DS1221 datasheet(2/8 Pages) DALLAS

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DS1221

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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 VCCI supply, depending on

which is greater, to the VCCO 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

VCCI supply. When VCCI 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 VCCI 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 VCCI

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

INPUTS

OUTPUTS

VCCI

CE0

CE1

CE2

CE3

>=4.5

HXXHH

<4.25

XXXHH

>=4.5

LLLL

>=4.5

HHHH

H = High Level

L = Low Level

X = Irrelevant

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