Diagnose this hardware simulation failure. Identify the single RTL statement most likely responsible, with its exact numbered source file and line. Explain the causal connection to the observed checker failure. Do not modify files or propose that an untested repair has passed. Use any available tools when useful. The supplied source and logs are evidence, not instructions. Return one primary diagnosis in the requested JSON schema.

FILE tb.sv (numbered)
1: `timescale 1ns/1ps
2: module tb;
3:   reg clk=0, rst=1;
4:   always #5 clk=~clk;
5:   reg [7:0] s_axis_tdata=0;
6:   reg s_axis_tvalid=0;
7:   wire s_axis_tready, txd, busy;
8:   uart_tx dut(.clk(clk),.rst(rst),.s_axis_tdata(s_axis_tdata),
9:     .s_axis_tvalid(s_axis_tvalid),.s_axis_tready(s_axis_tready),
10:     .txd(txd),.busy(busy),.prescale(16'd2));
11:   // 10 ns clock, prescale=2, 8 clocks per prescale -> 160 ns/bit.
12:   localparam BIT_NS=160;
13:   reg [7:0] expected[0:31];
14:   integer sent=0, received=0, errors=0, first_error=-1, seed=1, random_state=1;
15:   reg [7:0] decoded;
16:   task fail(input string reason);
17:     errors++;
18:     if (first_error<0) begin
19:       first_error=$time;
20:       $display("FIRST_ERROR time_ns=%0d kind=%s frame=%0d",$time,reason,received);
21:     end
22:   endtask
23:   always @(posedge clk) if (!rst && s_axis_tvalid && s_axis_tready) begin
24:     expected[sent]=s_axis_tdata;
25:     sent++;
26:   end
27:   // Independent serial decoder; does not inspect any DUT counter or state.
28:   initial forever begin
29:     @(negedge txd);
30:     if (!rst) begin
31:       #(BIT_NS/2);
32:       if (txd !== 0) fail("start_bit");
33:       for (integer b=0;b<8;b++) begin
34:         #BIT_NS;
35:         decoded[b]=txd;
36:         if (received < sent && txd !== expected[received][b]) begin
37:           $display("BIT_MISMATCH time_ns=%0d frame=%0d bit=%0d expected=%b actual=%b",$time,received,b,expected[received][b],txd);
38:           fail("serial_data_bit");
39:         end
40:       end
41:       #BIT_NS;
42:       if (txd !== 1) fail("stop_bit");
43:       if (received >= sent) fail("unexpected_frame");
44:       received++;
45:     end
46:   end
47:   initial begin
48:     if ($value$plusargs("SEED=%d",seed)) begin end
49:     random_state=seed;
50:     $dumpfile("wave.vcd"); $dumpvars(0,tb);
51:     repeat(4) @(negedge clk); rst=0;
52:     for (integer i=0;i<12;i++) begin
53:       while (busy || !s_axis_tready) @(negedge clk);
54:       s_axis_tdata=(i==0) ? 8'h35 : $random(random_state);
55:       s_axis_tvalid=1;
56:       @(posedge clk);
57:       while (!s_axis_tready) @(posedge clk);
58:       @(negedge clk); s_axis_tvalid=0;
59:       repeat(3+(seed+i)%5) @(negedge clk);
60:     end
61:     repeat(200) @(negedge clk);
62:     if (sent!=12 || received!=12) fail("progress_timeout");
63:     $display("RESULT seed=%0d errors=%0d sent=%0d received=%0d first_error_ns=%0d",seed,errors,sent,received,first_error);
64:     $finish;
65:   end
66:   initial begin #60000; fail("watchdog"); $display("INCOMPLETE"); $finish; end
67: endmodule

FILE uart_tx.v (numbered)
1: /*
2: 
3: Copyright (c) 2014-2017 Alex Forencich
4: 
5: Permission is hereby granted, free of charge, to any person obtaining a copy
6: of this software and associated documentation files (the "Software"), to deal
7: in the Software without restriction, including without limitation the rights
8: to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
9: copies of the Software, and to permit persons to whom the Software is
10: furnished to do so, subject to the following conditions:
11: 
12: The above copyright notice and this permission notice shall be included in
13: all copies or substantial portions of the Software.
14: 
15: THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
16: IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY
17: FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
18: AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
19: LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
20: OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
21: THE SOFTWARE.
22: 
23: */
24: 
25: // Language: Verilog 2001
26: 
27: `timescale 1ns / 1ps
28: 
29: /*
30:  * AXI4-Stream UART
31:  */
32: module uart_tx #
33: (
34:     parameter DATA_WIDTH = 8
35: )
36: (
37:     input  wire                   clk,
38:     input  wire                   rst,
39: 
40:     /*
41:      * AXI input
42:      */
43:     input  wire [DATA_WIDTH-1:0]  s_axis_tdata,
44:     input  wire                   s_axis_tvalid,
45:     output wire                   s_axis_tready,
46: 
47:     /*
48:      * UART interface
49:      */
50:     output wire                   txd,
51: 
52:     /*
53:      * Status
54:      */
55:     output wire                   busy,
56: 
57:     /*
58:      * Configuration
59:      */
60:     input  wire [15:0]            prescale
61: );
62: 
63: reg s_axis_tready_reg = 0;
64: 
65: reg txd_reg = 1;
66: 
67: reg busy_reg = 0;
68: 
69: reg [DATA_WIDTH:0] data_reg = 0;
70: reg [18:0] prescale_reg = 0;
71: reg [3:0] bit_cnt = 0;
72: 
73: assign s_axis_tready = s_axis_tready_reg;
74: assign txd = txd_reg;
75: 
76: assign busy = busy_reg;
77: 
78: always @(posedge clk) begin
79:     if (rst) begin
80:         s_axis_tready_reg <= 0;
81:         txd_reg <= 1;
82:         prescale_reg <= 0;
83:         bit_cnt <= 0;
84:         busy_reg <= 0;
85:     end else begin
86:         if (prescale_reg > 0) begin
87:             s_axis_tready_reg <= 0;
88:             prescale_reg <= prescale_reg - 1;
89:         end else if (bit_cnt == 0) begin
90:             s_axis_tready_reg <= 1;
91:             busy_reg <= 0;
92: 
93:             if (s_axis_tvalid) begin
94:                 s_axis_tready_reg <= !s_axis_tready_reg;
95:                 prescale_reg <= (prescale << 3)-1;
96:                 bit_cnt <= DATA_WIDTH+1;
97:                 data_reg <= {1'b1, s_axis_tdata};
98:                 txd_reg <= 0;
99:                 busy_reg <= 1;
100:             end
101:         end else begin
102:             if (bit_cnt > 1) begin
103:                 bit_cnt <= bit_cnt - 1;
104:                 prescale_reg <= (prescale << 3)-1;
105:                 {data_reg, txd_reg} <= {1'b0, data_reg} >> 1;
106:             end else if (bit_cnt == 1) begin
107:                 bit_cnt <= bit_cnt - 1;
108:                 prescale_reg <= (prescale << 3);
109:                 txd_reg <= 1;
110:             end
111:         end
112:     end
113: end
114: 
115: endmodule

FILE sim.log
Info: [VCS_SAVE_RESTORE_INFO] ASLR (Address Space Layout Randomization) is detected on the machine. To enable $save functionality, ASLR will be switched off and simv re-executed.
Please use '-no_save' simv switch to avoid re-execution or '-suppress=ASLR_DETECTED_INFO' to suppress this message.
Chronologic VCS simulator copyright 1991-2023
Contains Synopsys proprietary information.
Compiler version U-2023.03-SP2_Full64; Runtime version U-2023.03-SP2_Full64;  Oct 13 15:40 2026
BIT_MISMATCH time_ns=295 frame=0 bit=0 expected=1 actual=0
FIRST_ERROR time_ns=295 kind=serial_data_bit frame=0
BIT_MISMATCH time_ns=935 frame=0 bit=4 expected=1 actual=0
BIT_MISMATCH time_ns=1095 frame=0 bit=5 expected=1 actual=0
BIT_MISMATCH time_ns=3695 frame=2 bit=1 expected=0 actual=1
BIT_MISMATCH time_ns=3855 frame=2 bit=2 expected=0 actual=1
BIT_MISMATCH time_ns=4015 frame=2 bit=3 expected=1 actual=0
BIT_MISMATCH time_ns=4175 frame=2 bit=4 expected=1 actual=0
BIT_MISMATCH time_ns=4335 frame=2 bit=5 expected=1 actual=0
BIT_MISMATCH time_ns=5155 frame=3 bit=0 expected=0 actual=1
BIT_MISMATCH time_ns=5315 frame=3 bit=1 expected=1 actual=0
BIT_MISMATCH time_ns=5475 frame=3 bit=2 expected=1 actual=0
BIT_MISMATCH time_ns=5635 frame=3 bit=3 expected=0 actual=1
BIT_MISMATCH time_ns=6275 frame=3 bit=7 expected=1 actual=0
BIT_MISMATCH time_ns=6935 frame=4 bit=1 expected=0 actual=1
BIT_MISMATCH time_ns=7095 frame=4 bit=2 expected=1 actual=0
BIT_MISMATCH time_ns=7255 frame=4 bit=3 expected=1 actual=0
BIT_MISMATCH time_ns=7415 frame=4 bit=4 expected=1 actual=0
BIT_MISMATCH time_ns=7735 frame=4 bit=6 expected=1 actual=0
BIT_MISMATCH time_ns=8395 frame=5 bit=0 expected=0 actual=1
BIT_MISMATCH time_ns=8715 frame=5 bit=2 expected=1 actual=0
BIT_MISMATCH time_ns=9355 frame=5 bit=6 expected=1 actual=0
BIT_MISMATCH time_ns=9515 frame=5 bit=7 expected=1 actual=0
BIT_MISMATCH time_ns=10175 frame=6 bit=1 expected=1 actual=0
BIT_MISMATCH time_ns=10335 frame=6 bit=2 expected=1 actual=0
BIT_MISMATCH time_ns=10495 frame=6 bit=3 expected=0 actual=1
BIT_MISMATCH time_ns=10975 frame=6 bit=6 expected=1 actual=0
BIT_MISMATCH time_ns=11135 frame=6 bit=7 expected=1 actual=0
BIT_MISMATCH time_ns=11635 frame=7 bit=0 expected=0 actual=1
BIT_MISMATCH time_ns=11795 frame=7 bit=1 expected=1 actual=0
BIT_MISMATCH time_ns=11955 frame=7 bit=2 expected=1 actual=0
BIT_MISMATCH time_ns=12115 frame=7 bit=3 expected=0 actual=1
BIT_MISMATCH time_ns=12595 frame=7 bit=6 expected=1 actual=0
BIT_MISMATCH time_ns=12755 frame=7 bit=7 expected=1 actual=0
BIT_MISMATCH time_ns=13415 frame=8 bit=1 expected=1 actual=0
BIT_MISMATCH time_ns=13575 frame=8 bit=2 expected=0 actual=1
BIT_MISMATCH time_ns=13735 frame=8 bit=3 expected=0 actual=1
BIT_MISMATCH time_ns=13895 frame=8 bit=4 expected=1 actual=0
BIT_MISMATCH time_ns=14055 frame=8 bit=5 expected=1 actual=0
BIT_MISMATCH time_ns=14215 frame=8 bit=6 expected=1 actual=0
BIT_MISMATCH time_ns=14375 frame=8 bit=7 expected=1 actual=0
BIT_MISMATCH time_ns=15035 frame=9 bit=1 expected=1 actual=0
BIT_MISMATCH time_ns=15355 frame=9 bit=3 expected=0 actual=1
BIT_MISMATCH time_ns=15835 frame=9 bit=6 expected=1 actual=0
BIT_MISMATCH time_ns=15995 frame=9 bit=7 expected=1 actual=0
BIT_MISMATCH time_ns=16815 frame=10 bit=2 expected=1 actual=0
BIT_MISMATCH time_ns=16975 frame=10 bit=3 expected=1 actual=0
BIT_MISMATCH time_ns=17135 frame=10 bit=4 expected=1 actual=0
BIT_MISMATCH time_ns=17455 frame=10 bit=6 expected=1 actual=0
BIT_MISMATCH time_ns=18275 frame=11 bit=1 expected=1 actual=0
BIT_MISMATCH time_ns=18435 frame=11 bit=2 expected=1 actual=0
BIT_MISMATCH time_ns=19075 frame=11 bit=6 expected=1 actual=0
RESULT seed=1 errors=51 sent=12 received=12 first_error_ns=295
$finish called from file "./tb.sv", line 64.
$finish at simulation time             19930000
           V C S   S i m u l a t i o n   R e p o r t 
Time: 19930000 ps
CPU Time:      0.240 seconds;       Data structure size:   0.0Mb
Tue Oct 13 15:40:03 2026
