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CMSIS-DAP v2: send ZLP instead of a dummy byte to terminate OUT transfer #1882

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Merged
RobertRostohar merged 1 commit into from
Dec 18, 2025
Merged

CMSIS-DAP v2: send ZLP instead of a dummy byte to terminate OUT transfer #1882

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12 changes: 6 additions & 6 deletions pyocd/probe/pydapaccess/interface/pyusb_v2_backend.py
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
# pyOCD debugger
# Copyright (c) 2019-2021 Arm Limited
# Copyright (c) 2019-2021,2025 Arm Limited
# Copyright (c) 2021 mentha
# Copyright (c) 2021-2023 Chris Reed
# Copyright (c) 2025 Lars Häring
Expand Down Expand Up @@ -220,18 +220,18 @@ def get_all_connected_interfaces():

def write(self, data):
"""@brief Write data on the OUT endpoint."""

if self.ep_out:
if (len(data) > 0) and (len(data) < self.packet_size) and (len(data) % self.ep_out.wMaxPacketSize == 0):
data.append(0)

if TRACE.isEnabledFor(logging.DEBUG):
TRACE.debug(" USB OUT> (%d) %s", len(data), ' '.join([f'{i:02x}' for i in data]))

self.read_sem.release()

self.ep_out.write(data, timeout=self.DEFAULT_USB_TIMEOUT_MS)

# Send a zero-length packet (ZLP) to terminate the transfer when data length is less
# than DAP packet size but exactly a multiple of USB endpoint maximum packet size.
if (len(data) > 0) and (len(data) < self.packet_size) and (len(data) % self.ep_out.wMaxPacketSize == 0):
self.ep_out.write(b'', timeout=self.DEFAULT_USB_TIMEOUT_MS)

def read(self):
"""@brief Read data on the IN endpoint."""
if self.closed:
Expand Down
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