
88
Agere Systems Inc.
Data Sheet
May 2001
and Packet Payload Engine
Ambassador T8110 PCI-Based H.100/H.110 Switch
7 Clock Architecture
(continued)
7.7.2 Clock Failsafe
Clock failsafe provides a safety net for the APLL1 reference clock source and is controlled by three registers,
0x00114—0x00116; see Section 6.1.11 on page 54. A failsafe event overrides the active clock control registers and
forces the APLL1 clock selection to be a fixed 4.096 MHz, derived from the XTAL1 crystal, divided by four. Transi-
tion into one of the failsafe states is independent of clock fallback (i.e., can enter from any state other than INI-
TIAL). Transitions out of the failsafe states are by user command and allow re-entry into either a nonfallback
(primary or secondary) or a fallback (TO_SECONDARY or TO_PRIMARY) state. Refer to Table 66 and Figure 25.
7.7.2.1 Failsafe Events
Clock failsafe (transition from either clock register set to a forced XTAL1-div-4 APLL1 reference clock) can only
occur if the failsafe mode is enabled (register 0x00115, lower nibble), and a failsafe event occurs. A failsafe event
is triggered by a watchdog error on the APLL1 reference clock (i.e., loss-of-reference).
Additionally, an out-of-lock (OOL) condition is provided for debug purposes. This does not trigger a failsafe event,
but does indicate potential difficulty with the APLL1. A lock status flag is provided out of APLL1, and the OOL is
defined by exceeding a user-defined threshold value (register 0x00116). The lock status is a flag indicating when
APLL1 is making a correction to maintain synchronization. The flag is continuously sampled. If enough active flags
are sampled in a row to exceed the user-defined threshold, this condition is reported via the system status register
(0x00125).
5-9421 (F)
Figure 25. T8110 Clock Failsafe States
FS_2
FAILSAFE RETURN TO
FAILSAFE ENABLED AND
FAILSAFE ENABLED AND
FAILSAFE RETURN TO
FATYPE
A(X ITO_PRIMARY
(Y ISSECONDARY
A(Y TO_SECONDARY
(Y IS TINITIAL
RESET
USER COMMAND
FALLBACK ENABLED AND
FALLBACK ENABLED AND
USER COMMAND
ROTATING
FATYPE
SEMODE
USER COMMAND
RMODE
SEMODE
FS_1
FAILSAFE RETURN TO
FAILSAFE ENABLED AND
FAILSAFE ENABLED AND
FAILSAFE RETURN TO
(X IS TPRIMARY