S12S Debug Module (S12SDBGV2)
MC9S12G Family Reference Manual Rev.1.27
NXP Semiconductors 337
8.4.5.5 Reading Data from Trace Buffer
The data stored in the Trace Buffer can be read provided the DBG module is not armed, is configured for
tracing (TSOURCE bit is set) and the system not secured. When the ARM bit is written to 1 the trace buffer
is locked to prevent reading. The trace buffer can only be unlocked for reading by a single aligned word
write to DBGTB when the module is disarmed.
The Trace Buffer can only be read through the DBGTB register using aligned word reads, any byte or
misaligned reads return 0 and do not cause the trace buffer pointer to increment to the next trace buffer
address. The Trace Buffer data is read out first-in first-out. By reading CNT in DBGCNT the number of
valid lines can be determined. DBGCNT does not decrement as data is read.
Whilst reading an internal pointer is used to determine the next line to be read. After a tracing session, the
pointer points to the oldest data entry, thus if no rollover has occurred, the pointer points to line0, otherwise
it points to the line with the oldest entry. In compressed Pure PC mode on rollover the line with the oldest
data entry may also contain newer data entries in fields 0 and 1. Thus if rollover is indicated by the TBF
bit, the line status must be decoded using the INF bits in field3 of that line. If both INF bits are clear then
the line contains only entries from before the last rollover.
If INF0=1 then field 0 contains post rollover data but fields 1 and 2 contain pre rollover data.
If INF1=1 then fields 0 and 1 contain post rollover data but field 2 contains pre rollover data.
The pointer is initialized by each aligned write to DBGTBH to point to the oldest data again. This enables
an interrupted trace buffer read sequence to be easily restarted from the oldest data entry.
The least significant word of line is read out first. This corresponds to the fields 1 and 0 of Table 8-37. The
next word read returns field 2 in the least significant bits [3:0] and “0” for bits [15:4].
Reading the Trace Buffer while the DBG module is armed returns invalid data and no shifting of the RAM
pointer occurs.
8.4.5.6 Trace Buffer Reset State
The Trace Buffer contents and DBGCNT bits are not initialized by a system reset. Thus should a system
reset occur, the trace session information from immediately before the reset occurred can be read out and
the number of valid lines in the trace buffer is indicated by DBGCNT. The internal pointer to the current
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.
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