# Forum: FPGA, VHDL & Verilog Implement a VHDL program using with select for PAL

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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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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:
  with I3&I2&I1&I0 select O3 <= '1' when "0-11", '1' when "001-", '1' when "1--0", '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
  with I3&I2&I1&I0 select O3 <= '1' when "0011", '1' when "0111", '1' when "0010", '1' when "0011", '1' when "1000", '1' when "1010", '1' when "1100", '1' when "1110", '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.

: Edited by Moderator

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I see thanks a lot

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