MC9S12NE64_06 Datasheet, PDF (496/554 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 18 Debug Module (DBGV1)
18.4.2.6 Capture Modes
The DBG in DBG mode can operate in four capture modes. These modes are described in the following
subsections.
18.4.2.6.1 Normal Mode
In normal mode, the DBG module uses comparator A and B as triggering devices. Change-of-flow
information or data will be stored depending on TRG in DBGSC.
18.4.2.6.2 Loop1 Mode
The intent of loop1 mode is to prevent the trace buffer from being filled entirely with duplicate information
from a looping construct such as delays using the DBNE instruction or polling loops using
BRSET/BRCLR instructions. Immediately after address information is placed in the trace buffer, the DBG
module writes this value into the C comparator and the C comparator is placed in ignore address mode.
This will prevent duplicate address entries in the trace buffer resulting from repeated bit-conditional
branches. Comparator C will be cleared when the ARM bit is set in loop1 mode to prevent the previous
contents of the register from interfering with loop1 mode operation. Breakpoints based on comparator C
are disabled.
Loop1 mode only inhibits duplicate source address entries that would typically be stored in most tight
looping constructs. It will not inhibit repeated entries of destination addresses or vector addresses, because
repeated entries of these would most likely indicate a bug in the user’s code that the DBG module is
designed to help find.
NOTE
In certain very tight loops, the source address will have already been fetched
again before the C comparator is updated. This results in the source address
being stored twice before further duplicate entries are suppressed. This
condition occurs with branch-on-bit instructions when the branch is fetched
by the first P-cycle of the branch or with loop-construct instructions in
which the branch is fetched with the first or second P cycle. See examples
below:
LOOP INCX
; 1-byte instruction fetched by 1st P-cycle of BRCLR
BRCLR CMPTMP,#$0c,LOOP ; the BRCLR instruction also will be fetched by 1st P-cycle of BRCLR
LOOP2 BRN *
NOP
DBNE A,LOOP2
; 2-byte instruction fetched by 1st P-cycle of DBNE
; 1-byte instruction fetched by 2nd P-cycle of DBNE
; this instruction also fetched by 2nd P-cycle of DBNE
NOTE
Loop1 mode does not support paged memory, and inhibits duplicate entries
in the trace buffer based solely on the CPU address. There is a remote
possibility of an erroneous address match if program flow alternates
between paged and unpaged memory space.
MC9S12NE64 Data Sheet, Rev. 1.1
496
Freescale Semiconductor