MC68336/376
SYSTEM INTEGRATION MODULE
MOTOROLA
USER’S MANUAL
Rev. 15 Oct 2000
5-3
ity. 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 initial-
ization. Refer to 5.8 Interrupts for a discussion of interrupt arbitration.
5.2.3 Show Internal Cycles
A show cycle allows internal bus transfers to be monitored externally. The SHEN field
in SIMCR determines what the external bus interface does during internal transfer
operations. Table 5-1 shows whether data is driven externally, and whether external
5.2.4 Register Access
The CPU32 can operate at one of two privilege levels. Supervisor level is more privi-
leged than user level — all instructions and system resources are available at
supervisor 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.
5.2.5 Freeze Operation
The FREEZE signal halts MCU operations during debugging. FREEZE is asserted
internally by the CPU32 if a breakpoint occurs while background mode is enabled.
When FREEZE is asserted, only the bus monitor, software watchdog, and periodic
interrupt timer are affected. The halt monitor and spurious interrupt monitor continue
to operate normally. Setting the freeze bus monitor (FRZBM) bit in SIMCR disables the
bus monitor when FREEZE is asserted. Setting the freeze software watchdog
(FRZSW) bit disables the software watchdog and the periodic interrupt timer when
FREEZE is asserted.
5.3 System Clock
The system clock in the SIM provides timing signals for the IMB modules and for an
external peripheral bus. Because the MCU is a fully static design, register and memory
contents are not affected when the clock rate changes. System hardware and software
support changes in clock rate during operation.
Table 5-1 Show Cycle Enable Bits
SHEN[1:0]
Action
00
Show cycles disabled, external arbitration enabled
01
Show cycles enabled, external arbitration disabled
10
Show cycles enabled, external arbitration enabled
11
Show cycles enabled, external arbitration enabled;
internal activity is halted by a bus grant