S12S Debug Module (S12SDBGV2)
is disabled and the transition to Final State can only generate a breakpoint request. In this case or upon
completion of a tracing session when tracing is enabled, the ARM bit in the DBGC1 register is cleared,
returning the module to the disarmed state0. If tracing is enabled a breakpoint request can occur at the end
of the tracing session. If neither tracing nor breakpoints are enabled then when the final state is reached it
returns automatically to state0 and the debug module is disarmed.
6.4.5
Trace Buffer Operation
The trace buffer is a 64 lines deep by 20-bits wide RAM array. The DBG module stores trace information
in the RAM array in a circular buffer format. The system accesses the RAM array through a register
window (DBGTBH:DBGTBL) using 16-bit wide word accesses. After each complete 20-bit trace buffer
line is read, an internal pointer into the RAM increments so that the next read receives fresh information.
Data is stored in the format shown in Table 6-37 and Table 6-40. After each store the counter register
DBGCNT is incremented. Tracing of CPU activity is disabled when the BDM is active. Reading the trace
buffer whilst the DBG is armed returns invalid data and the trace buffer pointer is not incremented.
6.4.5.1
Trace Trigger Alignment
Using the TALIGN bit (see 6.3.2.3) it is possible to align the trigger with the end or the beginning of a
tracing session.
If end alignment is selected, tracing begins when the ARM bit in DBGC1 is set and State1 is entered; the
transition to Final State signals the end of the tracing session. Tracing with Begin-Trigger starts at the
opcode of the trigger. Using end alignment or when the tracing is initiated by writing to the TRIG bit whilst
configured for begin alignment, tracing starts in the second cycle after the DBGC1 write cycle.
6.4.5.1.1
Storing with Begin Trigger Alignment
Storing with begin alignment, data is not stored in the Trace Buffer until the Final State is entered. Once
the trigger condition is met the DBG module remains armed until 64 lines are stored in the Trace Buffer.
If the trigger is at the address of the change-of-flow instruction the change of flow associated with the
trigger is stored in the Trace Buffer. Using begin alignment together with tagging, if the tagged instruction
is about to be executed then the trace is started. Upon completion of the tracing session the breakpoint is
generated, thus the breakpoint does not occur at the tagged instruction boundary.
6.4.5.1.2
Storing with End Trigger Alignment
Storing with end alignment, data is stored in the Trace Buffer until the Final State is entered, at which point
the DBG module becomes disarmed and no more data is stored. If the trigger is at the address of a change
of flow instruction, the trigger event is not stored in the Trace Buffer. If all trace buffer lines have been
used before a trigger event occurrs then the trace continues at the first line, overwriting the oldest entries.
6.4.5.2
Trace Modes
Four trace modes are available. The mode is selected using the TRCMOD bits in the DBGTCR register.
Tracing is enabled using the TSOURCE bit in the DBGTCR register. The modes are described in the
following subsections.
S12P-Family Reference Manual, Rev. 1.13
Freescale Semiconductor
181
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