Table of Contents
Advertisement
12.4
Common Debug Port (DP) features
12.4.1
Sticky flags and DP error responses
ARM DDI 0337B
This section describes features that are implemented by both SW-DP and JTAG-DP.
These common features affect the way that a debugger is able to perform transactions
with the DAP. It contains the following:
•
Sticky flags and DP error responses.
Within both a SW-DP and a JTAG-DP, sticky flags are used to indicate error conditions
and to report the result of pushed compare and pushed verify operations. The different
sticky flags are described in the following sections:
•
Read and write errors on page 12-42
•
Overrun detection on page 12-42
•
Protocol errors, SW-DP only on page 12-43
•
Pushed compare and pushed verify operations on page 12-44.
Note
When set, a sticky flag remains set until it is explicitly cleared. Even if the condition
that caused the flag to be set no longer applies, the flag remains set until the debugger
clears it. The method for clearing sticky flags is different for the JTAG-DP and the
SW-DP. See The Control/Status Register, CTRL/STAT on page 12-53 for information
about how these flags are cleared.
Errors can be returned by the DP itself, or can come from a debug register access, for
example when the processor is powered down.
Within the Debug Port, errors are flagged by sticky flags in the DP Control/Status
Register (CTRL/STAT). When an error is flagged the current transaction is completed
and further APACC (AP Access) transactions are discarded until the sticky flag is
cleared.
The DP response to an error condition might be:
•
To signal an error response immediately. This happens with the SW-DP.
•
To immediately discard all transactions as complete. This happens with the
JTAG-DP.
This means that the debugger must check the Control/Status Register after performing
a series of APACC transactions, to check if an error occurred. If a sticky flag is set, the
debugger can clear the flag and then, if necessary, initiate more APACC transactions to
Copyright © 2005, 2006 ARM Limited. All rights reserved.
Debug Port
12-41
Advertisement
Table of Contents
