CODEWITHSUNDEEP
verilogfpgahdl
AZ123
AZ123

Reputation: 184

What do the round braces do when used outside the whole non-blocking assignment?

What are the round braces doing in this code in Verilog?

rx_wb_valid <= (wb_memaddr <= { rx_len[(10+1):2] });

rx_wb_valid is a 1-bit register

wb_memaddr is a 10-bit register

rx_len is a 12-bit register

Upvotes: 1

Views: 344

Answers (2)

dave_59
dave_59

Reputation: 42798

You really need to show more context than what you have written. If you had the statement:

begin
  some_var <= rx_wb_valid <= (wb_memaddr <= { rx_len[(10+1):2] });
end

The round braces change the default left-to-right precedence rules which would have been interpreted as

begin
  some_var <= (rx_wb_valid <= wb_memaddr) <= { rx_len[(10+1):2] };
end

The compiler sees the first variable name followed by an assignment operator (=, <=) at the beginning of a statement and it knows what follows must be an expression. You are not allowed to have assignment operators within an expression in Verilog (SystemVerilog does allow certain cases). Within an expression, round parenthesis () are used to change the default precedence, but can also be used for clarity as in your example.

Curly braces {} are the concatenation operator. Normally a concatenation has several operands, but there is one particular use with only one operand. Since each operand of a concatenation is self-determined, you can use it to prevent extensions before applying operations. For example, if you had the following:

rx_wb_valid <= wb_memaddr <= ~rx_len[3:2];

Since wb_memaddr is a 10-bit variable, it would extend rx_len[3:2] by padding 8 bits with 0 before negating the 10-bits. Those padded bits would become 1's. But by writing:

rx_wb_valid <= wb_memaddr <= {~rx_len[3:2]};

it would negate the 2-bits of rx_len[3:2] before padding the the 8-bits, leaving those bits 0's.

Upvotes: 1

toolic
toolic

Reputation: 62236

The parentheses, (), are optional in your line of code. Your code should behave the same without them. However, I think they make your code more readable since they clearly separate the left <= operator (nonblocking assignment) from the right <= operator (comparison).

The curly braces, {}, are also optional there. I think it is better to remove them in this case.

Upvotes: 2

Related Questions