latch warnings during synthesis of verilog code

[kumarswamy.hl]

Hi all,

Can you please help me to clear the following warnings....



WARNIG:Xst:737 - Found 1-bit latch for signal <$old_fsm/1/fsm_enable_misr_2>. Latches may be generated from incomplete case or if statements. We do not recommend the use of latches in FPGA/CPLD designs, as they may lead to timing problems.
WARNING:Xst:737 - Found 1-bit latch for signal <$old_fsm/1/fsm_enable_lfsr_3>. Latches may be generated from incomplete case or if statements. We do not recommend the use of latches in FPGA/CPLD designs, as they may lead to timing problems.
WARNING:Xst:737 - Found 3-bit latch for signal <$old_fsm/1/fsm_NEXT_STATE_5>. Latches may be generated from incomplete case or if statements. We do not recommend the use of latches in FPGA/CPLD designs, as they may lead to timing problems.


Code:

module BIST_controller(input test_mode,input clk,input reset,input lfsr_ready,input bist_run,
input bist_done,output enable_lfsr,output mux_sel,output enable_misr,
output enable_misr_28_bit
);

//Internal FSM state declarations
wire [1:0] NEXT_STATE;
reg [1:0] PRES_STATE = 2'b00;

//State encodings

parameter s0 = 2'b00;//Normal mode
parameter s1 = 2'b01;//Enable
parameter s2 = 2'b10;//BIST_RUN
parameter s3 = 2'b11;//BIST_DONE

//Combinational logic

function [5:0] fsm;

input fsm_test_mode,fsm_reset,fsm_lfsr_ready,fsm_bist_run,fsm_bist_done;
input [1:0] fsm_PRES_STATE;

reg fsm_enable_lfsr;
reg fsm_mux_sel,fsm_enable_misr,fsm_enable_misr_28_bit;
reg [1:0] fsm_NEXT_STATE;


begin
case (fsm_PRES_STATE)
s0:// state = s0 -> Normal mode
begin
if((fsm_reset) && (fsm_test_mode) && (fsm_bist_done))
begin
fsm_enable_lfsr = 1'b1;
fsm_mux_sel = 1'b1;
fsm_NEXT_STATE = s1;
end
else
begin
fsm_enable_lfsr = 1'b0;
fsm_mux_sel = 1'b0;
fsm_NEXT_STATE = s0;
fsm_enable_misr = 1'b0;
fsm_enable_misr_28_bit = 1'b0;
end
end

s1: //state s1 -> BIST Enable
begin
if(fsm_test_mode && fsm_reset && fsm_lfsr_ready)
begin
fsm_enable_misr = 1'b1;
fsm_enable_misr_28_bit = 1'b1;
fsm_NEXT_STATE = s2;
end
else
begin
fsm_enable_lfsr = 1'b0;
fsm_mux_sel = 1'b0;
fsm_enable_misr = 1'b0;
fsm_enable_misr_28_bit = 1'b0;
fsm_NEXT_STATE = s0;
end
end

s2:// state s2 -> BIST_RUN
begin
if(fsm_test_mode && fsm_reset && !fsm_bist_run)
begin
fsm_NEXT_STATE = s3;
end
else if (fsm_bist_run)
begin
fsm_NEXT_STATE = s2;
end
else
begin
fsm_enable_lfsr = 1'b0;
fsm_mux_sel = 1'b0;
fsm_enable_misr = 1'b0;
fsm_enable_misr_28_bit = 1'b0;
fsm_NEXT_STATE = s0;
end
end
s3: // state s3 -> BIST_DONE
begin
if (fsm_test_mode == 1'b1 && fsm_reset == 1'b1 && fsm_bist_done == 1'b1)
begin

fsm_NEXT_STATE = s0;
fsm_enable_lfsr = 1'b0;
fsm_mux_sel = 1'b0;
fsm_enable_misr = 1'b0;
fsm_enable_misr_28_bit = 1'b0;
end
else
fsm_NEXT_STATE = s3;
end


endcase

fsm = {fsm_mux_sel,fsm_enable_misr,fsm_enable_lfsr,fsm_enable_misr_28_bit,fsm_NEXT_STATE};
end
endfunction

//Reevaluate combinational logic

assign {mux_sel,enable_misr,enable_lfsr,enable_misr_28_bit,NEXT_STATE} = fsm(test_mode,reset,lfsr_ready,
bist_run,bist_done,PRES_STATE);

// clock the state flipflops
// use synchronous reset
always@(negedge clk or negedge reset)
begin
if(reset == 1'b0)
PRES_STATE <= s0;
else
PRES_STATE <= NEXT_STATE;
end
endmodule

 

[pedroromanvr]

Could't see the definition of fsm_NEXT_STATE_5, but for the other two (fsm_enable_misr_2 and fsm_enable_lfsr_3), you are forcing a constant value, which is implemented using latches.
A workaround can be to make the synthetizer to use Flip-flops, so instead of having

if((fsm_reset) && (fsm_test_mode) && (fsm_bist_done))

You can use a validation for edges (at least for one variable)

if((posedge fsm_reset) && (fsm_test_mode) && (fsm_bist_done))

 

[kumarswamy.hl]

we can't use "posedge" with in if-else condition right....?

 

[sagar.bavane]

u can use posedge with if-else...no issue of using that but u must not use incomplete if-else block.....

 

[DFT_designer]

In each state, all signals should be affected in the if and else branches.
I checked your code quickly, sometimes you forget some signals.
Example:
s0:// state = s0 -> Normal mode
begin
if((fsm_reset) && (fsm_test_mode) && (fsm_bist_done))
begin
fsm_enable_lfsr = 1'b1;
fsm_mux_sel = 1'b1;
fsm_NEXT_STATE = s1;
end
else
begin
fsm_enable_lfsr = 1'b0;
fsm_mux_sel = 1'b0;
fsm_NEXT_STATE = s0;
////
fsm_enable_misr = 1'b0;
fsm_enable_misr_28_bit = 1'b0;
////
end
end

These two signals were not mentioned in the if branch.
I think this is the cause of the warning,
You have to check your code.
As I said all signals should be affected in if and else branches in all states.
Regards