IBM PPC440X5 - page 209

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
Thetime 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)
TBU31 (233 clocks)
TBL3 (229 clocks)
TBL7 (225 clocks)
TBL11 (221 clocks)
TBL7 (225 clocks)
TBL11 (221 clocks)
TBL15 (217 clocks)
TBL19 (213 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