DS1213B
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OPERATION
The DS1213B SmartSocket performs five circuit functions required to battery back up a CMOS memory.
The first function involves switching between the battery and the V
CC
supply, depending on which is
greater. The switch has a voltage drop of less than 0.2 volts.
The second function is power-fail detection. The DS1213B constantly monitors the V
CC
supply. When
V
CC
falls below 4.75 volts, a precision comparator detects the condition and inhibits the RAM chip
enable.
The third function, write protection, is accomplished by holding the RAM chip enable signal to within 0.2
volts of V
CC
or the battery supply whichever is greater. If the incoming chip enable signal is active at the
time power fail detection occurs, write protection is delayed until after the current memory cycle is
complete to avoid corruption of data. Power fail detection occurs in the range of 4.75 to 4.5 volts. During
nominal power supply conditions the chip enable signal will be passed through from the socket pin to the
socket contact with a maximum propagation delay of 20 ns.
The fourth function the DS1213B performs is to check battery status to warn of potential data loss. Each
time that V
CC
power is restored to the SmartSocket the battery voltage is checked with a precision
comparator. If the battery supply is less than 2.0 volts, the second memory access to the SmartSocket is
inhibited. Battery status can, therefore, be determined by a three-step process. First, a read cycle is
performed to any location in the memory, in order to save the contents of that location. A subsequent
write cycle can then be executed to the same memory location, altering the data. If the next read cycle
fails to verify the written data, then the battery voltage is less than 2.0V and data is in danger of being
corrupted.
The fifth function the SmartSocket provides is battery redundancy. In many applications, data integrity is
paramount. In these applications it is desirable to use two batteries to ensure reliability. The DS1213B
SmartSocket provides two batteries and an internal isolation switch to select between them. During
battery back up, the battery with the highest voltage is selected for use. If one battery fails, the other
automatically takes over. The switch between batteries is transparent to the user. A battery status warning
will occur if both batteries are less than 2.0 volts. Each of the two internal lithium cells has a 45 mAh
capacity.
NOTE:
As shipped from Dallas Semiconductor, battery voltage cannot be measured on the V
CC
socket contact.
Only after V
CC
has been applied to the device for the first time and then removed will the battery voltage
be present on socket contacts 28, 26 and 20.
