XC886/888CLM
On-Chip Debug Support
User’s Manual
17-6
V1.3, 2010-02
OCDS, V 1.0
Note: In order to continue user program execution after the debug event, an external
Debugger must restore the original opcode at the address of the current software
breakpoint.
17.3.1.3
External Breaks
These debug events are of Break Now type and can be raised in two ways:
•
By a request via the JTAG interface - using a special sequence, an external device
connected to the JTAG can break the user program running on XC886/888 and start
a debug session;
•
By asserting low the dedicated Monitor and BootStrap loader Control line (MBC)
while the XC886/888 is running and this type of break is enabled - used for reaction
to asynchronous events from the external world.
17.3.1.4
NMI-mode priority over Debug-mode
While the core is in NMI-mode (after an NMI-request has been accepted and before the
RETI instruction is executed, i.e. the time during a NMI-servicing routine), certain debug
functions are blocked/restricted:
1. No external break is possible while the core is servicing an NMI.
External break requested inside a NMI-servicing routine will be taken only after RETI
is executed.
2. A breakpoint into NMI-servicing routine is taken, but single-step is not possible
afterwards.
If a step is requested, the servicing routine will run as coded and monitor mode will
be invoked again only after a RETI is executed.
Hardware breakpoints and software breakpoints proceed as normal while CPU is in NMI-
mode.
17.3.2
Debug Actions
In case of a debug event, the OCDS system can respond in two ways depending on the
current configuration.
17.3.2.1
Call the Monitor Program
XC886/888 comes with an on-chip Monitor program, factory-stored into the non-volatile
Monitor ROM (see
). Activating this program is the primary and basic OCDS
reaction to recognized debug events.
The OCDS hardware ensures that the Monitor is always safely started, and fully
independent of the current system status at the moment when the debug action is taken.
Also, interrupt requests optionally raised during Monitor-entry will not disturb the
firmware functioning.
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