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¥ PowerPC decrementerÑProvides a decrementer register/interrupt clocked at the
timebase frequency. This 32-bit decrementing counter is deÞned to be clocked by
TMBCLK. When it is driven by a 4-MHz oscillator the period for the decrementer
is 4,295 seconds (approximately 71.6 minutes).
¥ Real-time clock (RTC)ÑProvides time-of-day information to the operating system
or application software. It is composed of a 45-bit counter and an alarm register. A
maskable interrupt is generated when the counter reaches the value programmed in
the alarm register. The RTC is clocked by PITRTCLK.
¥ Freeze supportÑThe SIU determines whether the software watchdog timer, PIT,
timebase, decrementer, and real-time clock should continue to run in freeze mode.
Figure 11-1 is a block diagram of the system conÞguration and protection logic.
Figure 11-1. System Configuration and Protection Logic
11.3 Multiplexing SIU Pins
Due to the limited number of pins available in the MPC860 package, some of the
functionalities share pins. Table 11-1 shows how the functionality is controlled on each pin.
MOTOROLA
Module
ConÞguration
Bus
Monitor
Periodic Interrupt
Software
Watchdog Timer
PowerPC
Clock
Decrementer
PowerPC
Timebase Counter
Real-Time
Chapter 11. System Interface Unit
TEA
Interrupt
Timer
Interrupt or
System Reset
Interrupt
Interrupt
Interrupt
Clock
Part III. Configuration
11-3
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