I am working in a TOP file of a project in VHDL. I have a problem-question concerning the port mapping.
Below, is a part of the code that I am trying to implement and afterwards the description of the context.
generate_microfib:
for i in 0 to 5 generate
fib: microfib
Port Map (
InL => SignalNumber(num(i)*2 + i), --Input Left
OutR => SignalNumber(num(i)*2 + i+1) --Output Right
);
end generate generate_microfib;
The code above creates multiple components of microfib. The microfib, has an input and an output port. What I want to do is connect the left input of the current created component with the right output of the previous created one.
My initial thought was to do it inside the port mapping, but VHDL doesn't give many options for calculations in the for..generate statement. So, I decided to do it in two steps. Meaning that, I first create the desired components and afterwards do the connections (by setting the desired signals equal).
So, my question is: Is it possible to do the connections in a function for example, by setting:
SignalNumber(i)=SignalNumber(i-1)?
I know that the '<=' operator is accepted and not the '=' but I want to show you that my goal is to make those two signals equal and not assign the value of the one to the other.
When I've done things like this, I've used an array-type signal to act as an intermediate placeholder. Then, within the generate loop, you can connect the ports to the indexed locations on the array signal.
So you could try something like this, using std_ulogic as an example port type:
architecture rtl of entity is:
constant N_DEVICES : integer := 5;
signal port_connector : std_ulogic vector(N_DEVICES downto 0);
begin
-- Concurrent statements
generate_microfib : for i in N_DEVICES-1 downto 0 generate
fib : microfib
port map (
InL => port_connector(i+1)
OutR => port_connector(i)
)
end generate generate_microfib;
-- Other stuff
end architecture rtl;
Then, port_connector(N_DEVICES) acts as the input to the chain, and port_connector(0) acts as the output from the chain.
This isn't a function, but it is (relatively) clean and avoids too much of a headache with the generate statement. Functions in VHDL, though, are meant to produce calculated values, not circuit connections, so I would recommend against trying for that anyhow.
Related
Probably an easy one, but has been a while since I've dabbled in VHDL, but I'm trying to figure out how to properly change generic values of a sub-component after instantiation, when an event happens. Some pseudo code is below that jumps to the point:
U1: PARITY
generic map (N => 8)
port map (A => DATA_BYTE,
ODD => PARITY_BYTE);
....
process(Some_button_click)
begin
// Change generic map value for N (integer) in PARITY to 10 //
end process;
I'm not in front of the code, but I've tried using 'shared variables' to no avail.... any help is greatly appreciated as this has been strangely hard to google.
I understand what you want to do, but as far as I understand generics are mean to be used only to make components reusable and flexible. It does not make sense to change a constant value at run time. My suggestion is that if you need to change the value you better use a signal port
I have a VHDL entity with a generic parameter list. The architecture to this entity calculates several constants, which are needed to create the intended functionality.
Is it possible to access one of these constants from outside?
Example 1:
Let's say there is a FIFO that decides based on DEPTH and OUTREG what the best implementation is (register based, SRL based or BlockRAM based). Depending on this the minimum delay through the FIFO can vary from 1 to 2 cycles.
Example 2:
Consider the same FIFO to be cross clock compatible. Now the min delay depends also on the choosen sync circuits and the frequency difference.
Example 3:
A division entity needs N cycles to calculate a div b. N depends on BITS, RADIX, OUTREG, IS_SIGNED, ...
Further let's say each entity has a MIN_DELAY constant of type NATURAL which is of interest for other instances.
E.g. the instantiating entity needs to know how long it must at least wait for a result.
I can think of 2 solutions, but I think neither is a nice one.
Solution 1:
I could store the algorithmn for the computation in a package so it's globally accessable. But this is against the encapsulation principle :). The outside world only needs to know the delay value not the algorithmn.
Solution 2:
I could use a valid bit. That's always a good solution in dynamic, adaptive or pipelined systems, but this bit can not be used at synthesis time for further choices or optimizations.
Possible solution 3:
VHDL has the ability to define new attributes, but can they be accessed?
Example entity: alu_div:
constant MIN_DELAY : NATURAL := BITS / log2(RADIX) + 2;
attribute DELAY : NATURAL;
attribute DELAY of alu_div : entity is MIN_DELAY;
Example top:
mydiv : entity work.alu_div
generic map (....)
port map (....);
blk : block
constant my : NATURAL := mydiv'delay;
begin
....
end block;
NEW: Possible solution 4:
I found this SE question, where Jim Lewis noted that hierarchical references should also work with constants.
alias MY_DELAY is <<constant mydiv.DELAY : NATURAL >>;
Get internal signals of vhdl design in ncvhdl (alternative to modelsim's signal spy)
This is a modification of Morten's first entity declaration, in which for the 'module' instantiating alu_div I expect there's a component declaration which provides the declaration for the name alu_div.
There is no attribute decorating that declaration so the instantiation found at label alu_div_0 has no attribute DELAY.
If you were to use direct entity instantiation it might work:
entity alu_div is
constant MIN_DELAY : NATURAL := 42;
attribute DELAY : NATURAL;
attribute DELAY of alu_div : entity is MIN_DELAY;
end entity;
architecture foo of alu_div is
begin
end architecture;
entity test is
end entity;
architecture foo of test is
begin
alu_div_0:
entity work.alu_div ;
MONITOR:
process
begin
wait for 1 ns;
report "alu_div'DELAY = " & natural'image(work.alu_div'DELAY);
wait;
end process;
end architecture;
Which gives:
ghdl -a alu_div.vhdl
ghdl -e test
ghdl -r test
alu_div.vhdl:25:9:#1ns:(report note): alu_div'DELAY = 42
>
The idea is that if you use a direct entity instantiation with a selected name (an expanded name) you're using the declaration in the library noted by the prefix (in this case WORK).
The following demonstrates accessing the value of alu_div'DELAY can be done at elaboration:
entity alu_div is
generic (pickone: natural := 1);
constant MIN_DELAY : NATURAL := 42;
constant TARG_DELAY: natural := MIN_DELAY + pickone;
attribute DELAY : NATURAL;
attribute DELAY of alu_div: entity is MIN_DELAY;
-- attribute DELAY of alu_div : entity is TARG_DELAY;
end entity;
architecture foo of alu_div is
begin
end architecture;
entity test is
end entity;
architecture fie of test is
constant fumble: natural := work.alu_div'DELAY;
component alu_div is
generic (pickone: natural := 1);
end component;
begin
alu_div_0:
alu_div
generic map(1);
MONITOR:
process
begin
report "constant fumble = " & natural'image(fumble);
report "alu_div'DELAY = " & natural'image(work.alu_div'DELAY);
wait;
end process;
end architecture;
And that works:
ghdl -a alu_div.vhdl
david_koontz#Macbook: ghdl -e test
david_koontz#Macbook: ghdl -r test
alu_div.vhdl:60:9:#0ms:(report note): constant fumble = 42
alu_div.vhdl:61:9:#0ms:(report note): alu_div'DELAY = 42
Also following Jonathan's comment that the question was attempting to loop information through the instantiated component supplied by generics I tried switching the entity attribute to depend on a generic (commenting out the one with MIN_DELAY, uncommenting the one with TARG_DELAY) and that results in a different error than Morten supplied:
ghdl -a alu_div.vhdl
alu_div.vhdl:36:13: attribute expression for entity must be locally static
ghdl: compilation error
And that error is singularly helpful and easy to find in the 2008 LRM and is quite specific:
7.2 Attribute specification (paragraph 8):
The expression specifies the value of this attribute for each of the named entities inheriting the attribute as a result of this attribute specification. The type of the expression in the attribute specification shall be the same as (or implicitly convertible to) the type mark in the corresponding attribute declaration. If the entity name list denotes an entity declaration, architecture body, configuration declaration, or an uninstantiated package that is declared as a design unit, then the expression is required to be locally static (see 9.4.1)....
This requirement was introduced in the '93 LRM (5.1 Attribute specification). And researching that we find there was a proposal for out-mode generics in the -1992 standardization effort (approved in -1993).
Also in the '87 Issue Report 40 (IR00040.txt) following the first ISAC rationale report of a discussion of the problem as relates to setting the attribute from within an architecture:
Such a capability would greatly (and negatively) affect at least some
implementations. A straightforward approach to the implementation of
specifications is to decorate the named entity with the information
contained in the specification. However, when the entity appears in
one design unit and the applicable specification appears in another,
many problems result. One cannot analyze the specification without
modifying the library unit containing the entity, which can lead to
potential circular chains of dependence. Moreover, multiple
architectures corresponding to a given entity interface cannot each
supply a different value to the attribute of some interface-resident
entity. Finally, there is no LRM requirement that, if one architecture
attributes some interface-resident entity, then all must, which seems
desirable.
You could note the undesired circular dependencies are also possible with an attribute dependent on a generic. Or similarly with out-mode generics the issue moves from circular dependencies in analysis order (locally static expressions in attribute declarations) to elaboration order (evaluating globally static expressions) which is likely quite a bit harder. out-mode generics show sporadic mention in available records, the last time on the vhdl-200x-mp (Modeling and Productivity) email reflector.
It's not likely the status of either of these will change without someone defining how to deal with late binding (linking loader time) order dependencies.
In the mean time as Brian says the accepted method is to use a package commonly shared, which uses locally static constant declarations (and are declaration order dependent). You could also manage the issue with configuration.
Good question. I have sometimes felt the need for "OUT mode generics" too, whose actual value is computed within the architecture, again to allow the higher levels in a hierarchy to know (and adjust to) the pipeline depth of a processing unit.
It might be worth writing a proposal to allow something of the sort in VHDL-201x and submit it to the standards group but meanwhile we have to live with what we have.
My normal solution is to use a package associated with the unit, holding both the initial constant (instead of a generic) and the dependent quantities. This limits the "encapsulation breakage" to those compilation units that use the package, making them at least readily identifiable.
Within the package the constants are deferred where possible, or parameterless (impure) functions, which amount to the same thing.
A possible approach I haven't explored is that an entity declaration, after the PORT list, also allows zero or more entity_delarative_items. If these may include function declarations, then we could potentially return such information that way.
EDIT : David points out an LRM rule (8.3) that prevents this approach with current VHDL versions : a limited relaxation of that rule might be an alternative to "OUT mode generics".
An entity declaration may also include begin and some passive constructs - such as asserts that a set of generics and port widths are consistent. That way you would have to input all the required values, but at least the build would fail reporting errors if e.g. width and depth were inconsistent.
Agree that it is sometimes very useful with information about implementation
details from the entity, even though it breaks the encapsulation principle, but
for white box verification it can be a great help.
Tried to use entity attribute based on entity like:
entity alu_div is
generic(
BITS : positive;
RADIX : positive);
port(
...);
constant MIN_DELAY : NATURAL := BITS / log2(RADIX) + 2;
attribute DELAY : NATURAL;
attribute DELAY of alu_div : entity is MIN_DELAY;
end entity;
But the module where the alu_div is instantiated is not able to access it
using e.g. alu_div_0'DELAY, since ModelSim gives error:
No attribute specification with designator "DELAY" decorates label "alu_div_0".
One method that is useful for white box verification, where the verification
depends on the implementation, is to make an output port with information from
the implementation, like:
entity alu_div is
...
port(
...
DELAY_O : out natural);
...
end entity;
architecture syn of alu_div is
begin
DELAY_O <= MIN_DELAY;
...
It won't be a true constant, since for simulation it will need a delta cycle
before getting the value, but it may be a sufficient solution in many cases.
Another approach I have used is to live with the restriction that all "generic" information flows into a module by specifying as another generic, the result I want from the derived parameter.
For example,
entity alu_div is
generic(
BITS : positive;
RADIX : positive;
DELAY : positive);
port(
...);
Within the architecture, an ACTUAL_DELAY constant is derived from the other generics (plus port bus-widths, etc) and compared with the given DELAY generic.
If the requested DELAY and ACTUAL_DELAY are identical, all is well.
If the requested DELAY exceeds ACTUAL_DELAY, the architecture can insert pipeline stages to meet the request. The overall design will function as intended, though it may consume more registers than strictly necessary.
Otherwise the requested delay cannot be met, and the architecture asserts with severity FAILURE.
I want to map the lower bit (bit0) of a 32 bit output port to a signal and leave the upper bits unconnected (OPEN). Is there a way to treat this mapping as an aggregate?
I've tried the following to no avail:
port map (
some_output => ( 0 => sig_1, others => OPEN)
);
The below is not a valid answer, since it does not adhere to the VHDL standard. This "answer" is however kept here, and not deleted, since it shows what construction to avoid for tool compliance, since some tools apparently accept this non-standard compliant VHDL code.
Example of port mapping without association of all scalar subelement, which is accepted by Altera Quartus II and Mentor ModelSim in some cases, but is also likely to result in warning or error:
port map(
some_output(0) => sig_1,
-- some_output others are simply not included in port mapping
So, as David Koontzs point out in the comment, the VHDL standard (IEEE Std 1076-2008) section "6.5.7 Association lists" describes:
... every scalar subelement of the explicitly declared interface object shall be associated exactly once with an actual (or subelement thereof) in the same association list, and all such associations shall appear in a contiguous sequence within that association list. Each association element that associates a slice or subelement (or slice thereof) of an interface object shall identify the formal with a locally static name.
So it is not valid to only associate some of the scalar subelements, since the standard says "every scalar subelement", and open can't be used for the remaining scalar subelements, since the standard says "Each association element that associates a slice ... of an interface object shall identify the formal with a locally static name.".
Another solution is this:
signal my_vector : STD_LOGIC_VECTOR(3 downto 0);
signal my_vector_float : STD_LOGIC_VECTOR(3 downto 0);
port map (
[...]
some_output(3 downto 0) => my_vector,
some_output(7 downto 4) => my_vector_float,
[...]
);
You can now setup some report filter rules to silence all warning concerning *_float signals.
I am preparing for an exam by going through some old ones. One of the questions is:
Write the synthesizable behavioral VHDL code that implements the synchronous FSM in fig...
The FSM has one input, called request, that is of enumeration type with values (r1, r2, r3)...
That makes me want to write this code:
entity fsm is
port ( clk : in std_logic;
request : in my_enum_type
);
end fsm;
And somewhere have a:
type my_enum_type is (r1, r2, r3);
somewhere (I have tried right befor the port declaration and right after the architecture declaration).
But I can't seem to get that to work.
Can I have custom types as inputs or outputs like that?
Yes you can, and I regard it as best practice - it means least work, best understanding, easiest maintenance, and cleanest design.
The trick is to declare the types common to your whole design in a package (I usually call it "Common" :-) and add use work.Common.all before the entity declaration AND in every customer of that entity. More specialised components can have appropriate names, of course!
For example:
package Common is -- untested...
type my_enum_type is (r1, r2, r3);
-- (optional) useful tools
function to_slv (e : my_enum_type) return std_logic_vector;
function to_enum (s : std_logic_vector(my_enum'length downto 0))
return my_enum_type;
end Common;
package body Common is
-- subprogram bodies here
end Common;
Now when you add a value to the enumeration, you ONLY modify "Common" and rebuild the design, while those who follow conventional guidelines are still trying to identify every port and signal where they have to increase the range of their "std_logic_vector" by 1.
Works really well for bus interfaces too, where a record in each direction hides all the individual bus and handshaking signals.
You WILL have to fight brain-dead tools like Xilinx "automatic testbench generator" which will helpfully translate ALL your port types - integer or boolean as well as custom - into std_logic(_vector) and then fail to compile. Just translate them back again.
You can still make a case that at the very top level, all the external FPGA pins should still be std_logic based. And if you ever need to simulate a post-synthesis version of your design, then you will either need to live with std_logic_vector ports, or add a simple wrapper to convert from one form to the other.
Quick VHDL question, I don't have access to Xilinx at the moment due to dead laptop, so can't test this.
I was wondering if it's possible to use variables and arithmetic in 'downto' statements, e.g:
proc: process (x)
begin
y <= z(x downto 0) & z(7 downto x);
end process;
Thanks.
Yes, for reference look this page. Arrays allow integer expressions as a definition of the endpoints of a range.
My guess is that it would synthesize to a large and ugly multiplexer instead of a simple shift register which is looks like you are trying to create.