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User’s Manual
Preliminary PPC440x5 CPU Core
timers.fm.
September 12, 2002
Page 209 of 589
7. Timer Facilities
The PPC440x5 provides four timer facilities: a time base, a Decrementer (DEC), a Fixed Interval Timer (FIT),
and a Watchdog Timer. These facilities, which share the same source clock frequency, can support:
• Time-of-day functions
• General software timing functions
• Peripherals requiring periodic service
• General system maintenance
• System error recoverability
Figure 7-1 shows the relationship between these facilities and the clock source.
7.1 Time Base
The time base is a 64-bit register which increments once during each period of the source clock, and provides
a time reference. Access to the time base is via two Special Purpose Registers (SPRs). The Time Base
Upper (TBU) SPR contains the high-order 32 bits of the time base, while the Time Base Lower (TBL) SPR
contains the low-order 32 bits.
TBL (32 bits)
TBU
31
(2
33
clocks)
TBL
3
(2
29
clocks)
TBL
7
(2
25
clocks)
TBL
11
(2
21
clocks)
TBL
7
(2
25
clocks)
TBL
11
(2
21
clocks)
TBL
15
(2
17
clocks)
TBL
19
(2
13
clocks)
Watchdog Timer
Fixed Interval Timer
Time Base (Incrementer)
31
TBU (32 bits)
31
0
0
DEC (Decrementer)
(32 bits)
31
0
Zero Detect Decrementer exception
Figure 7-1. Relationship of Timer Facilities to the Time Base
Reference
Clock
Source
Period
Period