DS1221
CONTROLLER /DECODER OPERATION
The DS1221 nonvolatile controller performs six circuit functions required to decode and battery-backup a
bank of up to four CMOS RAMs. First, a 2-to-4 decoder provides selection of one of four RAMs (see
Figure 1). Second, a switch is provided to direct power from the battery or V
CCI
supply, depending on
which is greater, to the V
CCO
pin. This switch has a voltage drop of less than 0.2V. The third function
which the nonvolatile controller provides is power-fail detection. The DS1221 constantly monitors the
V
CCI
supply. When V
CCI
falls below 4.5 volts, a precision comparator detects the condition and inhibits
the RAM chip enables (
CE0
through
CE3
). The fourth function of write protection is accomplished by
holding all chip enable outputs (
CE0
through
CE3
) to within 0.2 volts of V
CCI
or battery supply. If the
Chip Enable Input (
CE
) is low at the time power-fail detection occurs, the chip enable outputs are kept in
their present state until
CE
is driven high. The delay of write protection until the current memory cycle is
completed prevents the corruption of data. Power failure detection occurs in the range of 4.5 to 4.25 volts.
During nominal supply conditions the chip enable outputs follow the logic of a 2-to-4 decoder. The fifth
function the DS1221 performs is to check battery status to warn of potential data loss. Each time that V
CCI
power is restored the battery voltage is checked with a precision comparator. If the connected battery
voltage is less than 2 volts, the second memory cycle is inhibited. Battery status can, therefore, be
determined by performing a read cycle after power-up to any location in memory, verifying that memory
location content. 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, the contents of the memories are questionable.
The sixth function of the nonvolatile controller provides for battery redundancy. In many applications,
data integrity is paramount. In these applications it is often desirable to use two batteries to ensure
reliability. The DS1221 provides an internal isolation switch which provides for connection of two
batteries. During battery back-up operation the battery with the highest voltage is selected for use. If one
battery should fail, the other will automatically take 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. If only one battery is
used, the second battery input must be grounded. Figure 2 illustrates the connections required for the
DS1221 in a typical application.
NONVOLATILE CONTROLLER/DECODER
Figure 1
V
CCI
>=4.5
<4.25
>=4.5
>=4.5
>=4.5
>=4.5
H = High Level
L = Low Level
X = Irrelevant
CE
H
X
L
L
L
L
INPUTS
B
X
X
L
L
H
H
OUTPUTS
A
X
X
L
H
L
H
CE0
CE1
CE2
CE3
H
H
L
H
H
H
H
H
H
L
H
H
H
H
H
H
L
H
H
H
H
H
H
L
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