m48t12 STMicroelectronics, m48t12 Datasheet - Page 13

• Manufacturer Part Number: m48t12
• Description: 5.0v, 16 Kbit 2kb X 8 Timekeeper Sram
• Manufacturer: STMicroelectronics

3.3
3.4
Table 5.
Keys:
Stopping and starting the oscillator
The oscillator may be stopped at any time. If the device is going to spend a significant
amount of time on the shelf, the oscillator can be turned off to minimize current drain on the
battery. The STOP Bit is the MSB of the seconds register. Setting it to a '1' stops the
oscillator. The M48T02/12 is shipped from STMicroelectronics with the STOP Bit set to a '1.'
When reset to a '0,' the M48T02/12 oscillator starts within one second.
Calibrating the clock
The M48T02/12 is driven by a quartz-controlled oscillator with a nominal frequency of
32,768 Hz. A typical M48T02/12 is accurate within 1 minute per month at 25°C without
calibration. The devices are tested not to exceed ± 35 ppm (parts per million) oscillator
frequency error at 25°C, which equates to about ±1.53 minutes per month.
The oscillation rate of any crystal changes with temperature.
frequency error that can be expected at various temperatures. Most clock chips compensate
for crystal frequency and temperature shift error with cumbersome “trim” capacitors. The
M48T02/12 design, however, employs periodic counter correction. The calibration circuit
adds or subtracts counts from the oscillator divider circuit at the divide by 256 stage, as
shown in
negative calibration) or split (added, positive calibration) depends upon the value loaded into
the five-bit Calibration Byte found in the Control Register. Adding counts speeds the clock
up, subtracting counts slows the clock down.
The Calibration Byte occupies the five lower order bits in the Control register. This byte can
be set to represent any value between 0 and 31 in binary form. The sixth bit is the Sign Bit;
Addres
7FE
7FD
7FC
7FB
7FF
7FA
7F9
7F8
s
S = SIGN Bit
FT = FREQUENCY TEST Bit (Set to '0' for normal clock operation)
R = READ Bit
W = WRITE Bit
ST = STOP Bit
0 = Must be set to '0'
Figure 9 on page
D7
ST
W
0
0
0
0
0
Register map
D6
FT
R
10 Years
0
0
0
10 Seconds
10 Minutes
D5
10 Hours
S
0
0
10 Date
15. The number of times pulses are blanked (subtracted,
10 M
D4
0
Data
D3
0
Calibration
D2
Seconds
Minutes
Month
Hours
Date
Year
Day
D1
D0
Figure 8 on page 15
Seconds
Minutes
Control
Month
Hours
Date
Year
Day
Function/Range
BCD Format
shows the
00-99
01-12
00-59
00-59
01-31
01-07
00-23
13/24