
MOTOROLA
8-34
DEVELOPMENT SUPPORT
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RCPU
REFERENCE MANUAL
8.3.8.1 Valid Data Output
The
valid data
encoding is used when data has been transferred from the CPU to
the development port shift register. This is the result of executing an instruction in
debug mode to move the contents of a general purpose register to the develop-
ment port data register (DPDR).
The valid data encoding has the highest priority of all status outputs and is reported
even if an exception occurs at the same time. Any exception that is recognized dur-
ing the transmission of valid data is not related to the execution of an instruction.
Therefore, a status of valid data is output and the CPU exception status is saved
for the next transmission. Since it is not possible for a sequencing error to occur
and for valid data to be received on the same transmission, there is no conflict be-
tween a valid data status and the sequencing error status.
8.3.8.2 Sequencing Error Output
The
sequencing error
encoding indicates that the inputs from the external develop-
ment tool are not what the development port or the CPU was expecting. Two cases
could cause this error:
1) the processor was trying to read instructions and data was shifted into the
development port, or
2) the processor was trying to read data and an instruction was shifted into the
development port.
When a sequencing error occurs, the port terminates the CPU read or fetch cycle
with a bus error. This bus error causes the CPU to signal the development port that
an exception occurred. Since a status of sequencing error has a higher priority than
a status of exception, the port reports the sequencing error. The development port
ignores the data being shifted in while the sequencing error is shifting out. The next
transmission to the port should be a new instruction or trap enable data.
Table 8-16
illustrates a typical sequence of events when a sequencing error oc-
curs. This example begins with CPU data being shifted into the shift register (con-
trol bit = 1) when the processor is expecting an instruction. During the next
Table 8-15 Status/Data Shifted Out of Shift Register
Ready
Status [0:1]
Data (7 or 32 Bits
1
)
NOTES:
1. Depending on input mode.
Indication
(0)
0
0
Data
Valid Data from CPU
(0)
0
1
Ones
Sequencing Error
(0)
1
0
Ones
CPU Exception
(0)
1
1
Ones
Null
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