M41T00S

Clock operation

13/26

Table 2.

Keys:

0 = must be set to '0'

CB = century bit

CEB = century enable bit

FT = frequency test bit

OF = oscillator fail bit

OUT = output level

S = sign bit

ST = stop bit

3.2

Calibrating the clock

The M41T00S is driven by a quartz-controlled oscillator with a nominal frequency of 32,768

Hz. The devices are tested not exceed ±35 ppm (parts per million) oscillator frequency error

When the calibration circuit is properly employed, accuracy improves to better than ±2 ppm

at 25°C.

The oscillation rate of crystals changes with temperature. The M41T00S design 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 11 on page 15. The

number of times pulses which are blanked (subtracted, negative calibration) or split (added,

positive calibration) depends upon the value loaded into the five calibration bits found in the

calibration 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 calibration register 07h.

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.

Addr

Function/range BCD format

D7

D6

D5

D4

D3

D2

D1

D0

00h

ST

10 seconds

Seconds

Seconds

00-59

01h

OF

10 minutes

Minutes

Minutes

00-59

02h

CEB

CB

10 hours

Hours (24-hour format)

Century/hours

0-1/00-23

03h

0000

0

Day of week

Day

01-7

04h

0

0

10 date

Date: day of month

Date

01-31

05h

0

0

0

10M

Month

Month

01-12

06h

10 years

Year

Year

00-99

07h

OUT

FT

S

Calibration

Calibration