
MOTOROLA
8-34
DEVELOPMENT SUPPORT
Rev. 15 June 98
MPC509
USER’S MANUAL
tion status is shifting out. The port terminates the current CPU access with a bus error.
The next transmission to the port should be a new instruction or trap enable data.
8.3.8.4 Null Output
Finally, the nullencoding is used to indicate that no data has been transferred from the
CPU to the development port shift register. It also indicates that the previous transmis-
sion did not have any associated errors.
8.3.9 Use of the Ready Bit
To minimize the overhead required to detect and correct errors, the external develop-
ment system should wait for the ready bit on DSDO before beginning each input
transmission. This ensures that all CPU activity (if any) relative to the previous trans-
mission has been completed and that any errors have been reported.
When the ready bit is used to pace the transmissions, the error status is reported dur-
ing the transmission following the error. Since any transmission into the port which
occurs while shifting out an error status is ignored by the port, the error handler in the
external development tool does not need to undo the effects of an intervening
instruction.
To improve system performance, however, an external development system may
begin transmissions before the ready bit is asserted. If the next transmission does not
wait until the port indicates ready, the port will not assert ready again until this next
transmission completes and all activity associated with it has finished. Transmissions
that begin before ready is asserted on DSDI are subject to the following limitations and
problems.
First, if the previous transmission results in a sequence error, or the CPU reports an
exception, that status may not be reported until two transmissions after the transmis-
sion that caused the error. (When the ready bit is used, the status is reported in the
following transmission.) This is because an error condition which occurs after the start
of a transmission cannot be reported until the next transmission.
Second, if a transmitted instruction causes the CPU to write to the DPDR and the
transmission that follows does not wait for the assertion of ready, the CPU data may
not be latched into the development port shift register, and the valid data status is not
output. Despite this, no error is indicated in the status outputs. To ensure that the CPU
has had enough time to write to the DPDR, there must be at least four CLKOUT cycles
between the time the last bit of the instruction (move to SPR) is clocked into the port
and the time the start bit for the next transmission is clocked into the port.
8.4 Debug Mode Functions
In debug mode, the CPU fetches all instructions from the development port. In addi-
tion, data can be read from and written to the development port. This allows memory
and registers to be read and modified by an external development tool (emulator) con-
nected to the development port.