is it okay to assign 1 bit reg data type element to 4 bit reg element?

Question

i am trying to implement a rtl code where i am giving 1 bit reg data type to an 4 bit reg data type under always block.

lets say X & Y are two reg data type.where X is 4 bit reg data type and Y is 1 bit reg data type.

module rtL
reg [3:0]X;
reg Y;
always@(posedge clk)begin
   X<=Y;
end 

Answer 1

It’s always ok to make assignments from smaller to larger width signals. Unsigned types get 0 extended and signed types get sign extended.

Going from larger to smaller widths get truncated, which is OK in Verilog simulation, but other tools may issue warnings.

Answer 2

In Verilog, it's technically allowed to assign a 1-bit register data type to a 4-bit register element, but it might not produce the intended behavior and could lead to unexpected results.

Here's why:

1. Width Mismatch: When you assign a 1-bit value to a 4-bit register, only the least significant bit (LSB) of the 4-bit register will receive the value, while the remaining bits will retain their previous values. This can lead to unintentional behavior if the 1-bit value is not properly aligned with the LSB of the 4-bit register.

2. Data Coherency: If you're intending to store a 4-bit value in a 4-bit register, assigning a 1-bit value might not make sense from a design perspective. It's important to ensure that the width of the data being assigned matches the width of the register to maintain data coherency and prevent potential issues.

Here's an example:

reg [3:0] four_bit_reg;
reg one_bit_reg;

Assigning a 1-bit value to a 4-bit register in form of block assign

four_bit_reg = one_bit_reg; // Only the LSB of four_bit_reg will receive the value of one_bit_reg

If your intention is to use the value of one_bit_reg to set a specific bit of four_bit_reg, you should properly align the bits using bitwise operations or concatenation.

Properly setting the LSB of four_bit_reg with the value of one_bit_reg

four_bit_reg[0] = one_bit_reg; // Set the LSB of four_bit_reg to the value of one_bit_reg

Overall, while assigning a 1-bit value to a 4-bit register is syntactically allowed, it's important to ensure that the assignment aligns with your design intentions and that it doesn't introduce unexpected behavior or data coherency issues.