Added JTAG interface with testbench

.gitignore

@@ -1,3 +1,5 @@
 out
 build
 env
+__pycache*
+_impactbatch.log

boards/mimas_v1/constraints.ucf

@@ -25,3 +25,28 @@ NET "r2r[2]" IOSTANDARD = LVCMOS33;
 NET "r2r[3]" IOSTANDARD = LVCMOS33;
 NET "r2r[4]" IOSTANDARD = LVCMOS33;
 NET "r2r[5]" IOSTANDARD = LVCMOS33;
+
+NET "LED[0]" LOC = P119;
+NET "LED[0]" IOSTANDARD = LVCMOS33;
+NET "LED[0]" DRIVE = 8;
+NET "LED[1]" LOC = P118;
+NET "LED[1]" IOSTANDARD = LVCMOS33;
+NET "LED[1]" DRIVE = 8;
+NET "LED[2]" LOC = P117;
+NET "LED[2]" IOSTANDARD = LVCMOS33;
+NET "LED[2]" DRIVE = 8;
+NET "LED[3]" LOC = P116;
+NET "LED[3]" IOSTANDARD = LVCMOS33;
+NET "LED[3]" DRIVE = 8;
+NET "LED[4]" LOC = P115;
+NET "LED[4]" IOSTANDARD = LVCMOS33;
+NET "LED[4]" DRIVE = 8;
+NET "LED[5]" LOC = P114;
+NET "LED[5]" IOSTANDARD = LVCMOS33;
+NET "LED[5]" DRIVE = 8;
+NET "LED[6]" LOC = P112;
+NET "LED[6]" IOSTANDARD = LVCMOS33;
+NET "LED[6]" DRIVE = 8;
+NET "LED[7]" LOC = P111;
+NET "LED[7]" IOSTANDARD = LVCMOS33;
+NET "LED[7]" DRIVE = 8;

project.cfg

@@ -4,6 +4,15 @@ version             = 0.1
 out_dir             = out
 build_dir           = build
 
+[target.ip]
+toolchain           = ISE_IP
+ise_settings        = /opt/Xilinx/14.7/ISE_DS/settings64.sh
+family              = spartan6
+device              = xc6slx9
+package             = tqg144
+speedgrade          = -2
+files_def           = boards/mimas_v1/ip/clk_gen.xco
+
 [target.synth]
 toolchain           = ISE
 ise_settings        = /opt/Xilinx/14.7/ISE_DS/settings64.sh
@@ -56,31 +65,25 @@ files_other         = rtl/util/rc_alpha_q15.vh
                       rtl/util/clog2.vh
                       sw/blinky/blinky.hex
 
-[target.ip]
+[target.jtag]
-toolchain           = ISE_IP
+toolchain           = ISE
 ise_settings        = /opt/Xilinx/14.7/ISE_DS/settings64.sh
 family              = spartan6
 device              = xc6slx9
 package             = tqg144
 speedgrade          = -2
-files_def           = boards/mimas_v1/ip/clk_gen.xco
+toplevel            = top_jtag
+xst_opts            = -vlgincdir rtl/util
+files_other         =
+files_con           = boards/mimas_v1/constraints.ucf
+files_verilog       = rtl/arch/spartan-6/jtag_if.v
+                      rtl/arch/spartan-6/clk_gen.v
+                      rtl/toplevel/top_jtag.v
 
-[target.sim]
+[target.svftest]
 toolchain           = iverilog
 runtime             = all
-toplevel            = tb_sigmadelta
+toplevel            = tb_svf
-ivl_opts            = -Irtl/util
+files_verilog       = sim/tb/tb_svf.v
-files_verilog       = sim/tb/tb_nco_q15.v
+                      sim/overrides/jtag_if.v
-                      sim/tb/tb_sigmadelta.v
+files_other         = sim/other/test.svf
-                      sim/tb/tb_mul_const.v
-                      rtl/core/nco_q15.v
-                      rtl/core/lvds_comparator.v
-                      rtl/core/sigmadelta_rcmodel_q15.v
-                      rtl/core/sigmadelta_input_q15.v
-                      rtl/core/mul_const.v
-                      rtl/core/lpf_iir_q15_k.v
-                      rtl/core/decimate_by_r_q15.v
-                      sim/overrides/sigmadelta_sampler.v
-                      sim/overrides/clk_gen.v
-files_other         = rtl/util/conv.vh
-                      rtl/util/rc_alpha_q15.vh

rtl/arch/spartan-6/jtag_if.v

@@ -0,0 +1,34 @@
+`timescale 1ns/1ps
+
+// =============================================================================
+// JTAG interface
+// Spartan-6 BSCAN primitive wrapper (USER1 chain).
+// =============================================================================
+module jtag_if #(
+    parameter chain = 1
+)(
+    input wire i_tdo,
+    output wire o_tck,
+    output wire o_tdi,
+    output wire o_drck,
+    output wire o_capture,
+    output wire o_shift,
+    output wire o_update,
+    output wire o_runtest,
+    output wire o_reset,
+    output wire o_sel
+);
+    BSCAN_SPARTAN6 #(
+        .JTAG_CHAIN(chain)
+    ) bscan_i (
+        .CAPTURE(o_capture),
+        .DRCK(o_drck),
+        .RESET(o_reset),
+        .RUNTEST(o_runtest),
+        .SEL(o_sel),
+        .SHIFT(o_shift),
+        .TCK(o_tck),
+        .TDI(o_tdi),
+        .UPDATE(o_update)
+    );
+endmodule

rtl/core/jtag_if.v

@@ -0,0 +1,34 @@
+`timescale 1ns/1ps
+
+// =============================================================================
+// JTAG interface
+// Generic stub model with inactive/tied-off outputs.
+// =============================================================================
+module jtag_if #(
+    parameter chain = 1
+)(
+    input wire i_tdo,
+    output wire o_tck,
+    output wire o_tdi,
+    output wire o_drck,
+    output wire o_capture,
+    output wire o_shift,
+    output wire o_update,
+    output wire o_runtest,
+    output wire o_reset,
+    output wire o_sel
+);
+    assign o_tck = 1'b0;
+    assign o_tdi = 1'b0;
+    assign o_drck = 1'b0;
+    assign o_capture = 1'b0;
+    assign o_shift = 1'b0;
+    assign o_update = 1'b0;
+    assign o_runtest = 1'b0;
+    assign o_reset = 1'b0;
+    assign o_sel = 1'b0;
+
+    // Keep lint tools quiet in generic builds where TDO is unused.
+    wire _unused_tdo;
+    assign _unused_tdo = i_tdo;
+endmodule

rtl/toplevel/top_jtag.v

@@ -0,0 +1,131 @@
+module top_jtag(
+    input wire aclk,
+    input wire aresetn,
+
+    output wire led_green,
+    output wire led_red,
+    output wire [7:0] LED,
+
+    output wire [5:0] r2r
+);
+    // Clocking
+    wire clk_100;
+    wire clk_15;
+    assign clk_100 = aclk;
+    clk_gen clocking(
+        .clk_in(clk_100),
+        .clk_out_15(clk_15)
+    );
+
+    localparam integer JTAG_DATA_BITS = 8;
+    wire [JTAG_DATA_BITS-1:0] jtag_data;
+
+    jtag_write_reg #(
+        .DATA_BITS(JTAG_DATA_BITS),
+        .CHAIN(1)
+    ) jtag_writer (
+        .clk_i(clk_15),
+        .rst_n_i(aresetn),
+        .data_o(jtag_data)
+    );
+
+    assign LED = jtag_data[7:0];
+    assign led_green = jtag_data[0];
+    assign led_red = jtag_data[7];
+    assign r2r = jtag_data[5:0];
+
+endmodule
+
+module jtag_write_reg #(
+    parameter integer DATA_BITS = 8,
+    parameter integer CHAIN = 1
+)(
+    input wire clk_i,
+    input wire rst_n_i,
+    output reg [DATA_BITS-1:0] data_o
+);
+    wire jtag_tck;
+    wire jtag_tdi;
+    wire jtag_drck;
+    wire jtag_capture;
+    wire jtag_shift;
+    wire jtag_update;
+    wire jtag_runtest;
+    wire jtag_reset;
+    wire jtag_sel;
+    reg [DATA_BITS-1:0] shift_q;
+    reg [DATA_BITS-1:0] data_jtag_q;
+    reg update_toggle_jtag_q;
+    reg update_toggle_meta_q;
+    reg update_toggle_sync_q;
+    reg update_toggle_sync_d_q;
+    reg [DATA_BITS-1:0] data_meta_q;
+    reg [DATA_BITS-1:0] data_sync_q;
+
+    jtag_if #(
+        .chain(CHAIN)
+    ) jtag (
+        .i_tdo(shift_q[0]),
+        .o_tck(jtag_tck),
+        .o_tdi(jtag_tdi),
+        .o_drck(jtag_drck),
+        .o_capture(jtag_capture),
+        .o_shift(jtag_shift),
+        .o_update(jtag_update),
+        .o_runtest(jtag_runtest),
+        .o_reset(jtag_reset),
+        .o_sel(jtag_sel)
+    );
+
+    always @(posedge jtag_drck or posedge jtag_reset or negedge rst_n_i) begin
+        if (!rst_n_i || jtag_reset) begin
+            shift_q <= {DATA_BITS{1'b0}};
+        end else if (jtag_sel && jtag_capture) begin
+            shift_q <= data_jtag_q;
+        end else if (jtag_sel && jtag_shift) begin
+            if (DATA_BITS == 1) begin
+                shift_q <= jtag_tdi;
+            end else begin
+                shift_q <= {jtag_tdi, shift_q[DATA_BITS-1:1]};
+            end
+        end
+    end
+
+    always @(posedge jtag_update or posedge jtag_reset or negedge rst_n_i) begin
+        if (!rst_n_i || jtag_reset) begin
+            data_jtag_q <= {DATA_BITS{1'b0}};
+            update_toggle_jtag_q <= 1'b0;
+        end else if (jtag_sel) begin
+            data_jtag_q <= shift_q;
+            update_toggle_jtag_q <= ~update_toggle_jtag_q;
+        end
+    end
+
+    // CDC into clk_i domain: toggle synchronize + stable data sampling.
+    always @(posedge clk_i or negedge rst_n_i) begin
+        if (!rst_n_i) begin
+            update_toggle_meta_q <= 1'b0;
+            update_toggle_sync_q <= 1'b0;
+            update_toggle_sync_d_q <= 1'b0;
+            data_meta_q <= {DATA_BITS{1'b0}};
+            data_sync_q <= {DATA_BITS{1'b0}};
+            data_o <= {DATA_BITS{1'b0}};
+        end else begin
+            update_toggle_meta_q <= update_toggle_jtag_q;
+            update_toggle_sync_q <= update_toggle_meta_q;
+            update_toggle_sync_d_q <= update_toggle_sync_q;
+            data_meta_q <= data_jtag_q;
+            data_sync_q <= data_meta_q;
+
+            if (update_toggle_sync_q ^ update_toggle_sync_d_q) begin
+                data_o <= data_sync_q;
+            end
+        end
+    end
+
+    // Not used for this simple write-register, but kept for completeness.
+    wire _unused_tck;
+    wire _unused_runtest;
+    assign _unused_tck = jtag_tck;
+    assign _unused_runtest = jtag_runtest;
+endmodule

scripts/jtag_write_user_impact.py

@@ -0,0 +1,167 @@
+#!/usr/bin/env python3
+"""Write USER JTAG data via iMPACT using SVF + play flow."""
+
+from __future__ import annotations
+
+import argparse
+import os
+import shutil
+import subprocess
+import sys
+import tempfile
+
+
+DEFAULT_IMPACT = "/opt/Xilinx/14.7/ISE_DS/ISE/bin/lin64/impact"
+DEFAULT_SETTINGS = "/opt/Xilinx/14.7/ISE_DS/settings64.sh"
+
+
+def _parse_int(value: str) -> int:
+    return int(value, 0)
+
+
+def _resolve_impact(binary: str | None) -> str | None:
+    if binary:
+        return binary if os.path.isfile(binary) else shutil.which(binary)
+    if os.path.isfile(DEFAULT_IMPACT):
+        return DEFAULT_IMPACT
+    return shutil.which("impact")
+
+
+def _fmt_hex_for_bits(value: int, bits: int) -> str:
+    masked = value & ((1 << bits) - 1)
+    nibbles = (bits + 3) // 4
+    return f"{masked:0{nibbles}X}"
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser(
+        description="Write USER JTAG data to Spartan-6 via iMPACT play (SVF)."
+    )
+    parser.add_argument(
+        "--input",
+        required=True,
+        help="Input file to stream over USER JTAG (written byte-by-byte).",
+    )
+    parser.add_argument("--output", type=str, default=None)
+    parser.add_argument("--dr-bits", type=int, default=8, help="DR width in bits (default: 8).")
+    parser.add_argument("--ir-value", type=_parse_int, default=0x02, help="IR opcode (default USER1: 0x02).")
+    parser.add_argument("--ir-bits", type=int, default=6, help="IR width in bits (default: 6).")
+    parser.add_argument("--serial", default=None, help="Cable ESN/serial (example: D306180FABCD).")
+    parser.add_argument("--impact", default=None, help=f"Path/name of impact binary (default: {DEFAULT_IMPACT}).")
+    parser.add_argument(
+        "--settings",
+        default=DEFAULT_SETTINGS,
+        help=f"Xilinx settings script sourced for this run only (default: {DEFAULT_SETTINGS}).",
+    )
+    parser.add_argument("--dry-run", action="store_true", help="Print generated SVF/CMD and exit.")
+    args = parser.parse_args()
+
+    if args.dr_bits <= 0 or args.ir_bits <= 0:
+        print("error: --dr-bits and --ir-bits must be > 0", file=sys.stderr)
+        return 2
+    if args.dr_bits != 8:
+        print("error: this file-stream mode currently supports only --dr-bits 8", file=sys.stderr)
+        return 2
+
+    impact = _resolve_impact(args.impact)
+    if impact is None:
+        print("error: impact binary not found", file=sys.stderr)
+        return 127
+    if not os.path.isfile(args.input):
+        print(f'error: input file not found: "{args.input}"', file=sys.stderr)
+        return 2
+
+    ir_hex = _fmt_hex_for_bits(args.ir_value, args.ir_bits)
+    with open(args.input, "rb") as f:
+        payload = f.read()
+    if len(payload) == 0:
+        print("error: input file is empty", file=sys.stderr)
+        return 2
+
+    svf_lines = [
+        "! Auto-generated by jtag_write_user_impact.py",
+        f"! Source file: {args.input} ({len(payload)} bytes)",
+        "TRST ABSENT;",
+        "ENDIR IDLE;",
+        "ENDDR IDLE;",
+        "STATE RESET;",
+        "STATE IDLE;",
+        f"SIR {args.ir_bits} TDI ({ir_hex});",
+    ]
+    for byte in payload:
+        svf_lines.append(f"SDR 8 TDI ({byte:02X});")
+    svf_lines += [
+        "RUNTEST 16 TCK;",
+        "STATE IDLE;",
+        "",
+    ]
+    svf_text = "\n".join(svf_lines)
+
+    cable_cmd = "setCable -port auto"
+    if args.serial:
+        cable_cmd += f" -esn {args.serial}"
+
+    if args.dry_run:
+        print("### SVF ###")
+        preview_limit = 64
+        if len(payload) <= preview_limit:
+            print(svf_text, end="")
+        else:
+            preview = "\n".join(svf_lines[:8 + preview_limit])
+            print(preview)
+            print(f"... ({len(payload) - preview_limit} more SDR lines omitted)")
+        print("### iMPACT CMD ###")
+        print("setMode -bs")
+        print(cable_cmd)
+        print("addDevice -p 1 -file <generated.svf>")
+        print("play")
+        print("quit")
+        return 0
+
+    svf_path = None
+    cmd_path = None
+    try:
+        with tempfile.NamedTemporaryFile("w", suffix=".svf", delete=False) as sf:
+            sf.write(svf_text)
+            svf_path = sf.name
+        if args.output is not None:
+            with open(args.output, "w") as sf:
+                sf.write(svf_text)
+
+        cmd_text = "\n".join(
+            [
+                "setMode -bs",
+                cable_cmd,
+                f'addDevice -p 1 -file "{svf_path}"',
+                "play",
+                "quit",
+                "",
+            ]
+        )
+        with tempfile.NamedTemporaryFile("w", suffix=".cmd", delete=False) as cf:
+            cf.write(cmd_text)
+            cmd_path = cf.name
+
+        shell_cmd = (
+            f'source "{args.settings}" >/dev/null 2>&1 && '
+            f'"{impact}" -batch "{cmd_path}"'
+        )
+        proc = subprocess.run(["bash", "-lc", shell_cmd], text=True, capture_output=True)
+        output = (proc.stdout or "") + (proc.stderr or "")
+        print(output, end="")
+        return proc.returncode
+    finally:
+        if cmd_path is not None:
+            try:
+                os.unlink(cmd_path)
+            except OSError:
+                pass
+        if svf_path is not None:
+            try:
+                os.unlink(svf_path)
+            except OSError:
+                pass
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())

sim/other/test.svf

@@ -0,0 +1,25 @@
+TRST ABSENT;
+ENDIR IDLE;
+ENDDR IDLE;
+STATE RESET;
+STATE IDLE;
+SIR 6 TDI (02);
+
+SDR 8 TDI (07);
+SDR 8 TDI (4C);
+SDR 8 TDI (43);
+SDR 8 TDI (D1);
+SDR 8 TDI (97);
+SDR 8 TDI (6B);
+SDR 8 TDI (3D);
+SDR 8 TDI (E4);
+SDR 8 TDI (20);
+SDR 8 TDI (3A);
+SDR 8 TDI (8E);
+SDR 8 TDI (1D);
+SDR 8 TDI (9D);
+SDR 8 TDI (E4);
+
+RUNTEST 16 TCK;
+STATE RESET;
+STATE IDLE;

sim/overrides/jtag_if.v

@@ -0,0 +1,196 @@
+`timescale 1ns/1ps
+
+// =============================================================================
+// JTAG interface (simulation override)
+// Behavioral SVF player for simple USER-chain simulation.
+//
+// Supported SVF commands (line-oriented, uppercase recommended):
+//   - SIR <n> TDI (<hex>);
+//   - SDR <n> TDI (<hex>);
+//   - RUNTEST <n> TCK;
+//   - STATE RESET;
+//   - STATE IDLE;
+// Other lines are ignored.
+// =============================================================================
+module jtag_if #(
+    parameter integer chain = 1,
+    parameter [8*256-1:0] SVF_FILE = "write_jtag.svf",
+    parameter integer TCK_HALF_PERIOD_NS = 50,
+    parameter [31:0] USER_IR_OPCODE = 32'h00000002
+)(
+    input wire i_tdo,
+    output reg o_tck,
+    output reg o_tdi,
+    output reg o_drck,
+    output reg o_capture,
+    output reg o_shift,
+    output reg o_update,
+    output reg o_runtest,
+    output reg o_reset,
+    output reg o_sel
+);
+    integer fd;
+    integer got;
+    integer nbits;
+    integer cycles;
+    integer i;
+    integer line_no;
+    integer scan_bits;
+
+    reg [8*1024-1:0] line;
+    reg [8*32-1:0] cmd;
+    reg [8*32-1:0] arg1;
+    reg [8*256-1:0] svf_file_path;
+    reg [4095:0] scan_data;
+    reg [31:0] current_ir;
+    reg selected;
+
+    // Keep linters quiet: TDO is not consumed by this simple player.
+    wire _unused_tdo;
+    assign _unused_tdo = i_tdo;
+
+    task pulse_tck;
+    begin
+        #TCK_HALF_PERIOD_NS o_tck = 1'b1;
+        #TCK_HALF_PERIOD_NS o_tck = 1'b0;
+    end
+    endtask
+
+    task pulse_drck_shift;
+        input bit_in;
+    begin
+        o_tdi = bit_in;
+        o_shift = 1'b1;
+        #TCK_HALF_PERIOD_NS begin
+            o_tck = 1'b1;
+            o_drck = 1'b1;
+        end
+        #TCK_HALF_PERIOD_NS begin
+            o_tck = 1'b0;
+            o_drck = 1'b0;
+        end
+    end
+    endtask
+
+    task pulse_capture;
+    begin
+        o_capture = 1'b1;
+        #TCK_HALF_PERIOD_NS begin
+            o_tck = 1'b1;
+            o_drck = 1'b1;
+        end
+        #TCK_HALF_PERIOD_NS begin
+            o_tck = 1'b0;
+            o_drck = 1'b0;
+            o_capture = 1'b0;
+        end
+    end
+    endtask
+
+    task pulse_update;
+    begin
+        o_shift = 1'b0;
+        #TCK_HALF_PERIOD_NS o_update = 1'b1;
+        #TCK_HALF_PERIOD_NS o_update = 1'b0;
+    end
+    endtask
+
+    initial begin
+        // Default output levels
+        o_tck = 1'b0;
+        o_tdi = 1'b0;
+        o_drck = 1'b0;
+        o_capture = 1'b0;
+        o_shift = 1'b0;
+        o_update = 1'b0;
+        o_runtest = 1'b0;
+        o_reset = 1'b0;
+        o_sel = 1'b0;
+        selected = 1'b0;
+        current_ir = 32'h0;
+        line_no = 0;
+        svf_file_path = SVF_FILE;
+
+        // Deterministic startup reset pulse before consuming SVF.
+        o_reset = 1'b1;
+        #TCK_HALF_PERIOD_NS o_tck = 1'b1;
+        #TCK_HALF_PERIOD_NS o_tck = 1'b0;
+        #TCK_HALF_PERIOD_NS o_tck = 1'b1;
+        #TCK_HALF_PERIOD_NS o_tck = 1'b0;
+        o_reset = 1'b0;
+
+        fd = $fopen(svf_file_path, "r");
+        if (fd == 0) begin
+            $display("jtag_if(sim): could not open SVF file '%0s'", svf_file_path);
+        end else begin
+            while (!$feof(fd)) begin
+                line = {8*1024{1'b0}};
+                got = $fgets(line, fd);
+                line_no = line_no + 1;
+                if (got > 0) begin
+                cmd = {8*32{1'b0}};
+                arg1 = {8*32{1'b0}};
+                scan_data = {4096{1'b0}};
+                nbits = 0;
+                cycles = 0;
+
+                got = $sscanf(line, "%s", cmd);
+                if (got < 1) begin
+                    // Empty line
+                end else if (cmd == "!") begin
+                    // Comment line
+                end else if (cmd == "SIR") begin
+                    got = $sscanf(line, "SIR %d TDI (%h);", nbits, scan_data);
+                    if (got == 2) begin
+                        current_ir = scan_data[31:0];
+                        selected = (current_ir == USER_IR_OPCODE);
+                        o_sel = selected;
+
+                        // Emit TCK pulses for IR shift activity.
+                        scan_bits = nbits;
+                        if (scan_bits > 4096) scan_bits = 4096;
+                        for (i = 0; i < scan_bits; i = i + 1) begin
+                            o_tdi = scan_data[i];
+                            pulse_tck;
+                        end
+                    end
+                end else if (cmd == "SDR") begin
+                    got = $sscanf(line, "SDR %d TDI (%h);", nbits, scan_data);
+                    if (got == 2) begin
+                        if (selected) begin
+                            pulse_capture;
+                        end
+                        scan_bits = nbits;
+                        if (scan_bits > 4096) scan_bits = 4096;
+                        for (i = 0; i < scan_bits; i = i + 1) begin
+                            pulse_drck_shift(scan_data[i]);
+                        end
+                        if (selected) begin
+                            pulse_update;
+                        end
+                    end
+                end else if (cmd == "RUNTEST") begin
+                    got = $sscanf(line, "RUNTEST %d TCK;", cycles);
+                    if (got == 1) begin
+                        o_runtest = 1'b1;
+                        for (i = 0; i < cycles; i = i + 1)
+                            pulse_tck;
+                        o_runtest = 1'b0;
+                    end
+                end else if (cmd == "STATE") begin
+                    got = $sscanf(line, "STATE %s", arg1);
+                    if (got == 1) begin
+                        if (arg1 == "RESET;") begin
+                            o_reset = 1'b1;
+                            #TCK_HALF_PERIOD_NS o_reset = 1'b0;
+                            selected = 1'b0;
+                            o_sel = 1'b0;
+                        end
+                    end
+                end
+                end
+            end
+            $fclose(fd);
+        end
+    end
+endmodule

sim/tb/tb_svf.v

@@ -0,0 +1,90 @@
+`timescale 1ns/1ps
+
+module tb_svf();
+    wire [7:0] led_out;
+    wire [31:0] update_count;
+
+    jtag_byte_sink #(
+        .SVF_FILE("sim/other/test.svf"),
+        .TCK_HALF_PERIOD_NS(50)
+    ) dut (
+        .o_led(led_out),
+        .o_update_count(update_count)
+    );
+
+    initial begin
+        // Optional waveform dump.
+        $dumpfile("out.vcd");
+        $dumpvars;
+
+        // Timeout for large SVF playback.
+        #20_000;
+        $finish;
+    end
+endmodule
+
+module jtag_byte_sink #(
+    parameter [8*256-1:0] SVF_FILE = "",
+    parameter integer TCK_HALF_PERIOD_NS = 50
+)(
+    output reg [7:0] o_led,
+    output reg [31:0] o_update_count
+);
+    wire jtag_tck;
+    wire jtag_tdi;
+    wire jtag_drck;
+    wire jtag_capture;
+    wire jtag_shift;
+    wire jtag_update;
+    wire jtag_runtest;
+    wire jtag_reset;
+    wire jtag_sel;
+    wire jtag_tdo;
+    reg [7:0] shift_reg;
+
+    assign jtag_tdo = shift_reg[0];
+
+    jtag_if #(
+        .chain(1),
+        .SVF_FILE(SVF_FILE),
+        .TCK_HALF_PERIOD_NS(TCK_HALF_PERIOD_NS),
+        .USER_IR_OPCODE(32'h0000_0002)
+    ) jtag (
+        .i_tdo(jtag_tdo),
+        .o_tck(jtag_tck),
+        .o_tdi(jtag_tdi),
+        .o_drck(jtag_drck),
+        .o_capture(jtag_capture),
+        .o_shift(jtag_shift),
+        .o_update(jtag_update),
+        .o_runtest(jtag_runtest),
+        .o_reset(jtag_reset),
+        .o_sel(jtag_sel)
+    );
+
+    always @(posedge jtag_drck or posedge jtag_reset) begin
+        if (jtag_reset) begin
+            shift_reg <= 8'h00;
+        end else if (jtag_sel && jtag_capture) begin
+            shift_reg <= o_led;
+        end else if (jtag_sel && jtag_shift) begin
+            shift_reg <= {jtag_tdi, shift_reg[7:1]};
+        end
+    end
+
+    always @(posedge jtag_update or posedge jtag_reset) begin
+        if (jtag_reset) begin
+            o_led <= 8'h00;
+            o_update_count <= 32'd0;
+        end else if (jtag_sel) begin
+            o_led <= shift_reg;
+            o_update_count <= o_update_count + 1'b1;
+        end
+    end
+
+    // Keep lint happy for currently unused outputs.
+    wire _unused_tck;
+    wire _unused_runtest;
+    assign _unused_tck = jtag_tck;
+    assign _unused_runtest = jtag_runtest;
+endmodule

sw/blinky/link.ld

@@ -3,7 +3,7 @@ ENTRY(_start)
 
 MEMORY
 {
-  RAM (rwx) : ORIGIN = 0x00000000, LENGTH = 8K
+  RAM (rwx) : ORIGIN = 0x00000000, LENGTH = 8064
 }
 
 SECTIONS

sw/sweep/link.ld

@@ -3,7 +3,7 @@ ENTRY(_start)
 
 MEMORY
 {
-  RAM (rwx) : ORIGIN = 0x00000000, LENGTH = 8K
+  RAM (rwx) : ORIGIN = 0x00000000, LENGTH = 8064
 }
 
 SECTIONS