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Timer Facilities 6-1
Chapter 6. Timer Facilities
The PPC405 provides four timer facilities: a time base, a Programmable Interval Timer (PIT), a fixed
interval timer (FIT), and a watchdog timer. The PIT is a Special Purpose Register (SPR). These
facilities, which are driven by the same base clock, can, among other things, be used for:
• Time-of-day functions
• Data logging functions
• Peripherals requiring periodic service
• Periodic task switching
Additionally, the watchdog timer can help a system to recover from faulty hardware or software.
Figure 6-1 shows the relationship of the timers and the clock source to the time base.
6.1 Time Base
The PPC405 implements a 64-bit time base as required in
The PowerPC Architecture
. The time base,
which increments once during each period of the source clock, provides a time reference. Read
access to the time base is through the mftb instruction. mftb provides user-mode read-only access to
TBU (32 bits)
Bit 3 (2
29
clocks)
Bit 7 (2
25
clocks)
Bit 11 (2
21
clocks)
Bit 15 (2
17
clocks)
Bit 11 (2
21
clocks)
Bit 15 (2
17
clocks)
Bit 19 (2
13
clocks)
Bit 23 (2
9
clocks)
Watchdog Timer Events
FIT Events
Time Base (Incrementer)
31
TBL (32 bits)
31 0
0
PIT (Decrementer)
(32 bits)
31
0
Zero Detect PIT Events
Figure 6-1. Relationship of Timer Facilities to the Time Base
External
Clock
Source