M68HC16 Z SERIES
SYSTEM INTEGRATION MODULE
USER’S MANUAL
5-43
Figure 5-16 LPSTOP Interrupt Mask Level
5.6.5 Bus Exception Control Cycles
An external device or a chip-select circuit must assert at least one of the DSACK[1:0]
signals or the AVEC signal to terminate a bus cycle normally. Bus exception control
cycles are used when bus cycles are not terminated in the expected manner. There
are two sources of bus exception control cycles.
• Bus error signal (BERR)
— When neither DSACK nor AVEC is asserted within a specified period after as-
sertion of AS, the internal bus monitor asserts internal BERR.
— The spurious interrupt monitor asserts internal BERR when an interrupt re-
quest is acknowledged and no IARB contention occurs. BERR assertion termi-
nates a cycle and causes the MCU to process a bus error exception.
— External devices can assert BERR to indicate an external bus error.
• Halt signal (HALT)
— HALT can be asserted by an external device to cause single bus cycle opera-
tion. HALT is typically used for debugging purposes.
To control termination of a bus cycle for a bus error condition properly, DSACK, BERR,
and HALT must be asserted and negated synchronously with the rising edge of
CLKOUT. This ensures that setup time and hold time requirements are met for the
same falling edge of the MCU clock when two signals are asserted simultaneously.
Refer to
APPENDIX A ELECTRICAL CHARACTERISTICS
for more information. Ex-
ternal circuitry that provides these signals must be designed with these constraints in
mind, or the internal bus monitor must be used.
is a summary of the acceptable bus cycle terminations for asynchronous
cycles in relation to DSACK assertion.
LPSTOP MASK LEVEL
15
8
7
0
IP MASK
14
13
12
11
10
9
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
0
0
0
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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