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AS
Address strobe input
AS serves to demultiplex the address/data
bus. The falling edge of AS latches the ad-
dress on AD0–AD7. This demultiplexing pro-
cess is independent of the CS signal
.
DS
Data strobe input
With MOT= VSS, the DS input is provideda sig-
nal similar to RD, MEMR, or I/OR in an
Intel-based system. The falling edge on DS
is used to enable the outputs during a read
cycle.
R/W
Read/write input
With MOT= VSS, R/W is provideda signal simi-
lar to WR, MEMW, or I/OW in an Intel-
based system.
The rising edge on R/W
latches data into the bq4285E/L.
INT
Interrupt request output
INT is an open-drain output. This allows
INT to be valid in battery-backup mode for
the alarm interrupt.
To use this feature,
INT must be connected to a power supply
other than VCC. INT is asserted low when
any event flag is set and the corresponding
event enable bit is also set. INT becomes
high-impedance whenever register C is read
(see the Control/Status Registers section).
RST
Reset input
The bq4285E/L is reset when RST is pulled
low.
When reset, INT becomes high-
impedance, and the bq4285E/L is not accessi-
ble. Table 4 in the Control/Status Registers
section lists the register bits that are cleared
by a reset.
Reset may be disabled by connecting RST to
VCC. This allows the control bits to retain their
states through power-down/power-up cycles.
SQW
Square-wave output
SQW may output a programmable fre-
quency square-wave signal during normal
(VCC valid) system operation. Any one of
the 13 specific frequencies may be selected
through register A.
This pin is held low
when the square-wave enable bit (SQWE)
in register B is 0 (see the Control/Status
Registers section).
A 32.768kHz output is enabled by setting
the SQWE bit in register B to 1 and the
32KE bit in register C to 1 after setting
OSC2–OSC0 in register A to 011 (binary).
BC
3V backup cell input
BC should be connected to a 3V backup cell
for RTC operation and storage register non-
volatility in the absence of power. When VCC
slews down past VBC (3V typical), the inte-
gral control circuitry switches the power
source to BC. When VCC returns above VBC,
the power source is switched to VCC.
Upon power-up, a voltage within the VBC
range must be present on the BC pin for
the oscillator to start up.
X1–X2
Crystal inputs
The X1–X2 inputs are provided for an ex-
ternal 32.768Khz quartz crystal, Daiwa
DT-26 or equivalent, with 6pF load capaci-
tance. A trimming capacitor may be neces-
sary for extremely precise time-base gen-
eration.
CEIN
External RAM chip enable input,
active low
CEIN should be driven low to enable the
controlled external RAM. CEIN is internally
pulled up with a 50K
resistor.
CEOUT
External RAM chip enable output,
active low
When power is valid, CEOUT reflects CEIN.
VOUT
Supply output
VOUT provides the higher of VCC or VBC,
switched internally, to supply external RAM.
VCC
Positive power supply
VSS
Ground
3
bq4285E/L
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