Datasheet

Clock operations M48T35AV
14/29 Doc ID 6845 Rev 9
Table 5. Register map
Keys:
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
Note: 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.
3.4 Calibrating the clock
The M48T35AV 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 M48T35AV improves to better than
+1/–2 ppm at 25°C.
The oscillation rate of any crystal changes with temperature (see Figure 9 on page 16).
Most clock chips compensate for crystal frequency and temperature shift error with
cumbersome “trim” capacitors. The M48T35AV 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 Figure 10 on page 16. The number of times pulses
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
Address
Data
Function/range
BCD format
D7 D6 D5 D4 D3 D2 D1 D0
7FFFh 10 years Year Year 00-99
7FFEh 0 0 0 10 M. Month Month 01-12
7FFDh 0 0 10 date Date Date 01-31
7FFCh 0 FT CEB CB 0 Day Century/day 00-01/01-07
7FFBh 0 0 10 hours Hours Hours 00-23
7FFAh 0 10 minutes Minutes Minutes 00-59
7FF9h ST 10 seconds Seconds Seconds 00-59
7FF8h W R S Calibration Control