Clock operations

M48T35AV

14/28

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 occurs within a 64 minute cycle. The first 62 minutes in the cycle may, once per

minute, have one second either shortened by 128 or lengthened by 256 oscillator cycles. If a

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