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Development Capabilities and Interface
20-38
MPC823 USER’S MANUAL
MOTOROLA
DEVELOPMENT
20
CAPABILITIES
&
INTERFACE
All transmissions from the debug port on DSDO begin with a zero or ready bit. This indicates
that the core is trying to read an instruction or data from the port. The external development
tool waits until it sees DSDO go low before it starts sending the next transmission. The
control bit differentiates between instructions and data that allow the development interface
port to detect an instruction that was entered when the core was expecting data and vice
versa. If this occurs, a sequence error indication is shifted out in the next serial transmission.
The trap enable function allows the development interface port to transfer data to the trap
enable control register. The DEBUG PORT command allows the development tool to either
negate breakpoint requests, reset the processor, or activate or deactivate the fast download
procedure. The NOP function provides a null operation to use when there is data or a
response to be shifted out of the data register. The next appropriate instruction or command
will be determined by the value of the response or data shifted out.
The encoding of data shifted out of the development interface port shift register in debug
mode is the same as for trap enable mode, as shown in
Table 20-10. The valid data
encoding is used when data has been transferred from the core to the development interface
port shift register. This results when an instruction to move the contents of a
general-purpose register to the DPDR occurs. The valid data encoding has the highest
priority of all status outputs and will be reported even if an interrupt occurs at the same time.
Since it is not possible for a sequencing error to occur that has valid data, there is no priority
conflict with the sequencing error status. Also, any interrupt that is recognized at the same
time that there is valid data, is not related to the execution of an instruction. Therefore, a
valid data status will be output and the interrupt status will be saved for the next
transmission.
Table 20-11. Debug Instructions/Data Shifted Into the DPS Register
START
MODE
CONTROL
INSTRUCTION / DATA (32 BITS)
FUNCTION
BITS 0:6
BITS 7:31
1
0
CPU Instruction
Transfer Instruction to Core
1
0
1
CPU Data
Transfer Data to Core
1
0
Trap Enable
Bits
Not Exist
Transfer Data to Trap Enable
Control Register
1
0011111
Not Exist
Negate Breakpoint Requests
to Core
1
0
Not Exist
NOP
NOTE: See
Table 20-8 for details on trap enable bits and
Table 20-9 for details on debug port commands.