Because the SIM routes external interrupt requests to the CPU32, the SIM IARB field
value is used for arbitration between internal and external interrupts of the same pri-
ority. The reset value of IARB for the SIM is %1111, and the reset IARB value for all
other modules is %0000, which prevents SIM interrupts from being discarded during
initialization. Refer to 4.7 Interrupts for a discussion of interrupt arbitration.
4.2.3 Show Internal Cycles
A show cycle allows internal bus transfers to be monitored externally. The SHEN field
in the SIMCR determines what the external bus interface does during internal transfer
operations. Table 4-1 shows whether data is driven externally, and whether external
bus arbitration can occur. Refer to 4.5.6.2 Show Cycles for more information.
Table 4-1 Show Cycle Enable Bits
SHEN
00
Action
Show cycles disabled, external arbitration enabled
Show cycles enabled, external arbitration disabled
Show cycles enabled, external arbitration enabled
01
10
11
Show cycles enabled, external arbitration enabled;
internal activity is halted by a bus grant
4
4.2.4 Factory Test Mode
The internal IMB can serve as slave to an external master for direct module testing.
This test mode is reserved for factory test. Slave mode is enabled by holding DATA11
low during reset. The slave enabled (SLVEN) bit is a read-only bit that shows the reset
state of DATA11.
4.2.5 Register Access
The CPU32 can operate at either of two privilege levels. Supervisor level is more priv-
ileged than user level — all instructions and system resources are available at super-
visor level, but access is restricted at user level. Effective use of privilege level can
protect system resources from uncontrolled access. The state of the S bit in the CPU
status register determines access level, and whether the user or supervisor stack
pointer is used for stacking operations. The SUPV bit places SIM global registers in
either supervisor or user data space. When SUPV = 0, registers with controlled access
are accessible from either the user or supervisor privilege level; when SUPV = 1, reg-
isters with controlled access are restricted to supervisor access only.
4.2.6 Reset Status
The reset status register (RSR) latches internal MCU status during reset. Refer to
4.6.9 Reset Status Register for more information.
4.2.7 Bus Monitor
The internal bus monitor checks data and size acknowledge (DSACK) or autovector
(AVEC) signal response times during normal bus cycles. The monitor asserts the in-
ternal bus error (BERR) signal when the response time is excessively long.
MOTOROLA
4-4
SYSTEM INTEGRATION MODULE
MC68331
USER’S MANUAL
MOTOROLA [ MOTOROLA ]
