
CPU16
REFERENCE MANUAL
DEVELOPMENT SUPPORT
MOTOROLA
10-17
Breakpoint requests are made by asserting BKPT in either of two ways. The preferred
method is to assert BKPT during the bus cycle for which an exception is desired. The
second method is to assert BKPT until the CPU16 responds by asserting FREEZE.
This method is useful for forcing a transition into BDM when the bus is not being mon-
itored. Both methods require logic that precludes spurious serial clocks.
Figure 10-12
shows timing for BKPT assertion during a single bus cycle.
Figure 10-
13
shows BKPT/FREEZE timing. In both cases, the serial clock output is left high after
the final shift of each transfer. This prevents tagging the prefetch initiated when BDM
terminates.
Figure 10-12 BKPT Timing for Single Bus Cycle
Figure 10-13 BKPT Timing for Forcing BDM
Figure 10-14
shows a sample circuit that accommodates either method of BKPT as-
sertion. FORCE_BGND can either be pulsed or remain asserted until FREEZE is as-
serted. Once FORCE_BGND is asserted, the set-reset latch holds BKPT low until the
first SHIFT_CLK is applied.
FREEZE
BKPT
BKPT_TAG
FORCE_BGND
SHIFT_CLK
SHIFT_CLK
FORCE_BGND
BKPT_TAG
BKPT
FREEZE
F
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