m48t35 STMicroelectronics, m48t35 Datasheet - Page 13

• Manufacturer Part Number: m48t35
• Description: 5v, 256 Kbit 32 Kb X 8 Timekeeper Sram
• Manufacturer: STMicroelectronics

Note:
3.4
Table 5.
Keys:
When CEB is set to '1,' CB will toggle from '0' to '1' or from '1' to '0' at the turn of the century
(dependent upon the initial value set).
When CEB is set to '0,' CB will not toggle. The WRITE Bit does not need to be set to write to
CEB.
Calibrating the clock
The M48T35/Y is driven by a quartz-controlled oscillator with a nominal frequency of 32,768
Hz. 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 M48T35/Y improves to better than +1/–2 ppm at 25°C.
The oscillation rate of any crystal changes with temperature (see
Most clock chips compensate for crystal frequency and temperature shift error with
cumbersome “trim” capacitors. The M48T35/Y 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
are blanked (subtracted, negative calibration) or split (added, positive calibration) depends
upon the value loaded into the five calibration bits 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 (D4-D0) in the Control Register
7FF8h. These bits can be set to represent any value between 0 and 31 in binary form. Bit
D5 is the Sign Bit; '1' indicates positive calibration, '0' indicates negative calibration.
Calibration occurs within a 64 minute cycle. The first 62 minutes in the cycle may, once per
Address
7FFDh
7FFCh
7FFFh
7FFEh
7FFBh
7FFAh
7FF9h
7FF8h
S = SIGN Bit
FT = FREQUENCY TEST Bit (Must be set to '0' upon power for normal operation)
R = READ Bit
W = WRITE Bit
ST = STOP Bit
0 = Must be set to '0'
CEB = Century Enable Bit
CB = Century Bit
D7
ST
W
Register map
0
0
0
0
0
D6
FT
0
0
0
R
10 Years
10 Minutes
10 Seconds
CEB
D5
10 Hours
0
S
10 Date
10 M.
CB
D4
Data
D3
Figure 9 on page
0
Calibration
D2
Minutes
Seconds
Month
Hours
Date
Year
Day
D1
15. The number of times pulses
D0
Figure 8 on page
Century/
Minutes
Seconds
Month
Hours
Function/Range
Control
Date
Year
Day
BCD Format
00-01/01-
00-99
01-12
01-31
00-23
00-59
00-59
07
15).
13/29