PowerPC e500 Core Family Reference Manual, Rev. 1
Freescale Semiconductor 9-1
Chapter 9
Timer Facilities
This chapter describes specific implementation details of the e500v1 and e500v2 implementations
of the Book E–defined timer facilities. These resources, which include the time base (TB),
decrementer (DEC), fixed-interval timer (FIT), and watchdog timer, are described in detail in the
EREF: A Reference for Freescale Book E and the e500 Core.
Section 9.3.2, “Performance Monitor Time Base Event,” describes the time base event
implemented by the e500v2 performance monitor.
9.1 Timer Facilities
The TB, DEC, FIT, and watchdog timer provide timing functions for the system. All of these must
be initialized during start-up.
• The TB provides a long-period counter driven by a frequency that is implementation
dependent.
• The decrementer, a counter that is updated at the same rate as the TB, provides a means of
signaling an exception after a specified amount of time has elapsed unless one of the
following occurs:
— DEC is altered by software in the interim.
— The TB update frequency changes.
The DEC is typically used as a general-purpose software timer.
• The clock source for the TB and the DEC is specified by two fields in HID0: time base
enable (TBEN), and select time base clock (SEL_TBCLK). If the TB is enabled
(HID0[TBEN] = 1) the clock source is determined as follows:
— If [SEL_TBCLK] = 0, the TB is updated every 8 core complex bus (CCB) clocks.
— If HID0[SEL_TBCLK] = 1, the time base is updated on the rising edge of tbclk (or a
clock input specified by the implementation). The exact frequency range is specified in
the hardware specification for the integrated device, but the maximum value should not
exceed 1/ 8
th
the core frequency.
See Section 2.10.1, “Hardware Implementation-Dependent Register 0 (HID0).”
• The fixed-interval timer is essentially a selected bit of the TB, which provides a means of
signaling an exception whenever the selected bit transitions from 0 to 1, in a repetitive
fashion. The fixed-interval timer is typically used to trigger periodic system maintenance
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