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12.4.4
Protocol errors, SW-DP only
ARM DDI 0337B
If the overrun detection mode is disabled, by clearing the ORUNDETECT flag, while
STICKYORUN is set, the subsequent value of STICKYORUN is Unpredictable. To
leave overrun detection mode a debugger must:
•
check the value of the STICKYORUN bit in the Control/Status register
•
clear the STICKYORUN bit, if it is set
•
clear the ORUNDETECT bit, to stop overrun detection mode.
Although these errors can only be detected with the SW-DP, they are described in this
chapter because they are part of the sticky flags error handling mechanism on the
SW-DP.
On the Serial Wire Debug interface, protocol errors can only occur, for example because
of wire-level errors. These errors might be detected by the parity checks on the data.
If the SW-DP detects a parity error in a message header, the Debug Port does not
respond to the message. The debugger must be aware of this possibility. If it does not
receive a response to a message, the debugger must back off. It must then request a read
of the IDCODE register, to ensure the Debug Port is responsive, before retrying the
original access. For details of the IDCODE register see The Identification Code
Register, IDCODE on page 12-52.
If the SW-DP detects a parity error in the data phase of a write transaction, it sets the
Sticky Write Data Error flag, WDATAERR, in the Control/Status (CTRL/STAT)
Register. Subsequent accesses from the debugger, other than IDCODE, CTRL/STAT or
ABORT, results in a FAULT response. For details of the CTRL/STAT register see The
Control/Status Register, CTRL/STAT on page 12-53.
On receiving a FAULT response from the SW-DP a debugger must read the
CTRL/STAT register and check the sticky flag values. The WDATAERR flag is cleared
by writing b1 to the WDERRCLR field of the Abort Register, see The Abort Register,
ABORT on page 12-49.
Copyright © 2005, 2006 ARM Limited. All rights reserved.
Debug Port
12-43
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