S12S Debug Module (S12SDBGV2)
trace buffer address is initialized by unlocking the trace buffer and points to the oldest valid data even if a
reset occurred during the tracing session. To read the trace buffer after a reset, TSOURCE must be set,
otherwise the trace buffer reads as all zeroes. Generally debugging occurrences of system resets is best
handled using end trigger alignment since the reset may occur before the trace trigger, which in the begin
trigger alignment case means no information would be stored in the trace buffer.
The Trace Buffer contents and DBGCNT bits are undefined following a POR.
NOTE
An external pin RESET that occurs simultaneous to a trace buffer entry can,
in very seldom cases, lead to either that entry being corrupted or the first
entry of the session being corrupted. In such cases the other contents of the
trace buffer still contain valid tracing information. The case occurs when the
reset assertion coincides with the trace buffer entry clock edge.
6.4.6
Tagging
A tag follows program information as it advances through the instruction queue. When a tagged instruction
reaches the head of the queue a tag hit occurs and can initiate a state sequencer transition.
Each comparator control register features a TAG bit, which controls whether the comparator match causes
a state sequencer transition immediately or tags the opcode at the matched address. If a comparator is
enabled for tagged comparisons, the address stored in the comparator match address registers must be an
opcode address.
Using Begin trigger together with tagging, if the tagged instruction is about to be executed then the
transition to the next state sequencer state occurs. If the transition is to the Final State, tracing is started.
Only upon completion of the tracing session can a breakpoint be generated. Using End alignment, when
the tagged instruction is about to be executed and the next transition is to Final State then a breakpoint is
generated immediately, before the tagged instruction is carried out.
R/W monitoring, access size (SZ) monitoring and data bus monitoring are not useful if tagging is selected,
since the tag is attached to the opcode at the matched address and is not dependent on the data bus nor on
the type of access. Thus these bits are ignored if tagging is selected.
When configured for range comparisons and tagging, the ranges are accurate only to word boundaries.
Tagging is disabled when the BDM becomes active.
6.4.7
Breakpoints
It is possible to generate breakpoints from channel transitions to final state or using software to write to
the TRIG bit in the DBGC1 register.
6.4.7.1
Breakpoints From Comparator Channels
Breakpoints can be generated when the state sequencer transitions to the Final State. If configured for
tagging, then the breakpoint is generated when the tagged opcode reaches the execution stage of the
instruction queue.
S12P-Family Reference Manual, Rev. 1.13
Freescale Semiconductor
187
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