S12X Debug (S12XDBGV3) Module
MC9S12XHY-Family Reference Manual, Rev. 1.01
246
Freescale Semiconductor
NOTE
Using this configuration, a byte access of ADDR[n] can cause a comparator match if the databus low byte
by chance contains the same value as ADDR[n+1] because the databus comparator does not feature access
size comparison and uses the mask as a “don’t care” function. Thus masked bits do not prevent a match.
Comparators A and C feature an NDB control bit to determine if a match occurs when the data bus differs
to comparator register contents or when the data bus is equivalent to the comparator register contents.
6.4.2.2
Exact Address Comparator Match (Comparators B and D)
Comparators B and D feature SZ and SZE control bits. If SZE is clear, then the comparator address match
qualification functions the same as for comparators A and C.
If the SZE bit is set the access size (word or byte) is compared with the SZ bit value such that only the
specified type of access causes a match. Thus if configured for a byte access of a particular address, a word
access covering the same address does not lead to match.
6.4.2.3
Data Bus Comparison NDB Dependency
Comparators A and C each feature an NDB control bit, which allows data bus comparators to be configured
to either trigger on equivalence or trigger on difference. This allows monitoring of a difference in the
contents of an address location from an expected value.
When matching on an equivalence (NDB=0), each individual data bus bit position can be masked out by
clearing the corresponding mask bit (DBGxDHM/DBGxDLM), so that it is ignored in the comparison. A
match occurs when all data bus bits with corresponding mask bits set are equivalent. If all mask register
bits are clear, then a match is based on the address bus only, the data bus is ignored.
When matching on a difference, mask bits can be cleared to ignore bit positions. A match occurs when any
data bus bit with corresponding mask bit set is different. Clearing all mask bits, causes all bits to be ignored
and prevents a match because no difference can be detected. In this case address bus equivalence does not
cause a match.
Table 6-37. Comparator Access Size Considerations
Comparator
Address
SZE
SZ8
Condition For Valid Match
Comparators
A and C
ADDR[n]
—
—
Word and byte accesses of ADDR[n]
(1)
MOVB #$BYTE ADDR[n]
MOVW #$WORD ADDR[n]
1. A word access of ADDR[n-1] also accesses ADDR[n] but does not generate a match.
The comparator address register must contain the exact address used in the code.
Comparators
B and D
ADDR[n]
0
X
Word and byte accesses of ADDR[n]
1
MOVB #$BYTE ADDR[n]
MOVW #$WORD ADDR[n]
Comparators
B and D
ADDR[n]
1
0
Word accesses of ADDR[n]
1
MOVW #$WORD ADDR[n]
Comparators
B and D
ADDR[n]
1
1
Byte accesses of ADDR[n]
MOVB #$BYTE ADDR[n]
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