m48t18 STMicroelectronics, m48t18 Datasheet - Page 15

• Manufacturer Part Number: m48t18
• Description: 5v, 64kbit 8 Kb X 8 Timekeeper Sram
• Manufacturer: STMicroelectronics

Note:
Stopping and starting the oscillator
Note:
Calibrating the clock
Table 5.
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'
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 (ST) is the MSB of the seconds register. Setting it to a '1' stops the
oscillator. The M48T08/18/08Y (in the PCDIP28 package) is shipped from
STMicroelectronics with the STOP Bit set to a '1.' When reset to a '0,' the M48T08/18/08Y
oscillator starts within one second.
To guarantee oscillator start-up after initial power-up, first write the STOP Bit (ST) to '1,'
then reset to '0.'
The M48T08/18/08Y is driven by a quartz-controlled oscillator with a nominal frequency of
32,768 Hz. A typical M48T08/18/08Y 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. With the
calibration bits properly set, the accuracy of each M48T08/18/08Y improves to better than
+1/–2 ppm at 25°C.
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
M48T08/18/08Y 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
Address
1FFDh
1FFCh
1FFFh
1FFEh
1FFBh
1FFAh
1FF9h
1FF8h
Figure 9 on page
Register map
D7
ST
W
0
0
0
0
0
D6
FT
R
0
0
0
10 Years
10 Seconds
10 Minutes
D5
10 Hours
S
0
0
10 Date
17. 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
Figure 8 on page 17
D0
Seconds
Minutes
Control
Month
Hours
Date
Year
Day
Function/range
BCD format
shows the
00-99
01-12
01-31
01-07
00-23
00-59
00-59
15/30