
8.0 Real-Time Clock (RTC)
(Continued)
Revision 1.2
147
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Figure 44. Typical Battery Current During Battery-Backed Power Mode
Figure 45. Typical Battery Current During Normal Operation Mode
8.2.10
During power On or power Off, spurious bus transactions from the host may occur. To protect the data in the RTC internal
registers from being corrupted, all inputs are automatically locked out. The lockout condition is asserted when V
SB
is lower
than V
SBON
at V
SB
power-on or V
SBOFF
at V
SB
power-off.
System Bus Lockout
8.2.11
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 ms (minimum) to 125 ms (maximum) after the RTC switches from battery to system power.
The lockout condition is switched off immediately in the following situations:
If the Divider Chain Control bits, DV0-2 (bits 6-4 in the CRA register), specify a normal operation mode (010), all input
signals are enabled immediately on detection of system voltage above V
SBON
.
When battery voltage is below V
BATDCT
and LRESET is 0, all input signals are enabled immediately on detection of sys-
tem voltage above V
SBON
. This also initializes registers at offsets 00h through 0Dh.
If bit 7 (VRT) of the CRD register is 0, all input signals are enabled immediately on detection of system voltage above
V
SBON
.
Power-Up Detection
8.2.12
The RTC oscillator is active if:
Oscillator Activity
G
V
SB
power supply is higher than V
SBON
regardless of the battery voltage, V
BAT
.
G
V
BAT
power supply is higher than V
BATMIN
whether or not V
SB
is present.
The RTC oscillator is disabled if:
G
During power-down (V
BAT
only), if the battery voltage drops below V
BATMIN
, Battery Fail state may be entered. In this
case, the oscillator may stop oscillating and memory contents may be corrupted or lost.
G
Software writes 00h to DV2-0 bits of the CRA register and V
SB
is removed. This disables the oscillator and decreases
the power consumption from the battery connected to the V
BAT
pin. When disabling the oscillator, the CMOS RAM is
not affected as long as the battery is present at a correct voltage level.
I
BAT
(
μ
A)
1.0
0.9
0.8
0.7
2.4 3.0 3.6
V
BAT (V)
I
BAT
(
μ
A)
0.20
0.15
0.10
0.05
3.0 3.3 3.6
V
SB
(V)
Note: Battery voltage in this test is 3.0V.