RTC initialization

AN3060

18/23

Doc ID 16253 Rev 2

8

RTC initialization

8.1

What is the best sequence for accessing the RTC at first

power-up? On subsequent power-ups?

This will vary from device to device, and hence a comprehensive answer is beyond the

scope of this document. Instead, we offer an example for the M41T83/M41T82.

Initializing and accessing the M41T83/M41T82

At first power-up, the part is in a benign state - the alarms and interrupts are disabled as are

the watchdog and timer. The OUT bit is high, and the frequency test bit, FT, is disabled.

Both the digital and analog calibration functions are set to the neutral points and the OTP bit

is set to 0 thus selecting the user analog calibration register over the factory value. (OTP

applies only to the M41T83 in the embedded crystal [MY] package.)

The squarewave output is enabled and 32.768 kHz is selected. (This allows devices using

this as a reference to have it at first power-up without need of user intervention.)

Figure 10.

M41T83/M41T82 power-up defaults

The clock registers - hours, minutes, seconds, year, month, date - are unknown. Only the

century bits, CB1:CB0, are defined (00).

The stop bit, ST, powers up cleared thus enabling the 32 KHz oscillator. But, since the

oscillator has not been running (prior to this first power-up) the oscillator fail detect bit, OF,

will be set.

Lastly, as is the case at all power-ups, the HT bit is set.

IRQ1/FT/OUT pin

OSC Fail

Alarm 1

Timer

Squarewave output

Alarm 2

ST HT CB1:CB0 OUT FT

Watchdog

BM4:BM0,

RB1:RB0

OFIE OF A1IE

ABE RPT15:11 TE

TIE TD1:TD0

Digital Cal

DCS, DC4:DC0

Analog Cal

ACS, AC5:AC0

SQWE

RS3:RS0 OTP AL2E A2IE RPT25:21

1st power-up

0

1

00

1

0

0000000

0

1

0

0

00000

0

0

0

11

000000

00000000

1

1000

0

0

0

00000

Power-up from

backup

UC 1

UC

UC

0

0000000

UC

UC

UC

UC

UC

0

UC

UC

UC

UC

UC

UC

UC

UC

UC

UC

UC

1st power-up

Power-up from

backup

TI/TP

AM04728v1