How to write to "wire" and passing parameters to arrays of instances in Verilog

Question

I have the following code:

module X;
    wire [11:0] mga [0:15]; 
    // other code omitted here for clarity

    reg [11:0] globalStartAddresses [0:15];
    reg [11:0] globalEndAddresses [0:15];

    localMMU mmu[0:15](clock, globalStartAddresses, globalEndAddresses, mrw, mdi, mdo, mga);

    task dispatcher;
        // parameters
        output reg dataReady;
        input readWrite;
        input [7:0] dataIn;
        output reg [7:0] dataOut;
        input [11:0] globalAddress;

        // local variables
        integer x, selected;

        begin
            for(x=0; x<16; x=x+1) begin
                if(globalAddress >= globalStartAddresses[x] && globalAddress <= globalEndAddresses[x]) selected=x;
            end

            mrw[selected]=readWrite;
            mdi[selected]=dataIn;       
            assign mga[selected]=globalAddress;

            if(readWrite==1)
                wait(mdo[selected]!=0);
            else
                wait(mdo[selected]!=dataIn);

            dataOut=mdo[selected];
        end
    endtask
endmodule

I get 2 errors in the code:

In the array instance declaration of "localMMU", it states that "globalStartAddress[ ] should be present", but I have declared it just above as is evident.
In the assign statement inside the task, it states that only variables are allowed on LHS.

(I am using VeritakWin 3.84F) Please tell me how can I rectify these 2 problems. Thanks.

Community · Accepted Answer · 2017-05-23 12:07:31Z

1

globalStartAddresses and globalEndAddresses are both of type array but array can not be passed directly between two modules. You must flatten the array into a vector and then pass it to a module. The problem is answered here :

How to pass array structure between two verilog modules
Left hand side of procedural continuous assignment can not be of type wire while in continuous assignment, left hand side must be just of type wire. As a solution, change mga type to reg. The same problem answered here:

Continuous assignment verilog

edited May 23, 2017 at 12:07

CommunityBot

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answered Feb 22, 2016 at 19:07

Mehran Torki

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7 Comments

user1637645 Over a year ago

1. If I flatten the array, I cannot use instance-array, I would have to declare 16 instances: mmu0, mmu1, ... like that, instead of mmu[0:15]. Is there a way around this? 2. The "mga" is used as a parameter to "inout" port of another module (not shown in the code), but if I change it to "reg", it gives an error that "reg" cannot be passed as output parameter. It is "inout" and not "output". Only when it is used as "in" mode, the "dispatcher" task is called (from another module [not shown]) and I need to assign a value to it using the problematic assign statement. How can I solve this?

Mehran Torki Over a year ago

1. The only way i know in this situation is to use generate statement to instantiate modules. Read here (Danielpoe's answer):stackoverflow.com/questions/1378159/… 2. Unfortunately i have no idea in that situation, i'll let you know if i found a solution.

Mehran Torki Over a year ago

2. For the second problem, as mga should be of type wire, there is no way to use procedural continuous assignment so you have to put assign statement out of dispatcher. My solution is to create a generate loop that assigns mga[i], like this: assign mga[i] = (write_bool) ? helper[i] : 'bz;. Define a global array of vectors named helper : reg [11:0] helper [0:15];. Then do this in dispatcher task: helper[selected] = globalAddress; write_bool = 1. This way you can put value on mga. By encountering this kind of problems, you'd better consider redesigning.

Mehran Torki Over a year ago

Here is a link containing a code with those two solutions i said (unnecessary parts of your code are removed to just show the main ideas): edaplayground.com/x/WAT

Mehran Torki Over a year ago

Yours :) if you want to have only mga[selected] to be updated, then you should create an array of write conditions instead of just one write_bool : reg write_bool [0:15];. When dispatcher is called, write_bool[i] becomes 1 so it assigns helper[i] to mga[selected]. For the purpose of education, do the modifications yourself.

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