M48T251Y Datasheet, PDF (13/24 Pages) STMicroelectronics – 5.0 or 3.3V, 4096K TIMEKEEPER SRAM with PHANTOM

M48T251Y, M48T251V
Clock Register Information
Clock information is contained in eight registers of
8 bits, each of which is sequentially accessed one
(1) bit at a time after the 64-bit pattern recognition
sequence has been completed. When updating
the clock registers, each must be handled in
groups of 8 bits. Writing and reading individual bits
within a register could produce erroneous results.
These READ/WRITE registers are defined in the
clock register map (see Table 5.).
Data contained in the clock registers is in Binary
Coded Decimal format (BCD). Reading and writing
the registers is always accomplished by stepping
through all eight registers, starting with Bit 0 of
Register 0 and ending with Bit 7 of Register 7.
Clock Accuracy
The RTC is guaranteed to keep time accuracy to
with ±1 minute per month at 25°C. The clock is fac-
tory-tuned with special calibration elements, and
does not require additional calibration. Moderate
temperature deviation will have a negligible effect
in most applications.
AM-PM/12/24 Mode
Bit 7 of the hours register is defined as the 12-hour
or 24-hour mode select bit. When it is high, the 12-
hour mode is selected. In the 12-hour mode, Bit 5
is the AM/PM bit with the logic high being “PM.” In
the 24-hour mode, Bit 5 is the second 10-hour bit
(20-23 hours).
Oscillator and Reset Bits
Bits 4 and 5 of the day register are used to control
the reset and oscillator functions. Bit 4 controls the
reset pin input. When the reset bit is set to logic '1,'
the Reset Input pin is ignored. When the reset bit
logic is set to '0,' a low input on the reset pin will
cause the device to abort data transfer without
changing data in the timekeeping registers. Reset
operates independently of all other inputs. Bit 5
controls the oscillator. When set to logic '0,' the os-
cillator turns on and the RTC/calendar begins to
increment.
Zero Bits
Registers 1, 2, 3, 4, 5, and 6 contain one (1) or
more bits that will always read logic '0.' When writ-
ing to these locations, either a logic '1' or '0' is ac-
ceptable.
Table 5. Phantom Clock Register Map
Register
D7
D6
D5
D4
0
0.1 Seconds
1
0
10 Seconds
2
0
10 Minutes
3
12/24
0
10 /
A/P
Hrs
4
0
0
OSC RST
5
0
0
10 date
6
0
0
0
10M
7
10 Years
Keys: A/P = AM/PM Bit
12/24 = 12 or 24-hour mode Bit
OSC = Oscillator Bit
D3
D2
D1
D0
0.01 Seconds
Seconds
Minutes
Hours (24 Hour Format)
0
Day of the Week
Date: Day of the Month
Month
Year
RST = Reset Bit
0 = Must be set to '0'
Function/Range
BCD Format
Seconds
Seconds
Minutes
Hours
Day
Date
Month
Year
00-99
00-59
00-59
01-12/
00-23
01-7
01-31
01-12
00-99
13/24