3

VT82885

Real Time Clock

VIA Technologies, Inc.

volts, the chip select input is internally forced

to an inac-tive level regardless of the value

of CS# at the input pin. The VT82885 is,

lithium energy source supplies power to the

Real Time Clock and the RAM memory.

SIGNAL DESCRIPTIONS

volts are applied within normal limits, the

device is fully accessible and data can be

written and read. When VCC is below 4.25

volts typical, reads and writes are inhibited.

However, the timekeeping function contin-

ues unaffected by the lower input voltage. As

timekeeper are switched over to the exter-

nal lithium energy source. The timekeeping

function maintains an accuracy of

± 1 minute

per month at 25

°C regardless of the voltage

lithium cell or energy source. Battery vol-

tage must be held between 2.5 and 3.4 volts

for proper operation. A maximum load of .5

should be used to size the external energy

source.

SQW (Square Wave Output)

− The SQW

pin can output a signal from one of 13 taps

provided by the internal divider stages of the

Real Time Clock. The frequency of the SQW

pin can be changed by programming

Register A as shown in Table 1. The SQW

signal can be turned on and off using the

SQWE bit in Register B. The SQW signal is

volts typical.

TABLE 1: PERIODIC INTERRUPT RATE AND SQUARE WAVE OUTPUT FREQUENCY

SELECT BITS REGISTER A

SQW OUTPUT

RS3

RS2

RS1

RS0

INTERRUPT RATE

FREQUENCY

0000

None

None

0001

3.90625 ms

256 Hz

0010

7.8125 ms

128 Hz

0011

122.070

µs

8.192 kHz

0100

244.141

µs

4.096 kHz

0101

488.281

µs

2.048 kHz

0110

976.5625

µs

1.024 kHz

0111

1.953125 ms

512 Hz

1000

3.90625 ms

256 Hz

1001

7.8125 ms

128 Hz

1010

15.625 ms

64 Hz

1011

31.25 ms

32 Hz

1100

62.5 ms

16 Hz

1101

125 ms

8 Hz

1110

250 ms

4 Hz

1111

500 ms

2 Hz