dap: add TransferProtocolError exception and do swd_reset when it occurs (#1784)

Author: TeoMahnic

pyocd/core/exceptions.py

@@ -1,5 +1,5 @@
 # pyOCD debugger
-# Copyright (c) 2018-2020 Arm Limited
+# Copyright (c) 2018-2020,2025 Arm Limited
 # SPDX-License-Identifier: Apache-2.0
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
@@ -61,6 +61,10 @@ class TransferTimeoutError(TransferError):
     """@brief An SWD or JTAG timeout occurred"""
     pass
 
+class TransferProtocolError(TransferError):
+    """@brief SWD transfer protocol error occurred"""
+    pass
+
 class TransferFaultError(TransferError):
     """@brief A memory fault occurred.
 

pyocd/coresight/dap.py

@@ -1,5 +1,5 @@
 # pyOCD debugger
-# Copyright (c) 2015-2020 Arm Limited
+# Copyright (c) 2015-2020,2025 Arm Limited
 # Copyright (c) 2021-2023 Chris Reed
 # Copyright (c) 2022 Clay McClure
 # Copyright (c) 2022 Toshiba Electronic Devices & Storage Corporation
@@ -1016,6 +1016,9 @@ def _handle_error(self, error: Exception, num: int) -> None:
             # This may put the AP that was aborted into an unpredictable state. Should consider
             # attempting to reset debug logic.
             self.write_reg(DP_ABORT, ABORT_DAPABORT)
+        elif isinstance(error, exceptions.TransferProtocolError):
+            if self.probe.wire_protocol == DebugProbe.Protocol.SWD:
+                self._swd_reset()
 
     def clear_sticky_err(self) -> None:
         self._invalidate_cache()
@@ -1028,6 +1031,28 @@ def clear_sticky_err(self) -> None:
         else:
             assert False
 
+    def _swd_reset(self) -> None:
+        """@brief Reset the SWD interface.
+
+        This method is used to reset the SWD interface when a protocol error occurs. It is not
+        intended to be used for resetting the target.
+        """
+        self._invalidate_cache()
+        try:
+            self.lock()
+            swj = SWJSequenceSender(self.probe, False)
+            for i in range(2):
+                try:
+                    swj.line_reset()                        # > 50 cycles SWDIO/TMS High.
+                    swj.idle_cycles(8)                      # At least 2 idle cycles (SWDIO/TMS Low).
+                    self.probe.read_dp(DP_IDR, now=True)    # Read DP IDR to take connection out of reset state.
+                    break                                   # Success, so break out of the loop.
+                except exceptions.TargetError as error:
+                    if i != 0:
+                        raise                               # Raise the error if this is the second attempt.
+        finally:
+            self.unlock()
+
 class APAccessMemoryInterface(memory_interface.MemoryInterface):
     """@brief Memory interface for performing simple APACC transactions.
 

pyocd/probe/cmsis_dap_probe.py

@@ -1,5 +1,5 @@
 # pyOCD debugger
-# Copyright (c) 2018-2020 Arm Limited
+# Copyright (c) 2018-2020,2025 Arm Limited
 # Copyright (c) 2021-2023 Chris Reed
 # SPDX-License-Identifier: Apache-2.0
 #
@@ -718,6 +718,8 @@ def _convert_exception(exc: Exception) -> Exception:
             return exceptions.TransferFaultError(*exc.args)
         elif isinstance(exc, DAPAccess.TransferTimeoutError):
             return exceptions.TransferTimeoutError(*exc.args)
+        elif isinstance(exc, DAPAccess.TransferProtocolError):
+            return exceptions.TransferProtocolError(*exc.args)
         elif isinstance(exc, DAPAccess.TransferError):
             return exceptions.TransferError(*exc.args)
         elif isinstance(exc, (DAPAccess.DeviceError, DAPAccess.CommandError)):

pyocd/probe/pydapaccess/dap_access_cmsis_dap.py

@@ -1,5 +1,5 @@
 # pyOCD debugger
-# Copyright (c) 2006-2013,2018-2021 Arm Limited
+# Copyright (c) 2006-2013,2018-2021,2025 Arm Limited
 # Copyright (c) 2020 Koji Kitayama
 # Copyright (c) 2021-2022 Chris Reed
 # SPDX-License-Identifier: Apache-2.0
@@ -373,7 +373,7 @@ def _check_response(self, response):
             else:
                 raise DAPAccessIntf.TransferError("Unexpected ACK value (%d) returned by probe" % ack)
         elif (response & DAPTransferResponse.PROTOCOL_ERROR_MASK) != 0:
-            raise DAPAccessIntf.TransferError("SWD protocol error")
+            raise DAPAccessIntf.TransferProtocolError("SWD protocol error")
 
     def _decode_transfer_data(self, data):
         """@brief Take a byte array and extract the data from it