M68HC16 Z SERIES
SYSTEM INTEGRATION MODULE
USER’S MANUAL
5-45
NOTE
The external bus interface does not latch data when an external bus
cycle is terminated by a bus error. When this occurs during an in-
struction prefetch, the IMB precharge state (bus pulled high, or $FF)
is latched into the CPU16 instruction register, with indeterminate re-
sults.
5.6.5.2 Double Bus Faults
Exception processing for bus error exceptions follows the standard exception process-
ing sequence. Refer to
for more information. However, two special
cases of bus error, called double bus faults, can abort exception processing.
BERR assertion is not detected until an instruction is complete. The BERR latch is
cleared by the first instruction of the BERR exception handler. Double bus fault occurs
in two ways:
1. When bus error exception processing begins, and a second BERR is detected
before the first instruction of the exception handler is executed.
2. When one or more bus errors occur before the first instruction after a RESET
exception is executed.
Multiple bus errors within a single instruction that can generate multiple bus cycles
cause a single bus error exception after the instruction has been executed.
Immediately after assertion of a second BERR, the MCU halts and drives the HALT
line low. Only a reset can restart a halted MCU. However, bus arbitration can still oc-
cur. Refer to
5.6.6 External Bus Arbitration
for more information. A bus error or ad-
dress error that occurs after exception processing has been completed (during the
execution of the exception handler routine, or later) does not cause a double bus fault.
The MCU continues to retry the same bus cycle as long as the external hardware re-
quests it.
5.6.5.3 Halt Operation
When HALT is asserted while BERR is not asserted, the MCU halts external bus ac-
tivity after negation of DSACK. The MCU may complete the current word transfer in
progress. For a long-word to byte transfer, this could be after S2 or S4. For a word to
byte transfer, activity ceases after S2.
Negating and reasserting HALT according to timing requirements provides single-step
(bus cycle to bus cycle) operation. The HALT signal affects external bus cycles only,
so that a program that does not use external bus can continue executing. During dy-
namically-sized 8-bit transfers, external bus activity may not stop at the next cycle
boundary. Occurrence of a bus error while HALT is asserted causes the CPU16 to pro-
cess a bus error exception.
When the MCU completes a bus cycle while the HALT signal is asserted, the data bus
goes into a high-impedance state and the AS and DS signals are driven to their inac-
tive states. Address, function code, size, and read/write signals remain in the same
state.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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