Implement a VHDL program using with select for PAL

James Guest #6372245
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Hello , I'm doing a task that got me a bit confused.
So basically as you can see on the picture I converted the PLA to PAL 
then I wrote the logical equations for each output . (O3 O2 O1 O0)

Now I want to implement a VHDL with the use of WITH select . Correct me 
if I'm wrong but if I want to write the VHDL program with the use of 
WITH select than I will need at first the truth table which contains 4 
Inputs (I3, I2, I1, I0)
and 4 outputs (O3, O2, O1, O0) .

The problem here is that if I want to extract the truth table from the 
PAL or the PLA then there will be a problem . Check for example the 
first line of PAL or PLA : 3 Inputs are defined (I0 = 1 , I1 = 1 , I3 = 
0) and one input is undefined .Their 4 outputs are (O3 = 1 , O2 =1 , O1 
=0 ,O0 = 1) .
In this case I can't know which state I2 will be .

If I can't extract the truth table from the PAL or PLA then normally I 
would extract it from the logical equations but I don't know how is this 
possible in our case . The equations are not easy to extract to find 
their truth table .
It's easy to extract the logical equations from the truth table and not 
the other way around because what I got in my case is simplfied logical 
equations .
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Lothar M. Moderator (Company: Titel) #6373327
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James wrote:
> I will need at first the truth table which contains 4 Inputs ...
> and 4 outputs (O3, O2, O1, O0) .
Much easier would be to split up the output vector into 4 seperate 
with-select blocks: each one for O3, O2, O1 and O0


> Check for example the first line of PAL or PLA : 3 Inputs are defined
> (I0 = 1 , I1 = 1 , I3 = 0) and one input is undefined
> (I0 = 1 , I1 = 1 , I3 = 0) and one input is undefined
That one input is not "undefined", it just "doesn't care".
So check out wether your toolchain is able to handle the '-' (don't 
care) for synthesis. It will make things easier:
1
 with I3&I2&I1&I0 select O3 <=

2
   '1' when "0-11",

3
   '1' when "001-",

4
   '1' when "1--0",

5
   '0' when others;


If you can't use '-' then you will have to write the don't care by hand 
like that:
I0=1, I1=1, I2=0, I3=0 and also the same for I0=1, I1=1, I2=1, I3=0
1
 with I3&I2&I1&I0 select O3 <=

2
   '1' when "0011",

3
   '1' when "0111",

4
   '1' when "0010",

5
   '1' when "0011",

6
   '1' when "1000",

7
   '1' when "1010",

8
   '1' when "1100",

9
   '1' when "1110",

10
   '0' when others;


James wrote:
> If I can't extract the truth table from the PAL or PLA then normally I
> would extract it from the logical equations but I don't know how is this
> possible in our case .
The truth table is directly and obvoiously visible in the PLA structure.
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