RTCs with Constant-Voltage Trickle Charger
Pin Description (continued)
PIN
EDIP
NAME
FUNCTION
SO
BGA
D5–D8,
E1–E8,
F5–F8
12, 16
12
GND
Ground
Chip-Select Input. The active-low chip-select signal must be asserted low for a bus cycle
in the DS12R885 to be accessed. CS must be kept in the active state during DS and AS
for Motorola timing and during DS and R/W for Intel timing. Bus cycles that take place
13
14
13
14
C1
C3
CS
without asserting CS latch addresses, but no access occurs. When V is below V volts,
CC
PF
the DS12R885 inhibits access by internally disabling the CS input. This action protects the
RTC data and the RAM data during power outages.
Address Strobe Input. A positive-going address-strobe pulse serves to demultiplex the
bus. The falling edge of AS causes the address to be latched within the DS12R885. The
next rising edge that occurs on the AS bus clears the address regardless of whether CS is
asserted. An address strobe must immediately precede each write or read access. If a
write or read is performed with CS deasserted, another address strobe must be performed
prior to a read or write access with CS asserted.
AS
Read/Write Input. The R/W pin has two modes of operation. When the MOT pin is
connected to V for Motorola timing, R/W is at a level that indicates whether the current
CC
cycle is a read or write. A read cycle is indicated with a high level on R/W while DS is high.
A write cycle is indicated when R/W is low during DS. When the MOT pin is connected to
GND for Intel timing, the R/W signal is an active-low signal. In this mode, the R/W pin
operates in a similar fashion as the write-enable signal (WE) on generic RAMs. Data are
latched on the rising edge of the signal.
15
22
15
C2
A3
R/W
2, 3, 16,
20–22
No Connection. This pin should remain unconnected. On the EDIP, these pins are missing
by design.
N.C.
Data Strobe or Read Input. The DS pin has two modes of operation depending on the level of
5C/DS12R87
the MOT pin. When the MOT pin is connected to V , Motorola bus timing is selected. In this
CC
mode, DS is a positive pulse during the latter portion of the bus cycle and is called data
strobe. During read cycles, DS signifies the time that the DS12R885 is to drive the
bidirectional bus. In write cycles, the trailing edge of DS causes the DS12R885 to latch the
written data. When the MOT pin is connected to GND, Intel bus timing is selected. DS
identifies the time period when the DS12R885 drives the bus with read data. In this mode, the
DS pin operates in a similar fashion as the output-enable (OE) signal on a generic RAM.
17
17
A1
DS
10
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