M48T59 datasheet(12/21 Pages) STMICROELECTRONICS

12 / 21 page

M48T59, M48T59Y, M48T59V

12/21

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 bi-

nary ’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 calibra-

tion step in the calibration register. Assuming that

the oscillator is in fact running at exactly 32,768Hz,

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.

Two methods are available for ascertaining how

much calibration a given M48T59/59Y/59V may

require. The first involves simply setting the clock,

letting it run for a month and comparing it to a

known accurate reference (like WWV broadcasts).

While that may seem crude, it allows the designer

to give the end user the ability to calibrate his clock

as his environment may require, even after the fi-

nal product is packaged in a non-user serviceable

enclosure. All the designer has to do is provide a

simple utility that accesses the Calibration byte.

The second approach is better suited to a manu-

facturing environment, and involves the use of the

IRQ/FT pin. The pin will toggle at 512Hz when the

Stop bit (D7 of 1FF9h) is '0', the FT bit (D6 of

1FFCh) is '1', the AFE bit (D7 of 1FF6h) is '0', and

the Watchdog Steering bit (D7 of 1FF7h) is '1' or

the Watchdog Register is reset (1FF7h = 0).

Any deviation from 512Hz indicates the degree

and direction of oscillator frequency shift at the test

temperature. For example, a reading of 512.01024

Hz would indicate a +20 ppm oscillator frequency

error, requiring a –10 (WR001010) to be loaded

into the Calibration Byte for correction. Note that

setting or changing the Calibration Byte does not

affect the Frequency test output frequency.

The IRQ/FT pin is an open drain output which re-

quires a pull-up resistor for proper operation. A

500-10k

Ω resistor is recommended in order to

control the rise time. The FT bit is cleared on pow-

er-down.

For more information on calibration, see the Appli-

cation Note AN934 "TIMEKEEPER Calibration".

SETTING ALARM CLOCK

Registers 1FF5h-1FF2h contain the alarm set-

tings. The alarm can be configured to go off at a

prescribed time on a specific day of the month or

repeat every month, day, hour, minute, or second.

It can also be programmed to go off while the

M48T59 is in the battery back-up mode of opera-

tion to serve as a system wake-up call.

RPT1-RPT4 put the alarm in the repeat mode of

operation. Table 12 shows the possible configura-

tions. Codes not listed in the table default to the

once per second mode to quickly alert the user of

an incorrect alarm setting.

Figure 11. Interrupt Reset Waveforms

AI01388B

15ns Min

A0-A12

ACTIVE FLAG BIT

IRQ/FT

ADDRESS 1FF0h

HIGH-Z

Table 12. Alarm Repeat Mode

RPT4

RPT3

RPT2

RPT1

Alarm Activated

Once per Second

Once per Minute

Once per Hour

Once per Day

Once per Month

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