VHDL How to define integer range based on result from procedures declaration - variables

I want to make my integer type signal range depend on output of some procedure i.e. something like this
procedure blah( signal A : in std_logic_vector(0 to 1); signal B : inout integer)
Begin
Case A is
When "00" => B <= 215;
When "01" => B <= 300;
When others => B <= 105;
end case;
End;
Next I want to use output from this procedure to define signal range of say type integer
Blah(A,myrange);
Signal myint: inout integer range 0 to myrange/23;
The difficulity is I can not use procedure in declaration of architecture, is there any way to define variable range for integer or is there a way to define integer range within architecture part after begin statement

As explained in the comments you cannot have a variable range for integer type. VHDL is a Hardware Description Language. On a pure hardware point of view, what would an integer type with variable range mean? Suppose you use the myint signal as an input of an adder. If its type changes during the lifetime of your hardware, it means that you have some dynamic silicon too. Some transistors would appear suddenly when the type range increases, and they would vanish when the type range decreases. It does not really make sense, does it?
So, if you understand this constraint, you must also understand that there is no need for varying signals to define your type. What you need is plain constant values that can be computed statically at the beginning of the simulation or logic synthesis. Two possibilities here:
First case: this A input that you use to compute the range is really an input of your design that can change during the lifetime of your hardware (switches of your board, for instance). Then, you must dimension your type to the worst case (300/23), such that it is always sufficient, even when A takes the worst case value "01":
signal myint: integer range 0 to 300 / 23;
Note: there is no mode specification for internal signal declarations, no inout. Internal signals can always be assigned and read. It is entity ports, for instance, that have a mode (in, out...) constraining how they can be used.
Second case: this A value is constant during the lifetime of your hardware. To make the point very clear, it means that if you decide to program an FPGA or to manufacture an Integrated Circuit (IC) with your design, you will fix the value of A before programming the FPGA or manufacturing the IC and the hardware will work only with this value. No way to change it afterwards without re-programming the FPGA or manufacturing another IC.
Then, if you are in this case, it would be much better to declare it as a generic parameter than as an input port or an internal signal. This is exactly what generic parameters are intended for. And, as explained in the comments, you cannot use a procedure here, but you can use a function. As your procedure has only one output, a function also does the job:
entity foo is
generic(A: std_logic_vector(0 to 1));
port(...);
end entity foo;
architecture rtl of foo is
function blah(val: in std_logic_vector(0 to 1)) return integer is
begin
case val is
when "00" => return 215;
when "01" => return 300;
when others => return 105;
end case;
end function blah;
signal myint: integer range 0 to blah(A) / 23;
If you really also need this procedure during operations (for a completely different purpose than declaring types), you can even use the function to define the procedure:
procedure blah(signal A: in std_logic_vector(0 to 1); signal B: out integer) is
begin
B <= blah(A);
end procedure blah;
Note that I changed the mode of B to out instead of inout because... it is only used as an output by your procedure, never as an input.

Related

Verilog: Use of register: When are the values actually updated?

When exactly are the variables inside an always-block in Verilog updated?
E.g. if the reg C is changed multiple times in an always-block: does the value of it always change? Or is the value only "physically written" to the reg at the end of the always-block?
Would it be better to use an extra intermediate register which is only actualized at the end of the always-block? Would it make any difference?
reg C;
always #(*)
C = 0;
C = A;
C = 1;
C = B;
end
Other example:
If I have a module with an always block as follows, could the multiple assignments of the output exhibit sort of a glitch, where the output quickly goes to 0 before getting the value of B? Does it make a difference if I use blocking (=) or non-blocking (<=) assignments?
module example1 (output C, input A, input B);
always #(*)
begin
C = 1’b0;
if (A==1)
C = B ;
end
endmodule
Example with intermediate register to avoid unwanted change of the output.
module example1 (output C, input A, input B);
reg intermediateReg;
always #(*)
begin
intermediateReg = 1’b0;
if (A==1)
intermediateReg = B ;
end
C <= intermediateReg;
endmodule
In the examples you provided the code behaves identically to the 'c' code. The variable gets re-assigned new values and the last one wins. In verilog this is true for blocking (=) or non-blocking (<=) assignments, but not for the mix of them.
As in 'c' it is up to the compiler optimization technique. It can eliminate unneeded assignments, so only the last one is relevant.
In your first example the value of 'C' will be 'B' at the end of the block. The value of 'C' in the second example will either be 0 or B, depending on A.
Now, for the purpose of a simplicity of explanation, mixing of blocking and non-blocking assignments will cause the last 'non-blocking' assignment win. The reason is that all nba's are executed after the block is finished.
always #(*) begin
C <= A;
C = B;
end
The value of C will be A at the end of the simulation cycle.
As for the question about using intermediate values, there is no difference. You use them as needed. The simulation compiler or synthesis will optimize your code in any case. The last example is absolutely normal, except that it does not change anything at all. It behaves the same way as this, which is more explicit and more readable in my opinion. The only problem is that you incorrectly use nba assignments int combinational logic (which i fixed).
always #(*)
begin
if (A==1)
C = B ;
else
C = 1'b0;
I guess there is another silent question in your post, will the intermediate value cause events on 'C', the answer in this case is no. There will be no events produced by the always block till it finishes its execution (or till it hits a delay or starts waiting on an event). So, only the last value is relevant.
In a testbench code you can see a situation like that:
always #* begin
C = A;
C = B;
#5 C = D;
end
In the above case The value of C will become B. The execution of the always block will stop for #5 delays. During that time other blocks will see the value B. In #5 ticks, the value will be changed to D.
If you want to know what blocking and non-blocking assignments do in standard Verilog (like you would have in simulation), you can easily look that up.
If you're talking about hardware, the type of hardware you get will depend on what's in the sensitivity list, not type type of assignments you use. The type of assignment will affect the combinational logic that is synthesized from your code. Assuming you aren't doing anything unsynthesizable, the logic you get should be equivalent to what the Verilog standard specifies but won't be a 1:1 direct mapping of your code.
So, no, adding intermediates won't necessarily help with glitches in the hardware. There will be other code and timing specifications, etc. for dealing with stuff like that.

Summation iterated over a variable length

I have written an optimization problem in pyomo and need a constraint, which contains a summation that has a variable length:
u_i_t[i, t]*T_min_run - sum (tnewnew in (t-T_min_run+1)..t-1) u_i_t[i,tnewnew] <= sum (tnew in t..(t+T_min_run-1)) u_i_t[i,tnew]
T is my actual timeline and N my machines
usually I iterate over t, but I need to guarantee the machines are turned on for certain amount of time.
def HP_on_rule(model, i, t):
return model.u_i_t[i, t]*T_min_run - sum(model.u_i_t[i, tnewnew] for tnewnew in range((t-T_min_run+1), (t-1))) <= sum(model.u_i_t[i, tnew] for tnew in range(t, (t+T_min_run-1)))
model.HP_on_rule = Constraint(N, rule=HP_on_rule)
I hope you can provide me with the correct formulation in pyomo/python.
The problem is that t is a running variable and I do not know how to implement this in Python. tnew is only a help variable. E.g. t=6 (variable), T_min_run=3 (constant) and u_i_t is binary [00001111100000...] then I get:
1*3 - 1 <= 3
As I said, I do not know how to implement this in my code and the current version is not running.
TypeError: HP_on_rule() missing 1 required positional argument: 't'
It seems like you didn't provide all your arguments to the function rule.
Since t is a parameter of your function, I assume that it corresponds to an element of set T (your timeline).
Then, your last line of your code example should include not only the set N, but also the set T. Try this:
model.HP_on_rule = Constraint(N, T, rule=HP_on_rule)
Please note: Building a Constraint with a "for each" part, you must provide the Pyomo Sets that you want to iterate over at the begining of the call for Constraint construction. As a rule of thumb, your constraint rule function should have 1 more argument than the number of Pyomo Sets specified in the Constraint initilization line.

When must/should I declare a variable with the range keyword in VHDL?

I am new to VHDL and have perhaps a basic question, but here goes:
When declaring a variable, say an integer, what is the benefit of
variable count_baud : integer range 0 to clk_freq/baud_rate - 1 := 0;
vs.
variable count_baud : integer := 0;
Is the point of using range (only) to limit the size of the synthesized real estate in the CPLD/FPGA?
There are two very good reasons:
Debugging. If you know that your integer shall stay in the [min..max] range, tell it to the simulator with a proper range declaration. In case there is a bug in your code and you try to assign an out-of-range value, the simulator will let you know with a very useful message. While if you just declared an integer the error could happen long after the bogus assignment.
Synthesis quality. A logic synthesizer, by default, will allocate 32 bits for an integer. Depending on the surrounding it may discover that less bits are sufficient... or not. So, telling the synthesizer what the real range is frequently saves hardware, power and increases the final performance (speed), especially if the real range can be represented on much less than 32 bits.

I want to make vhdl divider

i'm now using FPGA spartan3,
i want to calculate 'result' which is represented below formula.
and the 'result' should be returned as integer type. So i set up all the variables with integer type but it doesn't work.
result <=((a*b*7894*7)/(w*temp_constant));
i've set a,b,c,w,temp_constant as variables
variable a : integer range 0 to 99;
variable b : integer range 0 to 9999;
variable w : integer range 0 to 200;
variable temp_constant : integer range 0 to 99;
but the operator '/' doesn't work at this synthesis. the error msg was
'Operator '/' must have constant operands or first operand must be power of 2"'
The error message is almost (see the note below) 100% clear: divisions are not supported by your synthesis tool, except with constant operands (the result is computed by the synthesizer in the constant propagation phase) or with divisors that are powers of 2 (the division is a simple right shift).
One possible reason for this limitation of your synthesis tool is that there are many of ways to compute integer divisions in hardware and typing just / in a VHDL code is not enough to chose among them. There may be other reasons.
In your case where operands are not constants, and the divisor is not a power of 2, you must design this divider yourself at a lower level. If you have no idea about hardware implementations of integer dividers you will have to search a bit. This is a very classical topic, it should be easy to find good resources. Just a hint: pre-computing all inverses in fixed point representation, storing them in a read-only memory and using multiplications instead of divisions is an option.
Note: I find the error message you got (first operand must be power of 2) a bit surprising. Unless the term first operand is supposed to designate the divisor, which is not that common, it is probably a bug and the correct error message should be: second operand must be power of 2. Or, even better: divisor must be power of 2.

signal vs variable

VHDL provides two major object types to hold data, namel signal and variable, but I can't find anywhere that is clear on when to use one data-type over the other. Can anyone shed some light on their strengths/limitations/scope/synthesis/situations in which using one would be better than the other?
Signals can be used to communicate values between processes. Variables cannot. There are shared variables which can in older compilers, but you really are asking for problems (with race conditions) if you do that - unless you use protected types which are a bit like classes. Then they are same to use for communication, but not (as far as I know) synthesisable.
This fundamental restriction on communication comes from the way updates on signals and variables work.
The big distinction comes because variables update immediately they are assigned to (with the := operator). Signals have an update scheduled when assigned to (with the <= operator) but the value that anyone sees when they read the signal will not change until some time passes.
(Aside: That amount of time could be as small as a delta cycle, which is the smallest amount of time in a VHDL simuator - no "real" time passes. Something like wait for 0 ps; causes the simulator to wait for the next delta cycle before continuing.)
If you need the same logic to feed into multiple flipflops a variable is a good way of factoring that logic into a single point, rather than copying/pasting code.
In terms of logic, within a clocked process, signals always infer a flipflop. Variables can be used for both combinatorial logic and inferring a flipflop. Sometimes both for the same variable. Some think this confusing, personally, I think it's fine:
process (clk)
variable something : std_logic;
if rising_edge(clk) then
if reset = '1' then
something := '0';
else
output_b <= something or input c; -- using the previous clock's value of 'something' infers a register
something := input_a and input_b; -- comb. logic for a new value
output_a <= something or input_c; -- which is used immediately, not registered here
end if;
end if;
end process;
One thing to watch using variables is that because if they are read after they are written, no register output is used, you can get long chains of logic which can lead to missing your fmax target
One thing to watch using signals (in clocked processes) is that they always infer a register, and hence leads to latency.
As others have said signals get updated with their new value at the end of the time slice, but variables are updated immediately.
// inside some process
// varA = sigA = 0. sigB = 2
varA := sigB + 1; // varA is now 3
sigC <= varA + 1; // sigC will be 4
sigA <= sigB + 1; // sigA will be 3
sigD <= sigA + 1; // sigD will be 1 (original sigA + 1)
For hardware design, I use variables very infrequently. It's normally when I'm hacking in some feature that really needs the code to be re-factored, but I'm on a deadline. I avoid them because I find the mental model of working with signals and variables too different to live nicely in one piece of code. That's not to say it can't be done, but I think most RTL engineers avoid mixing... and you can't avoid signals.
Other points:
Signals have entity scoping. Variables are local to the process.
Both synthesize