MC68336/376
SYSTEM INTEGRATION MODULE
MOTOROLA
USER’S MANUAL
Rev. 15 Oct 2000
5-30
Figure 5-12 CPU Space Address Encoding
5.6.4.1 Breakpoint Acknowledge Cycle
Breakpoints stop program execution at a predefined point during system development.
Breakpoints can be used alone or in conjunction with background debug mode. In
M68300 microcontrollers, both hardware and software can initiate breakpoints.
The CPU32 BKPT instruction allows the user to insert breakpoints through software.
The CPU responds to this instruction by initiating a breakpoint acknowledge read cycle
in CPU space. It places the breakpoint acknowledge (%0000) code on ADDR[19:16],
the breakpoint number (bits [2:0] of the BKPT opcode) on ADDR[4:2], and %0 (indicat-
ing a software breakpoint) on ADDR1.
External breakpoint circuitry decodes the function code and address lines and
responds by either asserting BERR or placing an instruction word on the data bus and
asserting DSACK. If the bus cycle is terminated by DSACK, the CPU32 reads the
instruction on the data bus and inserts the instruction into the pipeline. (For 8-bit ports,
this instruction fetch may require two read cycles.)
If the bus cycle is terminated by BERR, the CPU32 then performs illegal instruction
exception processing: it acquires the number of the illegal instruction exception vector,
computes the vector address from this number, loads the content of the vector address
into the PC, and jumps to the exception handler routine at that address.
Assertion of the BKPT input initiates a hardware breakpoint. The CPU32 responds by
initiating a breakpoint acknowledge read cycle in CPU space. It places the breakpoint
acknowledge code of %0000 on ADDR[19:16], the breakpoint number value of %111
on ADDR[4:2], and ADDR1 is set to %1, indicating a hardware breakpoint.
External breakpoint circuitry decodes the function code and address lines, places an
instruction word on the data bus, and asserts BERR. The CPU32 then performs hard-
CPU SPACE CYC TIM
0000000000000000000
T0
BKPT#
19
23
16
000000111111111111111110
19
16
23
111
11111111111111111111
1
111
LEVEL
19
16
23
CPU SPACE CYCLES
FUNCTION
CODE
20
0
CPU SPACE
TYPE FIELD
ADDRESS BUS
BREAKPOINT
ACKNOWLEDGE
LOW POWER
STOP BROADCAST
INTERRUPT
ACKNOWLEDGE
2
41