VT82885 データシートの表示（PDF） - Unspecified
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VT82885 Datasheet PDF : 16 Pages
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VIA Technologies, Inc.
VT82885
Real Time Clock
power is applied. When VCC falls below 4.25
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,
therefore, write-protected. When VCC falls
below the level of VBAT, the external VCC
supply is switched off and the external VBAT
lithium energy source supplies power to the
Real Time Clock and the RAM memory.
SIGNAL DESCRIPTIONS
VCC − DC power is provided to the device on
thIs pin. VCC is the +5 volt input. When 5
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
VCC falls below VBAT, the RAM and
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
input on the VCC pin.
VBAT − 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 VCC power
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
not available when VCC is less than 4.25
volts typical.
TABLE 1: PERIODIC INTERRUPT RATE AND SQUARE WAVE OUTPUT FREQUENCY
SELECT BITS REGISTER A
RS3 RS2 RS1 RS0
tPI PERIODIC
INTERRUPT RATE
SQW OUTPUT
FREQUENCY
0
0
0
0
None
None
0
0
0
1
3.90625 ms
256 Hz
0
0
1
0
7.8125 ms
128 Hz
0
0
1
1
122.070 µs
8.192 kHz
0
1
0
0
244.141 µs
4.096 kHz
0
1
0
1
488.281 µs
2.048 kHz
0
1
1
0
976.5625 µs
1.024 kHz
0
1
1
1
1.953125 ms
512 Hz
1
0
0
0
3.90625 ms
256 Hz
1
0
0
1
7.8125 ms
128 Hz
1
0
1
0
15.625 ms
64 Hz
1
0
1
1
31.25 ms
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
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