Falling and Rising in same process

[YodaTheGreat]

Hi All,

I have 2 Signal INPUT_A and INPUT_B.

And I want Inc/Dec a variable with this signals

Process (INPUT_A,INPUT_B,Reset)
Begin
If Reset = '1' Then
-- Async reset etc
Else
If Falling_Edge(INPUT_A) Then
If INPUT_B='1' Then
Variable <= Variable + X"01";
Else
Variable <= Variable - X"01";
End If;
End If;
If Falling_Edge(INPUT_B) Then
If INPUT_A='1' Then
Variable <= Variable - X"01";
Else
Variable <= Variable + X"01";
End If;
End If;
If Rising_Edge(INPUT_A) Then
If INPUT_B='1' Then
Variable <= Variable + X"01";
Else
Variable <= Variable - X"01";
End If;
End If;
If Rising_Edge(INPUT_B) Then
If INPUT_A='1' Then
Variable <= Variable - X"01";
Else
Variable <= Variable + X"01";
End If;
End If;
End If;
End Process;


But This Code don't Work.....

Have you a Solution, Please.
Some Code for Me.

Best regards Yodathegreat.
VHDL Beginer..

 

[VHDLguy]

Firstly, if you plan on using code for real logic (as opposed to simulation) then you can only have one edge detection per process (be that a wait until, or a if clk'event etc).

so what you are trying to do here is going to take some thought. I'll come back with some ideas.

--

http://www.lateralsands.com/

 

[VHDLguy]

is there any other way you can do this?

And if not, how do A and B relate to each other? Are they synchronous to some clock but never change at the same time?

--

http://www.lateralsands.com/

 

[YodaTheGreat]

Hi VHDLGuy,

I know for one EVENT by process..
I have read some topic in this forum.

now, in my source code I have 4 Procres..
But it's a same probleme....

A is a square ware (10khz)
B is a square ware (A with 90°)

_____ ______
A => ___| |______| |_____
_____ ______
B => _____| |______| |_____

best regards christophe (Yodathegreat)

 

[VHDLguy]

ok that makes it easier. I take it that the square wave doesn't always exist (by exist i mean - the rise doesn't alway happen), but when they do exist that they follow the rule you mentioned. (and based on what you want to do, B can exist without A existing and visa verca - again with 'exist' meaning that the rise occurs or not).

Some questions:
do you have a clock? (list all of them) and is it synchronous to these 10khz signals? does the squarewave stay high for the same amount of time always?

A clock would make it much easier. If not I will have to think some more.

--

http://www.lateralsands.com/

 

[VHDLguy]

wait. I just realised. your rules don't match your counting logic.

if B is 90 degrees out of phase with A then this:
If Falling_Edge(INPUT_B) Then
If INPUT_A='1' Then
Variable <= Variable - X"01";
Else
Variable <= Variable + X"01";
End If;
End If;

doesn't have any use. A will always be '0' when B is falling.
Therefore there must be more that you haven't explained. (or your logic is wrong).

--

http://www.lateralsands.com/

 

[AllEars]

Yoda:

Is your application related with position-encoder? I have a way to work it out. VHDLguy is right, use more than one edge in one process can be dangerous.

We can design 4 similar entities, each is a counter.
Counter 1 works only on A's rising edge, it increases or decreases according to B's high or low. Counter 2 works only on A's falling edges. Counter 3 and 4 use B's edges and A's high/low.

Create a higher level entity that use the 4 counter as its components. The final output should be something like
CNT <= CNT1 - CNT2 + CNT3 - CNT4.

I am new in VHDL design. Hope to learn more form you guys.