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MM58274C Datasheet(PDF) 6 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. MM58274C
Description  Microprocessor Compatible Real Time Clock
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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MM58274C Datasheet(HTML) 6 Page - National Semiconductor (TI)

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Functional Description (Continued)
device which are synchronously clocked by this signal All
time data and data-changed flag change on the falling edge
of the clock setting pulse
Data-Changed Flag
The data-changed flag is set by the clock setting pulse to
indicate that the time data has been altered since the clock
was last read This flag occupies bit 3 of the control register
where it can be tested by the processor to sense data-
changed It will be reset by a read of the control register
See the section ‘‘Methods of Device Operation’’ for sug-
gested clock reading techniques using this flag
Seconds Counters
There are three counters for seconds
a) tenths of seconds
b) units of seconds
c) tens of seconds
The registers are accessed at the addresses shown in Ta-
ble I The tenths of seconds register is reset to 0 when the
clock startstop bit (bit 2 of the control register) is set to
logic 1 The units and tens of seconds are set up by the
processor giving time setting to the nearest second All
three registers can be read by the processor for time output
Minutes Counters
There are two minutes counters
a) units of minutes
b) tens of minutes
Both registers may be read to or written from as required
Hours Counters
There are two hours counters
a) units of hours
b) tens of hours
Both counters may be accessed for read or write operations
as desired
In 12-hour mode the tens of hours register has only one
active bit and the top three bits are set to logic 0 Data bit 1
of the clock setting register is the AMPM indicator logic 0
indicating AM logic 1 for PM
When 24-hour mode is programmed the tens of hours reg-
ister reads out two bits of data and the two most significant
bits are set to logic 0 There is no AMPM indication and bit
1 of the clock setting register will read out a logic 0
In both 1224-hour modes the units of hours will read out
four active data bits 12 or 24-hour mode is selected by bit 0
of the clock setting register logic 0 for 12-hour mode logic
1 for the 24-hour mode
Days Counters
There are two days counters
a) units of days
b) tens of days
The days counters will count up to 28 29 30 or 31 depend-
ing on the state of the months counters and the leap year
counter The microprocessor has full readwrite access to
these registers
Months Counters
There are two months counters
a) units of months
b) tens of months
Both these counters have full readwrite access
Years Counters
There are two years counters
a) units of years
b) tens of years
Both these counters have full readwrite access The years
will count up to 99 and roll over to 00
TABLE I Address Decoding of Real-Time Clock Internal Registers
Register Selected
Address (Binary)
(Hex)
Access
AD3
AD2
AD1
AD0
0
Control Register
0
0
0
0
0
Split Read and Write
1
Tenths of Seconds
0
0
0
1
1
Read Only
2
Units Seconds
0
0
1
0
2
RW
3
Tens Seconds
0
0
1
1
3
RW
4
Units Minutes
0
1
0
0
4
RW
5
Tens Minutes
0
1
0
1
5
RW
6
Unit Hours
0
1
1
0
6
RW
7
Tens Hours
0
1
1
1
7
RW
8
Units Days
1
0
0
0
8
RW
9
Tens Days
1
0
0
1
9
RW
10
Units Months
1
0
1
0
A
RW
11
Tens Months
1
0
1
1
B
RW
12
Units Years
1
1
0
0
C
RW
13
Tens Years
1
1
0
1
D
RW
14
Day of Week
1
1
1
0
E
RW
15
Clock Setting
1
1
1
1
F
RW
Interrupt Registers
6


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