If and else have been matched, and latches are also generated

qianguangsha

If and else have been matched, and latches are also generated [Copy link]

  In this code, if and else are already matched, why is there still a latch? When I change the number in the green box to all f, no latch is generated (I know how to fix other warnings, but I don’t know how to fix this latch problem)

qianguangsha

1. module exe( clk , rst_n , CFword , data ); input clk; input rst_n; input [31:0] CFword; output [31:0] data; reg [31:0]data; reg [31:0 ] Fword; //frequency step always@(posedge clk or negedge rst_n)begin if(rst_n==1'b0)begin Fword <= 32'b0; end else Fword <= Fword + 21'h100000; end always@(posedge clk or negedge rst_n)begin if(rst_n==1'b0)begin data <= 32'b0; end else data < = Fword; end endmodule

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This is the code I wrote

youzizhile

Fword <= Fword + 21'h100000; The subsequent addend uses 21 bits. Is it because of the bit width?

qianguangsha

youzizhile posted on 2018-6-8 20:40 Fword

Now I know the reason, it is because the last 20 bits are always zero, which is a fixed value, so the compiler will warn, but I don’t know how to change this program

haoshuai

This depends on whether your code will be modified. If the last few digits remain unchanged, you can directly write them as 0, then add the changed digits, and then splice them with 0. However, in this case, the code cannot be modified at will.

coyoo

qianguangsha posted on 2018-6-9 15:00 Now I know the reason, it is because the last 20 bits are always zero, which is a fixed value, so the compiler will warn, but I don’t know how to change this program

Adding a constant is no different from accumulating 1; you can try replacing 21'10000 with 1048576; this seems to be 32bit by default

heningbo

Direct Fword <= Fword + 32'h100000;

eeword_zhy

Direct Fword <= Fword + 32'h100000; Because your Fword is 32 bits, your direct Fword <= Fword + 21'h100000; is equivalent to the lower 21 bits being a D flip-flop, and the [31:22] bits are not assigned, so Fword[31:22] is recognized as a latch