🐛 fix: recover JTAG DAP after flash programming with system reset

- Add preemptive rst -system at Step 3 start to clean post-flash DAP state
- Replace ineffective SLCR write with targets 1 → rst -system recovery
- Exit with clear power-cycle instructions when DAP recovery fails
- Show prominent DAP error guidance in GUI for Step 3 and Run All mode

Verified: Step 1→2→3 now passes end-to-end (340s) without board power cycle.

src/bitstream_programmer.py

@@ -589,14 +589,27 @@ def full_bitstream_program(
     if callback:
         callback("start", "Initializing Zynq via JTAG (FSBL override)...")
 
-    # Build TCL with DAP error recovery
+    # Build TCL with preemptive system reset + DAP error recovery
     tcl_lines = [
         'puts "INFO: Starting Zynq initialization via JTAG override..."',
         "connect",
         "after 500",
     ]
 
-    # DAP error check — if ARM cores not visible, try recovery
+    # Preemptive system reset (safe on clean board; may help after flash)
+    tcl_lines.extend([
+        '# Preemptive system reset to clean up any leftover state',
+        'if {[catch {',
+        '  targets -set -nocase -filter {name =~ "arm*#0"}',
+        '  rst -system',
+        '  after 1000',
+        '  stop',
+        '  puts "INFO: Preemptive system reset OK"',
+        '}]} { puts "INFO: Preemptive system reset skipped (no ARM target)" }',
+        '',
+    ])
+
+    # DAP error check — if ARM cores not visible, try recovery via DAP
     tcl_lines.extend([
         '# Check for DAP errors and attempt recovery',
         'set target_list [targets]',
@@ -606,19 +619,29 @@ def full_bitstream_program(
         '  puts "WARNING: ARM cores not found — DAP may be in error state"',
         '  # Show DAP status for logging',
         '  foreach t $target_list { puts "  TARGET: $t" }',
-        '  # Attempt to recover by reinitializing via DAP',
+        '  # Attempt recovery: select DAP (target 1) and issue system reset',
-        '  catch { targets 1 }',
+        '  puts "INFO: Attempting DAP recovery via system reset..."',
-        '  after 500',
-        '  # If DAP is accessible, try writing to system reset register',
         '  catch {',
-        '    puts "Attempting SLCR system reset..."',
+        '    targets 1',
-        '    mwr 0xF8000200 0x00000001',
+        '    rst -system',
         '    after 1000',
-        '    disconnect',
+        '    stop',
-        '    after 1000',
-        '    connect',
-        '    after 500',
         '  }',
+        '  # Disconnect and reconnect to refresh JTAG chain',
+        '  catch { disconnect; after 1000; connect; after 500 }',
+        '  # Check if ARM cores reappeared',
+        '  set target_list2 [targets]',
+        '  set has_arm2 0',
+        '  foreach t $target_list2 {',
+        '    puts "  AFTER RECOVERY: $t"',
+        '    if {[regexp -nocase {arm|cortex} $t]} { set has_arm2 1 }',
+        '  }',
+        '  if {!$has_arm2} {',
+        '    puts "FATAL: DAP recovery failed — board requires power cycle"',
+        '    puts "FATAL: Unplug/replug power, then retry Step 3"',
+        '    exit 1',
+        '  }',
+        '  puts "INFO: ARM cores recovered after DAP reset"',
         '}',
         '',
     ])

src/gui/main_window.py

@@ -871,9 +871,22 @@ class MainWindow(ctk.CTk):
                 self._bitstream_callback,
             )
 
+            dap_fail = False
             for result in results:
                 status = "✓" if result.success else "✗"
                 self._log_message(f"  {status} {result.step}: {result.message}")
+                if "DAP" in result.message and not result.success:
+                    dap_fail = True
+
+            if dap_fail:
+                self._log_message("")
+                self._log_message("  ╔══════════════════════════════════════════╗")
+                self._log_message("  ║  DAP ERROR — JTAG debug port hung       ║")
+                self._log_message("  ║  This is caused by prior flash programming║")
+                self._log_message("  ║  → Power-cycle the board (unplug/replug) ║")
+                self._log_message("  ║  → Then re-run Step 3                    ║")
+                self._log_message("  ╚══════════════════════════════════════════╝")
+                self._log_message("")
 
             return all(r.success for r in results)
         except Exception as e:
@@ -1138,6 +1151,17 @@ class MainWindow(ctk.CTk):
 
             # Stop on first failure when running all steps
             if not single_step and not success:
+                # Special case: Step 2 pass → Step 3 DAP fail = board needs power cycle
+                if i == 2 and results[0] and not success:
+                    self._log_message("")
+                    self._log_message("  ┌─────────────────────────────────────────┐")
+                    self._log_message("  │ Step 3 failed after flash programming.  │")
+                    self._log_message("  │ This is normal — program_flash leaves   │")
+                    self._log_message("  │ the JTAG DAP in error state.            │")
+                    self._log_message("  │ → Power-cycle the board, then re-run    │")
+                    self._log_message("  │   Step 3 (Load Bootloader) alone.       │")
+                    self._log_message("  └─────────────────────────────────────────┘")
+                    self._log_message("")
                 break
 
             # Inter-step delay (skip after last step)