Clock operations

M48T512Y, M48T512V

12/23

Doc ID 5747 Rev 6

Table 5.

Register map

Keys:

S = SIGN bit

R = READ bit

W = WRITE bit

ST = STOP bit

0 = Must be set to '0'

3.4

Calibrating the clock

The M48T512Y/V is driven by a quartz controlled oscillator with a nominal frequency of

32,768 Hz. The devices are factory calibrated at 25 °C and tested for accuracy. Clock

accuracy will not exceed 35 ppm (parts per million) oscillator frequency error at 25 °C, which

equates to about ±1.53 minutes per month. When the Calibration circuit is properly

employed, accuracy improves to better than +1/–2 ppm at 25 °C. The oscillation rate of

crystals changes with temperature. The M48T512Y/V design employs periodic counter

correction. The calibration circuit adds or subtracts counts from the oscillator divider circuit

at the divide by 256 stage (see Figure 8 on page 13).

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 bits occupy the five lower order bits (D4-D0) in the control register

7FFF8h. These bits can be set to represent any value between 0 and 31 in binary form. Bit

D5 is a 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 binary '1' is

loaded into the register, only the first 2 minutes in the 64 minute cycle will be modified; if a

binary 6 is loaded, the first 12 will be affected, and so on. Therefore, each calibration step

has the effect of adding 512 or subtracting 256 oscillator cycles for every 125, 829, 120

actual oscillator cycles; that is, +4.068 or –2.034 ppm of adjustment per calibration step in

the calibration register.

Assuming that the oscillator is running at exactly 32,768 Hz, each of the 31 increments in

the calibration byte would represent +10.7 or –5.35 seconds per month which corresponds

to a total range of +5.5 or –2.75 minutes per month.

Address

Data

Function/range

BCD format

D7

D6

D5

D4

D3

D2

D1

D0

7FFFFh

10 years

Year

Year

00-99

7FFFEh

0

0

0

10 M

Month

Month

01-12

7FFFDh

0

0

10 date

Date

Date

01-31

7FFFCh

0

0

0

0

0

Day

Day

01-07

7FFFBh

0

0

10 hours

Hours

Hours

00-23

7FFFAh

0

10 minutes

Minutes

Minutes

00-59

7FFF9h

ST

10 seconds

Seconds

Seconds

00-59

7FFF8h

W

R

S

Calibration

Control