
MOTOROLA
8-36
DEVELOPMENT SUPPORT
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RCPU
REFERENCE MANUAL
When the ready bit is used to pace the transmissions, the error status is reported
during 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 han-
dler in the external development tool does not need to undo the effects of an inter-
vening 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. Trans-
missions 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
transmission that caused the error. (When the ready bit is used, the status is re-
ported in the following transmission.) This is because an error condition which oc-
curs 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 sta-
tus 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 when 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 ad-
dition, 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) connected to the development port.
8.4.1 Enabling Debug Mode
Debug mode is enabled by asserting the DSCK pin during reset. The state of this
pin is sampled immediately before the negation of RESETOUT. If the DSCK pin is
sampled low, debug mode is disabled until a subsequent reset when the DSCK pin
is sampled high. When debug mode is disabled, the internal watchpoint/breakpoint
hardware is still operational and can be used by a software monitor program for de-
bugging purposes.
The DSCK pin is sampled again eight clock cycles following the negation of
RESETOUT. If DSCK is negated following reset, the processor jumps to the reset
vector and begins normal execution. If DSCK is asserted following reset and debug
mode is enabled, the processor enters debug mode before executing any instruc-
tions.
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