Datasheet
DS1315 Phantom Time Chip
6 of 21
Nonvolatile Controller Operation
The operation of the nonvolatile controller circuits within the Time Chip is determined by the level of the
ROM/
RAM
select pin. When ROM/
RAM
is connected to ground, the controller is set in the RAM mode
and performs the circuit functions required to make CMOS RAM and the timekeeping function
nonvolatile. A switch is provided to direct power from the battery inputs or V
CCI
to V
CCO
with a
maximum voltage drop of 0.3 volts. The V
CCO
output pin is used to supply uninterrupted power to CMOS
SRAM. The DS1315 also performs redundant battery control for high reliability. On power-fail, the
battery with the highest voltage is automatically switched to V
CCO
. If only one battery is used in the
system, the unused battery input should be connected to ground.
The DS1315 safeguards the Time Chip and RAM data by power-fail detection and write protection.
Power-fail detection occurs when V
CCI
falls below V
PF
which is set by an internal bandgap reference. The
DS1315 constantly monitors the V
CCI
supply pin. When V
CCI
is less than V
PF
, power-fail circuitry forces
the chip enable output (
CEO
) to V
CCI
or V
BAT
-0.2 volts for external RAM write protection. During
nominal supply conditions,
CEO
will track
CEI
with a propagation delay. Internally, the DS1315 aborts
any data transfer in progress without changing any of the Time Chip registers and prevents future access
until V
CCI
exceeds V
PF
. A typical RAM/Time Chip interface is illustrated in Figure 3.
When the ROM/
RAM
pin is connected to V
CCO
, the controller is set in the ROM mode. Since ROM is a
read-only device that retains data in the absence of power, battery backup and write protection is not
required. As a result, the chip enable logic will force
CEO
low when power fails. However, the Time
Chip does retain the same internal nonvolatility and write protection as described in the RAM mode. A
typical ROM/Time Chip interface is illustrated in Figure 4.
Figure 3. DS1315-to-RAM/Time Chip Interface