Typewriter User Manual

4- 6 MC68340 USER’S MANUAL MOTOROLA
There are eight arbitration levels for access to the intermodule bus (IMB). The SIM40 is
fixed at the highest level (above the programmable level 7), and the CPU32 is fixed at the
lowest level (below level 0). The direct memory access (DMA) module is the only other
module that can become bus master and arbitrate for the bus. It must be initialized with a
level other than 0 or 7.
The AVR contains bits that correspond to external interrupt levels that require an
autovector response. The SIM40 supports up to seven discrete external interrupt
requests. If the bit corresponding to an interrupt level is set in the AVR, the SIM40 returns
an autovector in response to the IACK cycle servicing that external interrupt request.
Otherwise, external circuitry must either return an interrupt vector or assert the external
AVEC signal.
4.2.2.2 INTERNAL BUS MONITOR. The internal bus monitor continually checks for the
bus cycle termination response time by checking the
DSACK≈, BERR, and HALT status or
the
AVEC status during an IACK cycle. The monitor initiates a bus error if the response
time is excessive. The bus monitor feature cannot be disabled for internal accesses to an
internal module. The internal bus monitor cannot check the
DSACK≈ response on the
external bus unless the MC68340 is the bus master. The BME bit in the system protection
control register (SYPCR) enables the internal bus monitor for internal-to-external bus
cycles. If the system contains external bus masters whose bus cycles must be monitored,
an external bus monitor must be implemented. In this case, the internal-to-external bus
monitor option must be disabled.
The bus cycle termination response time is measured in clock cycles, and the maximum-
allowable response time is programmable. The bus monitor response time period ranges
from 8 to 64 system clocks (see Table 4-8). These options are provided to allow for
different response times of peripherals that might be used in the system.
4.2.2.3 DOUBLE BUS FAULT MONITOR. A double bus fault is caused by a bus error or
address error during the exception processing sequence. The double bus fault monitor
responds to an assertion of
HALT on the internal bus. Refer to Section 3 Bus Operation
for more information. The DBF bit in the reset status register (RSR) indicates that the last
reset was caused by the double bus fault monitor. The double bus fault monitor reset can
be enabled by the DBFE bit in the SYPCR.
4.2.2.4 SPURIOUS INTERRUPT MONITOR. The spurious interrupt monitor issues
BERR
if no interrupt arbitration occurs during an IACK cycle. Normally, during an IACK cycle,
one or more internal modules recognize that the CPU32 is responding to interrupt
request(s) and arbitrate for the privilege of returning a vector or asserting
AVEC. (The
SIM40 reports and arbitrates for externally generated interrupts.) This feature cannot be
disabled.
4.2.2.5 SOFTWARE WATCHDOG. The SIM40 provides a software watchdog option to
prevent system lock-up in case the software becomes trapped in loops with no controlled
exit. Once enabled by the SWE bit in the SYPCR, the software watchdog requires a
special service sequence to be executed on a periodic basis. If this periodic servicing
action does not occur, the software watchdog times out and issues a reset or a level 7
Frees
cale Semiconductor,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
nc...