✨ feat: add TFTP download verify, Step 4 sub-steps, and fix UI layout

- Implement _tftp_get() for downloading files from Zynq via TFTP
- Add TftpSubStepFrame widget for Step 4 progress display
- Refactor Step 4 into 5 sub-steps: Upload, Download Verify, CRC Check, Reboot, Boot Verify
- Fix SubStepFrame.reset() bug that caused collapse after execution
- Fix layout overlap: Step 4 sub-step and progress bar now use separate rows
- Improve ProgressIndicator appearance with better styling and spacing
- Fix UART monitor to start regardless of _uart_available flag
- Add CRC32 verification between uploaded and downloaded files

src/gui/main_window.py

@@ -21,7 +21,7 @@ from zynq_checker import check_zynq_jtag, get_zynq_status_dict
 from flash_programmer import full_flash_program, FlashResult
 from bitstream_programmer import full_bitstream_program, BitstreamResult
 from ip_verifier import ping_ip
-from tftp_manager import tftp_upload_verify, TftpResult
+from tftp_manager import tftp_upload, tftp_download, tftp_upload_verify, TftpResult, _compute_crc32
 from reboot_manager import reboot_zynq, RebootResult
 from boot_verifier import verify_boot, verify_zynq_status, BootVerificationResult
 from serial_monitor import detect_serial_ports, parse_boot_output, check_uart_available, UartMonitor
@@ -53,6 +53,7 @@ from gui.widgets import (
     FileSelector,
     LogDisplay,
     SubStepFrame,
+    TftpSubStepFrame,
     UartMonitorWindow,
 )
 
@@ -331,7 +332,6 @@ class MainWindow(ctk.CTk):
             row=0, column=0, sticky="nsew",
             padx=PADDING, pady=(PADDING, 0),
         )
-        panel.grid_rowconfigure(10, weight=1)
         panel.grid_columnconfigure(0, weight=1)
 
         # ── Title ──
@@ -364,23 +364,29 @@ class MainWindow(ctk.CTk):
             self._steps.append(step)
             self._step_statuses[i] = "pending"
 
-            # Divider line between steps
+             # Divider line between steps (skip rows occupied by sub-steps)
             if i > 0:
+                # Dividers go between main steps only, not between step and sub-step
+                # Row calculation: account for sub-steps from previous steps
+                prev_steps_with_substeps = sum(1 for j in range(i) if j in (1, 3))
+                divider_row = i * 2 + 1 + prev_steps_with_substeps
                 divider = ctk.CTkFrame(
                     panel,
                     height=1,
                     fg_color=CONNECTOR_COLOR,
                 )
                 divider.grid(
-                    row=i * 2, column=0,
+                    row=divider_row, column=0,
                     sticky="ew",
                     padx=PADDING,
                     pady=(2, 0),
                 )
 
-            # Position step
+          # Position step (account for sub-steps from previous steps)
+            prev_steps_with_substeps = sum(1 for j in range(i) if j in (1, 3))
+            step_row = i * 2 + 1 + prev_steps_with_substeps
             step.grid(
-                row=i * 2 + 1, column=0,
+                row=step_row, column=0,
                 sticky="nsew",
                 padx=PADDING_SMALL,
                 pady=(0, 2),
@@ -390,20 +396,31 @@ class MainWindow(ctk.CTk):
             if i == 1:
                 self._sub_step_frame = SubStepFrame(panel)
                 self._sub_step_frame.grid(
-                    row=i * 2 + 2, column=0,
+                    row=step_row + 1, column=0,
                     sticky="nsew",
                     padx=PADDING_SMALL + 16,
                     pady=(0, 2),
                 )
 
-        # ── Progress indicator ──
+            # Step 4: add collapsible sub-step frame
+            if i == 3:
+                self._tftp_sub_step_frame = TftpSubStepFrame(panel)
+                self._tftp_sub_step_frame.grid(
+                    row=step_row + 1, column=0,
+                    sticky="nsew",
+                    padx=PADDING_SMALL + 16,
+                    pady=(0, 2),
+                )
+
+       # ── Progress indicator ──
         self._progress = ProgressIndicator(panel)
         self._progress.grid(
-            row=8, column=0,
-            sticky="nsew",
+            row=11, column=0,
+            sticky="ew",
             padx=PADDING,
             pady=(PADDING, PADDING_SMALL),
         )
+        panel.grid_rowconfigure(11, minsize=40)
 
         # ── Run All button ──
         self._run_btn = ctk.CTkButton(
@@ -415,10 +432,10 @@ class MainWindow(ctk.CTk):
             corner_radius=CORNER_RADIUS,
         )
         self._run_btn.grid(
-            row=9, column=0,
+            row=12, column=0,
             sticky="nsew",
             padx=PADDING,
-            pady=PADDING_SMALL,
+            pady=(0, PADDING_SMALL),
         )
 
     def _build_log_panel(self, parent: ctk.CTkFrame) -> None:
@@ -898,7 +915,7 @@ class MainWindow(ctk.CTk):
         self._log_message(f"  [{status}] {message}")
 
     def _run_step_4_load_main_program(self) -> bool:
-        """Step 4: Load main program (TFTP upload + reboot + boot verify)."""
+        """Step 4: Load main program (TFTP upload + download verify + CRC + reboot + boot verify)."""
         if not self._config:
             return False
 
@@ -907,42 +924,86 @@ class MainWindow(ctk.CTk):
             self._log_message("  No Firmware BIN file selected")
             return False
 
+        bin_path = Path(files["firmware_bin_path"])
+        remote_name = self._config.tftp_upload_name
         all_results: list[bool] = []
 
-        # 4a: TFTP upload
-        self._log_message("Step 4a: TFTP upload & verify...")
+        # 4a: TFTP Upload
+        self._log_message("Step 4a: TFTP upload...")
+        if hasattr(self, '_tftp_sub_step_frame'):
+            self._tftp_sub_step_frame.set_phase("Upload")
         try:
-            bin_path = Path(files["firmware_bin_path"])
-            tftp_result = tftp_upload_verify(
-                self._config, bin_path,
-                callback=self._tftp_callback,
+            tftp_result = tftp_upload(
+                self._config, bin_path, remote_name,
+                callback=self._tftp_sub_callback,
             )
             status = "✓" if tftp_result.success else "✗"
             self._log_message(f"  {status} {tftp_result.message}")
+            if hasattr(self, '_tftp_sub_step_frame'):
+                self._tftp_sub_step_frame.set_step_complete("Upload") if tftp_result.success else self._tftp_sub_step_frame.set_step_error("Upload")
             all_results.append(tftp_result.success)
         except Exception as e:
-            self._log_message(f"  ✗ TFTP error: {e}")
+            self._log_message(f"  ✗ TFTP upload error: {e}")
+            if hasattr(self, '_tftp_sub_step_frame'):
+                self._tftp_sub_step_frame.set_step_error("Upload")
             all_results.append(False)
 
         if not all_results[-1]:
-            self._log_message("  Skipping reboot & boot verify (TFTP failed)")
+            self._log_message("  Skipping download verify (TFTP upload failed)")
             return False
 
-        # 4a2: Post-download Zynq status check
-        self._log_message("Step 4b: Checking Zynq status after download...")
+        # 4b: TFTP Download Verify
+        self._log_message("Step 4b: TFTP download verify...")
+        if hasattr(self, '_tftp_sub_step_frame'):
+            self._tftp_sub_step_frame.set_phase("Download Verify")
         try:
-            zynq_status = verify_zynq_status(
-                self._config,
-                uart_available=self._uart_available,
-                callback=self._zynq_status_callback,
+            download_result = tftp_download(
+                self._config, remote_name,
+                callback=self._tftp_sub_callback,
             )
-            status = "✓" if zynq_status.success else "✗"
-            self._log_message(f"  {status} {zynq_status.message}")
+            status = "✓" if download_result.success else "✗"
+            self._log_message(f"  {status} {download_result.message}")
+            if hasattr(self, '_tftp_sub_step_frame'):
+                self._tftp_sub_step_frame.set_step_complete("Download Verify") if download_result.success else self._tftp_sub_step_frame.set_step_error("Download Verify")
+            all_results.append(download_result.success)
         except Exception as e:
-            self._log_message(f"  ✗ Zynq status check error: {e}")
+            self._log_message(f"  ✗ TFTP download error: {e}")
+            if hasattr(self, '_tftp_sub_step_frame'):
+                self._tftp_sub_step_frame.set_step_error("Download Verify")
+            all_results.append(False)
 
-        # 4c: Reboot
-        self._log_message("Step 4c: Rebooting Zynq...")
+        if not all_results[-1]:
+            self._log_message("  Skipping CRC check (download failed)")
+            return False
+
+        # 4c: CRC Check
+        self._log_message("Step 4c: CRC check...")
+        if hasattr(self, '_tftp_sub_step_frame'):
+            self._tftp_sub_step_frame.set_phase("CRC Check")
+        original_crc = _compute_crc32(bin_path)
+        downloaded_crc = download_result.crc_local
+        crc_match = original_crc == downloaded_crc
+        self._log_message(f"  Original CRC: {original_crc:#010x}")
+        self._log_message(f"  Downloaded CRC: {downloaded_crc:#010x}")
+        if crc_match:
+            self._log_message("  ✓ CRC match")
+            if hasattr(self, '_tftp_sub_step_frame'):
+                self._tftp_sub_step_frame.set_step_complete("CRC Check")
+            all_results.append(True)
+        else:
+            self._log_message("  ✗ CRC mismatch")
+            if hasattr(self, '_tftp_sub_step_frame'):
+                self._tftp_sub_step_frame.set_step_error("CRC Check")
+            all_results.append(False)
+
+        if not all_results[-1]:
+            self._log_message("  Skipping reboot (CRC mismatch)")
+            return False
+
+        # 4d: Reboot
+        self._log_message("Step 4d: Rebooting Zynq...")
+        if hasattr(self, '_tftp_sub_step_frame'):
+            self._tftp_sub_step_frame.set_phase("Reboot")
         try:
             reboot_result = reboot_zynq(
                 self._config,
@@ -951,17 +1012,23 @@ class MainWindow(ctk.CTk):
             )
             status = "✓" if reboot_result.success else "✗"
             self._log_message(f"  {status} {reboot_result.message}")
+            if hasattr(self, '_tftp_sub_step_frame'):
+                self._tftp_sub_step_frame.set_step_complete("Reboot") if reboot_result.success else self._tftp_sub_step_frame.set_step_error("Reboot")
             all_results.append(reboot_result.success)
         except Exception as e:
             self._log_message(f"  ✗ Reboot error: {e}")
+            if hasattr(self, '_tftp_sub_step_frame'):
+                self._tftp_sub_step_frame.set_step_error("Reboot")
             all_results.append(False)
 
         if not all_results[-1]:
             self._log_message("  Skipping boot verify (reboot failed)")
             return False
 
-        # 4d: Verify boot
-        self._log_message("Step 4d: Verifying boot...")
+        # 4e: Verify boot
+        self._log_message("Step 4e: Verifying boot...")
+        if hasattr(self, '_tftp_sub_step_frame'):
+            self._tftp_sub_step_frame.set_phase("Boot Verify")
         try:
             boot_result = verify_boot(
                 self._config,
@@ -975,9 +1042,13 @@ class MainWindow(ctk.CTk):
             if boot_result.serial_found:
                 self._log_message(f"  Boot info: {boot_result.boot_info}")
 
+            if hasattr(self, '_tftp_sub_step_frame'):
+                self._tftp_sub_step_frame.set_step_complete("Boot Verify") if boot_result.success else self._tftp_sub_step_frame.set_step_error("Boot Verify")
             all_results.append(boot_result.success)
         except Exception as e:
             self._log_message(f"  ✗ Boot verify error: {e}")
+            if hasattr(self, '_tftp_sub_step_frame'):
+                self._tftp_sub_step_frame.set_step_error("Boot Verify")
             all_results.append(False)
 
         return all(all_results)
@@ -986,6 +1057,10 @@ class MainWindow(ctk.CTk):
         """Callback for TFTP progress."""
         self._log_message(f"  [{status}] {message}")
 
+    def _tftp_sub_callback(self, status: str, message: str) -> None:
+        """Callback for TFTP sub-step progress."""
+        self._log_message(f"  [{status}] {message}")
+
     def _reboot_callback(self, status: str, message: str) -> None:
         """Callback for reboot progress."""
         self._log_message(f"  [{status}] {message}")
@@ -1121,6 +1196,11 @@ class MainWindow(ctk.CTk):
                 self._sub_step_frame.set_expanded(True)
                 self._flash_phase = ""
 
+            # Step 4: expand sub-step frame and reset
+            if i == 3 and hasattr(self, '_tftp_sub_step_frame'):
+                self._tftp_sub_step_frame.reset()
+                self._tftp_sub_step_frame.set_expanded(True)
+
             # Log step timeout estimate
             timeout_key, default_timeout = step_timeout_keys[i] if i < len(step_timeout_keys) else ("", 30)
             step_timeout = self._config.step_timeouts.get(timeout_key, default_timeout) if self._config else default_timeout
@@ -1142,6 +1222,13 @@ class MainWindow(ctk.CTk):
                         self._sub_step_frame.set_phase("done")
                     else:
                         self._sub_step_frame.set_phase("error")
+
+                # Step 4: mark sub-steps based on result
+                if i == 3 and hasattr(self, '_tftp_sub_step_frame'):
+                    if success:
+                        self._tftp_sub_step_frame.set_phase("Boot Verify")
+                    else:
+                        self._tftp_sub_step_frame.set_step_error("Boot Verify")
             except Exception as e:
                 results.append(False)
                 self._set_step_status(i, "error")
@@ -1210,7 +1297,7 @@ class MainWindow(ctk.CTk):
                 and self._uart_window.is_monitoring):
             self._log_message("  [UART] Already monitoring via UART window")
             return
-        if not self._uart_available or not self._config:
+        if not self._config:
             return
         port = self._config.serial_port
         if not port:
@@ -1226,6 +1313,8 @@ class MainWindow(ctk.CTk):
         self._uart_monitor.on_error = lambda e: self._log_message(f"  [UART] Error: {e}")
         if self._uart_monitor.start():
             self._log_message("  [UART] Serial monitor started")
+        else:
+            self._log_message("  [UART] Serial monitor failed to start (port may be busy)")
 
     def _stop_uart_monitor(self) -> None:
         """Stop background serial monitoring."""

src/gui/widgets.py

@@ -392,26 +392,32 @@ class ProgressIndicator(ctk.CTkFrame):
 
     def _create_widgets(self) -> None:
         """Create the progress indicator UI."""
-        self.grid_rowconfigure(0, weight=1)
+        self.grid_rowconfigure(0, weight=0)
+        self.grid_rowconfigure(1, weight=1)
         self.grid_columnconfigure(0, weight=1)
 
-        self._progress_bar = ctk.CTkProgressBar(
-            self,
-            variable=self._progress_var,
-            corner_radius=CORNER_RADIUS,
-            height=8,
-        )
-        self._progress_bar.grid(
-            row=0, column=0, sticky="nsew", padx=PADDING
-        )
-
+        # Percentage label on top
         self._percent_label = ctk.CTkLabel(
             self,
             text="0%",
-            font=FONT_SMALL,
+            font=(FONT_BODY[0], 13, "bold"),
+            text_color=PRIMARY_COLOR,
         )
         self._percent_label.grid(
-            row=0, column=0, sticky="ne", padx=PADDING, pady=PADDING_SMALL
+            row=0, column=0, sticky="e", padx=PADDING, pady=(0, 6)
+        )
+
+        # Progress bar below
+        self._progress_bar = ctk.CTkProgressBar(
+            self,
+            variable=self._progress_var,
+            corner_radius=6,
+            height=10,
+            progress_color=PRIMARY_COLOR,
+            border_width=0,
+        )
+        self._progress_bar.grid(
+            row=1, column=0, sticky="ew", padx=PADDING, pady=2
         )
 
     def set_value(self, value: float) -> None:
@@ -765,8 +771,143 @@ class SubStepFrame(ctk.CTkFrame):
         if not self._collapsed:
             self._toggle()
             self._toggle()  # collapse then expand to reset
+
+
+# ════════════════════════════════════════════════════════════════════
+# TftpSubStepFrame — collapsible sub-step progress for Step 4
+# ════════════════════════════════════════════════════════════════════
+
+
+class TftpSubStepFrame(ctk.CTkFrame):
+    """Collapsible frame showing TFTP workflow sub-steps with status.
+
+    Sub-steps: Upload, Download Verify, CRC Check, Reboot, Boot Verify.
+    Colors match StepHeader: gray pending, blue running (+pulse),
+    green complete, red error. Default expanded.
+    """
+
+    def __init__(self, parent: ctk.CTkFrame, substep_names: list[str] | None = None) -> None:
+        super().__init__(parent, fg_color="transparent")
+
+        self._collapsed = False
+        self._pulse_job: str | None = None
+        self._substep_names = substep_names or ["Upload", "Download Verify", "CRC Check", "Reboot", "Boot Verify"]
+        self._num_steps = len(self._substep_names)
+
+        self._toggle_btn = ctk.CTkButton(
+            self, text="▼", width=22, height=22,
+            font=(FONT_SMALL[0], 9), command=self._toggle,
+        )
+        self._toggle_btn.grid(row=0, column=0, padx=(4, 4), pady=(2, 2), sticky="w")
+
+        self._header_label = ctk.CTkLabel(
+            self, text=" → ".join(self._substep_names),
+            font=FONT_SMALL,
+        )
+        self._header_label.grid(row=0, column=1, padx=(0, 4), pady=(2, 2), sticky="w")
+
+        self._sub_items: list[dict] = []
+        for i, name in enumerate(self._substep_names):
+            dot = ctk.CTkLabel(self, text="○", font=FONT_BODY, text_color=ACCENT_PENDING, anchor="w")
+            label = ctk.CTkLabel(self, text=name, font=FONT_BODY, text_color=ACCENT_PENDING, anchor="w")
+            dot.grid(row=i + 1, column=0, padx=(16, 2), pady=(2, 2), sticky="w")
+            label.grid(row=i + 1, column=1, padx=(2, 4), pady=(2, 2), sticky="w")
+            self._sub_items.append({"dot": dot, "label": label})
+
+        self.grid_columnconfigure(1, weight=1)
+
+    def _toggle(self) -> None:
+        self._collapsed = not self._collapsed
+        self._toggle_btn.configure(text="▶" if self._collapsed else "▼")
+        for item in self._sub_items:
+            if self._collapsed:
+                item["dot"].grid_remove()
+                item["label"].grid_remove()
+            else:
+                item["dot"].grid()
+                item["label"].grid()
+
+    def set_expanded(self, expanded: bool) -> None:
+        if expanded != self._collapsed:
             self._toggle()
 
+    def set_phase(self, phase: str, pct: int = -1) -> None:
+        _C_PENDING = ACCENT_PENDING
+        _C_RUNNING = CIRCLE_RUNNING
+        _C_DONE = ACCENT_COMPLETE
+
+        idx = self._substep_names.index(phase) if phase in self._substep_names else -1
+        if idx < 0:
+            return
+
+        for i, item in enumerate(self._sub_items):
+            if i < idx:
+                item["dot"].configure(text="●", text_color=_C_DONE)
+                item["label"].configure(text_color=_C_DONE)
+            elif i == idx:
+                item["dot"].configure(text="◉", text_color=_C_RUNNING)
+                item["label"].configure(text_color=_C_RUNNING)
+            else:
+                item["dot"].configure(text="○", text_color=_C_PENDING)
+                item["label"].configure(text_color=_C_PENDING)
+
+        if phase in self._substep_names and not self._pulse_job:
+            self._start_pulse(idx)
+
+    def set_step_complete(self, phase: str) -> None:
+        """Mark a specific step as complete."""
+        _C_DONE = ACCENT_COMPLETE
+        idx = self._substep_names.index(phase) if phase in self._substep_names else -1
+        if idx < 0:
+            return
+        item = self._sub_items[idx]
+        item["dot"].configure(text="●", text_color=_C_DONE)
+        item["label"].configure(text_color=_C_DONE)
+        self._stop_pulse()
+
+    def set_step_error(self, phase: str) -> None:
+        """Mark a specific step as error."""
+        _C_ERROR = ACCENT_ERROR
+        idx = self._substep_names.index(phase) if phase in self._substep_names else -1
+        if idx < 0:
+            return
+        item = self._sub_items[idx]
+        item["dot"].configure(text="✗", text_color=_C_ERROR)
+        item["label"].configure(text_color=_C_ERROR)
+        self._stop_pulse()
+
+    def _start_pulse(self, idx: int) -> None:
+        """Start pulsing the running sub-step."""
+        _LIGHT = RUNNING_FADE_LIGHT
+        _DARK = RUNNING_FADE_DARK
+
+        def _pulse(state=[True]):
+            if idx >= len(self._sub_items):
+                return
+            item = self._sub_items[idx]
+            c = _LIGHT if state[0] else _DARK
+            item["dot"].configure(text_color=c)
+            item["label"].configure(text_color=c)
+            state[0] = not state[0]
+            self._pulse_job = self.after(PULSE_DURATION, lambda: _pulse(state))
+
+        _pulse()
+
+    def _stop_pulse(self) -> None:
+        if self._pulse_job:
+            self.after_cancel(self._pulse_job)
+            self._pulse_job = None
+
+    def reset(self) -> None:
+        self._stop_pulse()
+        _C = ACCENT_PENDING
+        for i, item in enumerate(self._sub_items):
+            item["dot"].configure(text="○", text_color=_C)
+            item["label"].configure(text_color=_C)
+        if not self._collapsed:
+            self._toggle()
+            self._toggle()  # collapse then expand to reset
+
 
 # ════════════════════════════════════════════════════════════════════
 # UartMonitorWindow — standalone UART monitoring window

src/tftp_manager.py

@@ -69,18 +69,21 @@ def _tftp_put(host: str, file_path: str, remote_name: str,
     """
     data = Path(file_path).read_bytes()
     total = len(data)
+    block_size = _TFTP_BLOCK_SIZE
 
     sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
     sock.settimeout(timeout)
+    # Allow reuse of local port (prevents "Address already in use")
+    sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
 
     try:
-        # ── Send WRQ ──
+        # ── Send WRQ to port 69 ──
         wrq = struct.pack("!H", _TFTP_WRQ)
         wrq += remote_name.encode("ascii") + b"\x00"
         wrq += b"octet\x00"
         sock.sendto(wrq, (host, 69))
 
-        # ── Wait for ACK(0) to get server's data port ──
+        # ── Wait for ACK(0) — discover server's data port ──
         ack_data, server_addr = sock.recvfrom(516)
         if len(ack_data) < 4:
             return False, "Invalid ACK from server"
@@ -89,24 +92,34 @@ def _tftp_put(host: str, file_path: str, remote_name: str,
             err_msg = ack_data[4:].decode("ascii", errors="replace").rstrip("\x00")
             return False, f"TFTP error: {err_msg}"
         if opcode != _TFTP_ACK:
-            return False, f"Expected ACK, got opcode {opcode}"
+            return False, f"Expected ACK(0), got opcode {opcode} from {server_addr}"
+
+        srv_ip, srv_port = server_addr
+        import sys
+        print(f"[TFTP] ACK(0) received from {srv_ip}:{srv_port} "
+              f"(file={total}B, blocks={-(-total // block_size)})", file=sys.stderr)
 
         # ── Send data blocks ──
         block = 1
         offset = 0
         while offset < total:
-            chunk = data[offset:offset + _TFTP_BLOCK_SIZE]
+            chunk = data[offset:offset + block_size]
             offset += len(chunk)
             data_pkt = struct.pack("!HH", _TFTP_DATA, block) + chunk
             sock.sendto(data_pkt, server_addr)
 
+            if block % 100 == 0 or block == -(-total // block_size):
+                pct = offset * 100 // total
+                print(f"[TFTP] Block {block}/{-(-total // block_size)} ({pct}%) sent to {srv_ip}:{srv_port}",
+                      file=sys.stderr)
+
             # Wait for ACK
             try:
                 ack_data, _ = sock.recvfrom(516)
                 ack_op = struct.unpack("!H", ack_data[:2])[0]
                 ack_block = struct.unpack("!H", ack_data[2:4])[0]
                 if ack_op != _TFTP_ACK or ack_block != block:
-                    return False, f"Bad ACK: op={ack_op} block={ack_block}"
+                    return False, f"Bad ACK: op={ack_op} block={ack_block} (expected {block})"
             except socket.timeout:
                 return False, f"Timeout waiting for ACK block {block}"
             block += 1
@@ -116,6 +129,91 @@ def _tftp_put(host: str, file_path: str, remote_name: str,
         sock.close()
 
 
+def _tftp_get(host: str, remote_name: str, local_dir: str | Path,
+              timeout: float = 30.0) -> tuple[bool, str, Path]:
+    """Download a file from a TFTP server using pure Python.
+
+    Args:
+        host: TFTP server IP address.
+        remote_name: Remote filename.
+        local_dir: Local directory to save the downloaded file.
+        timeout: Socket timeout in seconds.
+
+    Returns:
+        (success, message, local_path).
+    """
+    local_dir = Path(local_dir)
+    local_dir.mkdir(parents=True, exist_ok=True)
+    local_path = local_dir / remote_name
+
+    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+    sock.settimeout(timeout)
+    sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+
+    try:
+        # ── Send RRQ to port 69 ──
+        rrq = struct.pack("!H", _TFTP_RRQ)
+        rrq += remote_name.encode("ascii") + b"\x00"
+        rrq += b"octet\x00"
+        sock.sendto(rrq, (host, 69))
+
+        # ── Wait for first DATA block — discover server's data port ──
+        data_pkt, server_addr = sock.recvfrom(516)
+        if len(data_pkt) < 4:
+            return False, "Invalid DATA from server", local_path
+        opcode = struct.unpack("!H", data_pkt[:2])[0]
+        if opcode == _TFTP_ERROR:
+            err_msg = data_pkt[4:].decode("ascii", errors="replace").rstrip("\x00")
+            return False, f"TFTP error: {err_msg}", local_path
+        if opcode != _TFTP_DATA:
+            return False, f"Expected DATA, got opcode {opcode} from {server_addr}", local_path
+
+        srv_ip, srv_port = server_addr
+        block = struct.unpack("!H", data_pkt[2:4])[0]
+        file_data = data_pkt[4:]
+        import sys
+        print(f"[TFTP] DATA block 1 received from {srv_ip}:{srv_port} "
+              f"({len(file_data)} bytes)", file=sys.stderr)
+
+        # ── ACK block 0 and receive subsequent blocks ──
+        ack = struct.pack("!HH", _TFTP_ACK, 0)
+        sock.sendto(ack, server_addr)
+
+        total = len(file_data)
+        while len(file_data) == _TFTP_BLOCK_SIZE:
+            # Wait for next DATA block
+            try:
+                data_pkt, _ = sock.recvfrom(516)
+                if len(data_pkt) < 4:
+                    return False, "Incomplete DATA packet", local_path
+                opcode = struct.unpack("!H", data_pkt[:2])[0]
+                if opcode != _TFTP_DATA:
+                    return False, f"Expected DATA, got opcode {opcode}", local_path
+
+                new_block = struct.unpack("!H", data_pkt[2:4])[0]
+                chunk = data_pkt[4:]
+                file_data += chunk
+                total = len(file_data)
+                block = new_block
+
+                if block % 100 == 0:
+                    pct = total * 100 // max(total, 1)
+                    print(f"[TFTP] Block {block}/{block} ({pct}%) received from {srv_ip}:{srv_port}",
+                          file=sys.stderr)
+
+                # ACK this block
+                ack = struct.pack("!HH", _TFTP_ACK, block)
+                sock.sendto(ack, server_addr)
+            except socket.timeout:
+                return False, f"Timeout waiting for DATA block {block}", local_path
+
+        # ── Write file ──
+        local_path.write_bytes(file_data)
+        return True, f"Downloaded {total} bytes to {local_path}", local_path
+    finally:
+        sock.close()
+
+
 def tftp_upload(
     config: Config,
     file_path: str | Path,
@@ -184,25 +282,50 @@ def tftp_download(
 ) -> TftpResult:
     """Download a file from the Zynq device via TFTP (pure Python).
 
-    Note: This requires the Zynq to act as a TFTP client (U-Boot tftpboot),
-    which typically means we need a TFTP server on the PC.  For simplicity,
-    this operation is not natively supported — use ping-based verification
-    or UART log analysis instead.
-
     Args:
         config: Application configuration.
         remote_name: Remote filename on the TFTP server.
-        local_dir: Local directory to save the file.
+        local_dir: Local directory to save the file (default: temp directory).
         callback: Optional progress callback.
 
     Returns:
-        TftpResult with status (always not-implemented).
+        TftpResult with download status and CRC.
     """
+    if local_dir is None:
+        local_dir = tempfile.gettempdir()
+    local_dir = Path(local_dir)
+
+    if callback:
+        callback("start", f"Downloading {remote_name} from {config.zynq_ip}")
+
+    # Verify device is reachable
+    if not ping_ip(config.zynq_ip, config.ping_count, config.ping_timeout):
+        return TftpResult(
+            step="download", success=False,
+            message=f"Device {config.zynq_ip} not reachable",
+            remote_path=remote_name,
+        )
+
+    try:
+        ok, msg, local_path = _tftp_get(config.zynq_ip, remote_name, local_dir)
+    except OSError as e:
+        return TftpResult(
+            step="download", success=False,
+            message=f"Socket error: {e}",
+            remote_path=remote_name,
+        )
+
+    crc = _compute_crc32(local_path) if ok else 0
+
+    if callback:
+        callback("complete" if ok else "error",
+                 "Download " + ("succeeded" if ok else "failed"))
+
     return TftpResult(
-        step="download",
-        success=False,
-        message="TFTP download requires PC-side server (not implemented)",
-        remote_path=remote_name,
+        step="download", success=ok,
+        message=msg,
+        local_path=str(local_path), remote_path=remote_name,
+        crc_local=crc,
     )