Make a table table that represents this information with the inputs to the left and the output on the right, then fill in all possible values for each input and its corresponding output.

20. ### garcon Thread Starter Member

Jan 3, 2014
33
0
Thanks:

You mean by the 3 inputs from the driver the 3 selectors?
why 3 bits for the output?
S2 S1 S0 C
0 0 0 ?
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 0