SC1100 Datasheet, PDF (84/348 Pages) National Semiconductor (TI) – Geode™ Information Appliance On a Chip

SuperI/O Module (Continued)
4.5.3.4 System Power States
The system power state may be No Power, Power On,
Power Off, or Power Failure. Table 4-16 indicates the
power-source combinations for each state. No other power-
source combinations are valid.
In addition, the power sources and distribution for the entire
system are illustrated in Figure 4-8 on page 83.
Table 4-16. System Power States
V_DIGITAL
V_SB
V_{BAT Power State}
−
−
−
No Power
−
−
+
Power Failure
−
+
+ or - Power Off
+
+
+ or - Power On
No Power
This state exists when no external or battery power is con-
nected to the device. This condition does not occur once a
backup battery has been connected, except in the case of
a malfunction.
Power On
This is the normal state when the system is active. This
state may be initiated by various events in addition to the
normal physical switching on of the system. In this state,
the system power supply is powered by external AC power
and produces V_DIGITAL and V_SB. The system and the part
are powered by V_DIGITAL, with the exception of the RTC log-
ical device, which is powered by V_SB.
Power Off (Suspended)
This is the normal state when the system has been
switched off and is not required to be active, but is still con-
nected to a live external AC input power source. This state
may be initiated directly or by software. The system is pow-
ered down. The RTC logical device remains active, pow-
ered by V_SB.
Power Failure
This state occurs when the external power source to the
system stops supplying power, due to disconnection or
power failure on the external AC input power source. The
RTC continues to maintain timekeeping and RAM data
under battery power (V_BAT), unless the oscillator stop bit
was set in the RTC. In this case, the oscillator stops func-
tioning if the system goes to battery power, and timekeep-
ing data becomes invalid.
System Bus Lockout
During power on or power off, spurious bus transactions
from the host may occur. To protect the RTC internal regis-
ters from corruption, all inputs are automatically locked out.
The lockout condition is asserted when V_SB is lower than
V_SBON.
Power-Up Detection
When system power is restored after a power failure or
power off state (V_SB = 0), the lockout condition continues
for a delay of 62 msec (minimum) to 125 msec (maximum)
after the RTC switches from battery to system power.
The lockout condition is switched off immediately in the fol-
lowing situations:
• If the Divider Chain Control bits, DV[2:0], (CRA bits [6:4])
specify a normal operation mode (01x or 100), all input
signals are enabled immediately upon detection of
system voltage above V_SBON.
• When battery voltage is below V_BATDCT and HMR is 1,
all input signals are enabled immediately upon detection
of system voltage above V_SBON. This also initializes
registers at offsets 00h through 0Dh.
• If bit 7 (VRT) of CRD is 0, all input signals are enabled
immediately upon detection of system voltage above
V_SBON.
4.5.3.5 Oscillator Activity
The RTC oscillator is active if:
• V_SB power supply is higher than V_SBON, independent of
the battery voltage, V_BAT.
-or-
• V_BAT power supply is higher than V_BATMIN, regardless if
V_SB is present or not.
The RTC oscillator is disabled if:
• During power-down (V_BAT only), the battery voltage
drops below V_BATMIN. When this occurs, the oscillator
may be disabled and its functionality cannot be guaran-
teed.
-or-
• Software wrote 00x to DV[2:0] bits of the CRA register
and V_SB is removed. This disables the oscillator and
decreases the power consumption from the battery
connected to V_BAT. When disabling the oscillator, the
CMOS RAM is not affected as long as the battery is
present at a correct voltage level.
If the RTC oscillator becomes inactive, the following fea-
tures are dysfunctional/disabled:
• Timekeeping
• Periodic interrupt
• Alarm
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