VT82885
Real Time Clock
VIA Technologies, Inc.
power is applied. When V falls below 4.25
per month at 25°C regardless of the voltage
CC
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,
input on the V pin.
CC
V
BAT
− Battery input for any standard 3 volt
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
µA at 25°C in the absence of V power
should be used to size the external energy
source.
therefore, write-protected. When V falls
CC
below the level of V
, the external V
BAT
CC
supply is switched off and the external V
BAT
CC
lithium energy source supplies power to the
Real Time Clock and the RAM memory.
SIGNAL DESCRIPTIONS
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
V
CC
− DC power is provided to the device on
thIs pin. V is the +5 volt input. When 5
CC
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
not available when V is less than 4.25
CC
VCC falls below V
, the RAM and
BAT
volts typical.
timekeeper are switched over to the exter-
nal lithium energy source. The timekeeping
function maintains an accuracy of ± 1 minute
TABLE 1: PERIODIC INTERRUPT RATE AND SQUARE WAVE OUTPUT FREQUENCY
SELECT BITS REGISTER A
t
PERIODIC
SQW OUTPUT
FREQUENCY
None
PI
RS3
0
RS2
0
RS1
0
RS0
0
INTERRUPT RATE
None
0
0
0
1
3.90625 ms
7.8125 ms
122.070 µs
244.141 µs
488.281 µs
976.5625 µs
1.953125 ms
3.90625 ms
7.8125 ms
15.625 ms
31.25 ms
256 Hz
0
0
1
0
128 Hz
0
0
1
1
8.192 kHz
4.096 kHz
2.048 kHz
1.024 kHz
512 Hz
0
1
0
0
0
1
0
1
0
1
1
0
0
1
1
1
1
0
0
0
256 Hz
1
0
0
1
128 Hz
1
0
1
0
64 Hz
1
0
1
1
32 Hz
1
1
0
0
62.5 ms
16 Hz
1
1
0
1
125 ms
8 Hz
1
1
1
0
250 ms
4 Hz
1
1
1
1
500 ms
2 Hz
3
ETC [ ETC ]
