T-8110L AGERE [Agere Systems], T-8110L Datasheet - Page 64

• Manufacturer Part Number: T-8110L
• Manufacturer: AGERE [Agere Systems]

Ambassador T8110L H.100/H.110 Switch
6 Clock Architecture
6.7 Clock Circuit Operation—Fallback and Failsafe
Fallback is a means to alter the reference source to APLL1 by switching between two clock control register sets
upon detection of a fallback event. Failsafe is a feature to provide a safety net for the reference source to APLL1,
independent of clock fallback.
6.7.1 Clock Fallback
Clock fallback is a means to alter the APLL1 reference clock source upon detection of a fallback event and is con-
trolled by eight registers, 0x00108—0x0010F (refer to Section 5.1.4 on page 33). These registers enable and con-
trol the state transitions that determine which of two clock register sets is used to control the APLL1 reference clock
source (see Section 6.1 on page 45 through Section 6.3, Table 54 on page 67, and Figure 10 on page 66).
6.7.1.1 Fallback Events
Clock fallback (transition from primary to secondary clock sets) can only occur if the fallback mode is enabled (reg-
ister 0x00109, lower nibble) and a fallback event occurs. When enabled, there are three ways to trigger the fallback
event:
!
!
!
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Software, via a FORCE_FALLBACK command. The user sets bit 2 of the fallback control register, 0x00108, cre-
ating a software-invoked fallback event.
Hardware via the fallback trigger enable registers, 0x0010A—0x0010B. User may enable specific watchdog tim-
ers and corresponding fallback trigger enable bits. If a watchdog timer indicates a clock error, and its correspond-
ing trigger enable bit is set, a hardware-invoked fallback event is produced.
Hardware, legacy modes, via the fallback type select register, 0x00109, upper nibble. The legacy modes are
included to maintain backwards compatibility with earlier Ambassador devices. User may enable specific watch-
dog timers, but the fallback trigger enable registers are ignored. Instead, the watchdogs which are allowed to
trigger a fallback event are automatically selected based on the state of the main input selector register, 0x00200
(refer to Table 53). If a watchdog timer indicates a clock error, and its corresponding trigger enable is selected via
the main input selector, a hardware-invoked fallback event is produced.
(continued)
Agere Systems Inc.
February 2004