DS1501WN datasheet(12/20 Pages) DALLAS

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DS1501/DS1511 Y2KC Watchdog Real-Time Clocks

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DATA RETENTION MODE

The DS1501/DS1511 are fully accessible, and data can be written and read only when VCC is greater than VPF.

However, when VCC falls below the power-fail point VPF (point at which write protection occurs) the internal clock

registers and SRAM are blocked from any access. While in the data retention mode, all inputs are don’t cares and

outputs go to a high-Z state, with the possible exception of

KS, PWR, SQW, and RST. If VPF is less than VBAT and

VBAUX, the device power is switched from VCC to the greater of VBAT and VBAUX when VCC drops below VPF. If VPF is

greater than VBAT and VBAUX, the device power is switched from VCC to the larger of VBAT and VBAUX when VCC drops

below the larger of VBAT and VBAUX. RTC operation and SRAM data are maintained from the battery until VCC is

returned to nominal levels (Table 1). If the square-wave and battery-backup 32kHz functions are enabled, VBAUX

always provides power for the square-wave output, when the device is in battery-backup mode.

AUXILIARY BATTERY

The VBAUX input is provided to supply power from an auxiliary battery for the DS1501/DS1511 kickstart and square-

wave output features in the absence of VCC. This power source must be available to use these auxiliary features

when VCC is not applied to the device.

This auxiliary battery can be used as the primary backup power source for maintaining the clock/calendar and

external user RAM. This occurs if the VBAT pin is at a lower voltage than VBAUX. If the DS1501/DS1511 are to be

backed up using a single battery with the auxiliary features enabled, then VBAUX should be used and connected to

VBAT should be grounded (DS1501). If VBAUX is not to be used, it must be grounded.

OSCILLATOR CONTROL BIT

When the DS1511 is shipped from the factory, the internal oscillator is turned off. This feature prevents the lithium

energy cell from being used until it is installed in a system. The oscillator is automatically enabled when power is

first applied.

POWER-ON RESET

A temperature-compensated comparator circuit monitors the level of V

CC. When VCC falls to the power-fail trip point,

the

RST signal (open drain) is pulled low. When V

CC returns to nominal levels, the RST signal continues to be pulled

low for a period of tREC. The power-on reset function is independent of the RTC oscillator and therefore operational

whether or not the oscillator is enabled.

TIME AND DATE OPERATION

The time and date information is obtained by reading the appropriate register bytes. Table 2 shows the RTC

registers. The time and date are set or initialized by writing the appropriate register bytes. The contents of the time

and date registers are in BCD format. Hours are in 24-hour mode. The day-of-week register increments at

midnight. Values that correspond to the day of week are user defined but must be sequential (i.e., if 1 equals

Sunday, then 2 equals Monday, and so on). Illogical time and date entries result in undefined operation.

READING THE CLOCK

When reading the clock and calendar data, it is possible to access the registers while an update (once per second)

occurs. There are three ways to avoid using invalid time and date data.

The first method uses the transfer enable (TE) bit in the control B register. Transfers are halted when a 0 is written

to the TE bit. Setting TE to 0 halts updates to the user-accessible registers, while allowing the internal registers to

advance. After the registers are read, the TE bit should be written to 1. TE must be kept at 1 for at least 366

ms to

ensure a user register update.

The time and date registers can be read and stored in temporary variables. The time and date registers are then

read again, and compared to the first values. If the values do not match, the time and date registers should be read

a third time and compared to the previous values. This should be done until two consecutive reads of the time and

date registers match. The TE bit should always be enabled when using this method for reading the time and date,.

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