Page-3
D
0
-D
3
11-14
14,15,16,19
Bi-
(Data bus)
direction
A
0
-A
3
4-7
5,7,9,10
Input
(Address bus)
ALE
3
4
Input
(Address Latch Enable)
WR
10
13
Input
(WRite)
RD
8
11
Input
(ReaD)
CS
1
, CS
0
15,2
20,2
Input
(Chip Select)
STD.P
1
1
Output
(STanDard Pulse)
V
DD
18
24
GND
9
12
(V
DD
)
16,17
22,23
N.C.
-
3,6,8,17,18,21
Connect these pins to a bidirectional data bus or CPU data bus. Use
this bus to read to and write from the internal counter and registers.
Address input pins used for connection to CPU addresses, etc. Used
to select the RTC's internal counter and registers (address selection).
When the RTC is connected to a multiplexed-bus type of CPU, these
pins can also be used in combination with the ALE described below.
Reads in address data and
CS
0
state for internal latching.
When the ALE is high, the address data and
CS
0
state is read into
the RTC. When the (through-mode) ALE falls, the address data and
CS
0
state at that point are held. The held address data and
CS
0
status
are maintained while the ALE is low.
If the RTC is connected to a CPU that does not have an ALE pin and
thus there is no need to use this ALE pin, fix it to V
DD
.
Writes the data on D
0
to D
3
into the register of the address specified
by A
0
to A
3
, at the leading edge of
WR
.
Make sure that
RD
and
WR
are never low at the same time.
Outputs data to D
0
to D
3
from the register at the address specified by
A
0
to A
3
, while
RD
is low.
Make sure that
RD
and
WR
are never low at the same time.
When CS
1
is high and
CS
0
is low, the RTC's chip-select function is
valid and read and write are enabled.
When the RTC is connected to a multiplexed-bus type of CPU,
CS
0
requires the operation of the ALE (see the description of the ALE).
Use CS
1
connected to a power voltage detection circuit. When CS
1
is
high, the RTC is enabled; when it is low, the RTC is on standby.
When CS
1
goes low, the HOLD and RESET bits in the RTC registers
are cleared to 0.
This is an N-channel open drain output pin.
Depending on the setting of the C
E
register, a fixed-period interrupt
signal and a pulse signal are output.
The output from this pin cannot be inhibited by the CS
1
and
CS
0
signals.
Use a load voltage that is less than or equal to V
DD
. If not using this
pin, keep it open-circuit.
An example of STD.P connection is shown below.
If the STD.P output is not be used during standby operation,
connecting the pull-up resistor to +5 V provides a reduction in current
consumption. If the STD.P output is to be used even during standby,
connect the pull-up resistor to the RTC's V
DD
. In this case, the current
consumption will be increased by the amount of current flowing
through the pull-up resistor.
Connect this pin to power source. Supply to 5 V ±10% to this pin
during normal operation; at least 2 V during battery back-up
operation.
Connect this pin to ground.
These pins are connected internally to V
DD
. Leave them open circuit.
These pins are not connected internally. Ground them.
■
Pin functions
Signal
Pin No.
Input/
Function
RTC-72421 RTC-72423 Output
CS-1
CS
0
RD
WR
Mode of D
0
to D
3
H
L
L
H
Output mode (read mode)
H
L
H
L
Input mode (write mode)
H
L
L
L
Do not use
L
H or L
High impedance (back-up mode)
H
H
H or L
High impedance (RTC not selected)
ALE
Address data and CS
0
status
H
Read into the RTC to set address data
L
Held in the RTC (latched at the trailing edge of the ALE)
