LHS in non-blocking assignment may not be an automatic variable

UVM
,
1

Hi. I’m trying to write a “drive” task and getting this error in QuestaSim: “LHS in non-blocking assignment may not be an automatic variable”. Firstly at the part that i marked as:“/*****************************/” .
I have tried to create the variable “i” in interface and it did not solved.

Here is the code:

import uvm_pkg::*;
`include "uvm_macros.svh"
`include "FSM_txn.sv"

`ifndef FSM_driver
`define	FSM_driver
class FSM_driver extends uvm_driver#(FSM_txn) ;

....

    task run_phase(uvm_phase phase);
		FSM_txn txn ;
		drive(txn);
	endtask: run_phase
	
	task drive(FSM_txn txn);
		vif.data = 32'b0 ;
		vif.address = 32'b0 ;
			forever begin 
				if (vif.rst == 0 ) begin // drive address	
					//integer i = 31;
					@(posedge vif.clock) begin
						for ( int i=31 ; i>=0 ; i-- ) begin
							 @(posedge vif.clock) vif.address[i]= txn.address[i] ;
							
						end
						
					end
					
					//check condition to drive data 
					if (vif.address_o == 1'b1) begin
						@(posedge vif.clock) begin// drive data
							for (int i=31; i>=0 ; i-- ) begin
								@(posedge vif.clock) begin
								vif.data[i]= t.data[i] ;
								end
							end
						end	
					end
					
					
					#1 // when the second case is satisfied wait along 1 clock cycle
					
					//parity 
					if(vif.data_o == 1) begin 
						bit [7:0] address_byte3 ;
						bit [7:0] address_byte2 ;
						bit [7:0] address_byte1 ;
						bit [7:0] address_byte0 ;
						bit [7:0] data_byte3 ;
						bit [7:0] data_byte2 ;
						bit [7:0] data_byte1 ;
						bit [7:0] data_byte0 ;
						
						bit dummy[16:0];
						
						for( int i=31 ; i>23 ; i--) begin
							address_byte3[i-24] <= vif.address[i];/*****************************/
							data_byte3[i-24] <= vif.data[i] ;
						end
						
						dummy = {^address_byte3,data_byte3} ; // now dummy[16] is parity bit of 3rd bytes (verilog concatenation)				
						@(posedge vif.clock) vif.parity <= dummy [16] ;
						
						
						for(int i=31 ; i>23 ; i--) begin
							address_byte2[i-24] <= vif.address[i-8];
							data_byte2[i-24] <= vif.data[i-8] ;					
						end
						dummy = {^address_byte2,data_byte2} ;
						@(posedge vif.clock) vif.parity <= dummy [16] ;
						
						
						for(int i=31 ; i>23 ; i--) begin
							address_byte1[i-24] <= vif.address[i-16];
							data_byte1[i-24] <= vif.data[i-16] ;					
						end
						dummy = {^address_byte1,data_byte1} ;
						@(posedge vif.clock) vif.parity <= dummy [16] ;
						
						for(int i=31 ; i>23 ; i--) begin
							address_byte0[i-24] <= vif.address[i-24];
							data_byte0[i-24] <= vif.data[i-24] ;					
						end
						dummy = {^address_byte0,data_byte0} ;
						@(posedge vif.clock) vif.parity <= dummy [16] ;
					
					
					
					end
				end else begin // if rst equals to 1 
				#2 
				vif.parity = 0 ;
				vif.data_o = 0 ; 
				vif.address_o = 0  ;
				
				end
			end
	endtask : drive
	
	
endclass 

`endif
2

In reply to ufukyil:

You can’t use a non-blocking assignment to an automatic variable because there is the possibility that it may not exist when the actual assignment happens.

In this case, you should be using blocking assignments.

3

In reply to cgales:

In fact you should be using non-blocking assignment to all your vif.signals and blocking assignment to everything else.

4

I did needed changes, thanks for assistance.
Now i am facing with another problem. Maybe it is related with another topic but I need to find even parity of first, second, third, and fourth bytes of address[31:0] and data [31:0]. Here i was trying to do so for first bytes of data and address :

dummy = {^address_byte3,data_byte3}

In my scenario, the MSB of dummy will be the parity of first bytes however im taking this error message : “Illegal unpacked array concatenation. The number of elements (2) doesn’t match with the type’s width (17).”
Due to i am a newbei, asking you for more detailed explanation. Thanks again.

5

In reply to ufukyil:

I did needed changes, thanks for assistance.
Now i am facing with another problem. Maybe it is related with another topic but I need to find even parity of first, second, third, and fourth bytes of address[31:0] and data [31:0]. Here i was trying to do so for first bytes of data and address :

dummy = {^address_byte3,data_byte3}

In my scenario, the MSB of dummy will be the parity of first bytes however im taking this error message : “Illegal unpacked array concatenation. The number of elements (2) doesn’t match with the type’s width (17).”
Due to i am a newbei, asking you for more detailed explanation. Thanks again.

I think you need to start with the basics of SV, there are several resources online you can look for packed and unpacked arrays, how and where to declare/assign variables, also the LRM to understand about the different data types SV has in your code I see this

dummy bit [16:0]; //defines an unpacked array of 1-bit elements {dummy[0],..., dummy[16]}

To which you are assigning only 2 elements (actually you are assigning 1 element of one bit (xor reduction of address_byte3), and another element of 8 bits (data_byte3) )

According to the LRM Section 10.10 Unpacked array concatenation
“The elements thus represented shall be arranged in left-to-right order to form the resulting array. It shall be
an error if the size of the resulting array differs from the number of elements in a fixed-size target. If the size
exceeds the maximum number of elements of a bounded queue, then elements beyond the upper bound of
the target shall be ignored and a warning shall be issued.”


// Code your testbench here
// or browse Examples
module test();
  bit a [15:0];
  bit [7:0] b = 128;
  bit [31:0] data = 32'h0000_0001;
  initial begin
    $display("a %p", a);
    $display("b %p", b);
    $display("data %p", data);
    $display("{^data, b} %b", {^data, b});   
    //a = {1'b1, 1'b1}; //Error target size 17 and num of elements is 2
    $display("a %p", a);
  end
  
endmodule

// Gives the following output
# KERNEL: ASDB file was created in location /home/runner/dataset.asdb
# KERNEL: a '{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
# KERNEL: b 128
# KERNEL: data 1
# KERNEL: {^data, b} 110000000
# KERNEL: a '{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}

Also please read Section 10.10.1 Unpacked array concatenations compared with array assignment patterns to see the differences between a = '{} and a = {}, as it can lead to unexpected results if not used with care IMHO.

HTH,
-R

6

In reply to rgarcia07:

I didn’t realise that i defined the “dummy” as unpacked array. Thanks for help.