DS17285(1998) View Datasheet(PDF) - Dallas Semiconductor -> Maxim Integrated

Part Name

Description

Manufacturer

DS17285
(Rev.:1998)

3 Volt/5 Volt Real Time Clock

Dallas Semiconductor -> Maxim Integrated 

DS17285/DS17287

VBAUX – Auxiliary battery input required for kickstart

and wake up features. This input also supports clock/

calendar and user RAM if VBAT is at lower voltage or is

not present. A standard +3 volt lithium cell or other

energy source can be used. Battery voltage must be held

between +2.5 and +3.7 volts for proper operation. If

VBAUX is not going to be used it should be grounded and

auxiliary battery enable bit bank 1, register 4BH,

should=0.

DS17285 ONLY

X1, X2 – Connections for a standard 32.768 KHz quartz

crystal. For greatest accuracy, the DS17285 must be

used with a crystal that has a specified load capacitance

of either 6 pF or 12.5 pF. The Crystal Select (CS) bit in

Extended Control Register 4B is used to select opera-

tion with a 6 pF or 12.5 pF crystal. The crystal is

attached directly to the X1 and X2 pins. There is no

need for external capacitors or resistors. Note: X1 and

X2 are very high impedance nodes. It is recommended

that they and the crystal be guard–ringed with ground

and that high frequency signals be kept away from the

crystal area.

For more information on crystal selection and crystal

layout considerations, please consult Application Note

58, “Crystal Considerations with Dallas Real Time

Clocks”. The DS17285 can also be driven by an exter-

nal 32.768 KHz oscillator. In this configuration, the X1

pin is connected to the external oscillator signal and the

X2 pin is floated.

VBAT – Battery input for any standard 3 Volt lithium cell

or other energy source. Battery voltage must be held

between 2.5 and 3.7 volts for proper operation.

POWER–DOWN/POWER–UP

CONSIDERATIONS

The real–time clock function will continue to operate and

all of the RAM, time, calendar, and alarm memory loca-

tions remain nonvolatile regardless of the level of the

VCC input. When VCC is applied to the

DS17285/DS17287 and reaches a level of greater than

VPF (power fail trip point), the device becomes accessi-

ble after tREC, provided that the oscillator is running and

the oscillator countdown chain is not in reset (see Regis-

ter A). This time period allows the system to stabilize

after power is applied.

The DS17285/DS17287 is available in either a 3 volt or

a 5 volt device.

The 5 volt device is fully accessible and data can be writ-

ten and read only when VCC is greater than 4.5 volts.

When VCC is below 4.5 volts, read and writes are inhib-

ited. However, the timekeeping function continues

unaffected by the lower input voltage. As VCC falls

below the greater of VBAT and VBAUX, the RAM and

timekeeper are switched over to a lithium battery con-

nected either to the VBAT pin or VBAUX pin.

The 3 volt device is fully accessible and data can be writ-

ten or read only when VCC is greater than 2.7 volts.

When VCC falls below VPF, access to the device is inhib-

ited. If VPF is less than VBAT and VBAUX, the power sup-

ply is switched from VCC to the backup supply (the

greater of VBAT and VBAUX) when VCC drops below VPF.

If VPF is greater than VBAT and VBAUX, the power supply

is switched from VCC to the backup supply when VCC

drops below the larger of VBAT and VBAUX.

When VCC falls below VPF, the chip is write-protected.

With the possible exception of the KS, PWR, RCLR, and

SQW pins, all inputs are ignored and all outputs are in a

high impedance state.

RTC ADDRESS MAP

The address map for the RTC registers of the

DS17285/DS17287 is shown in Figure 2. The address

map consists of the 14 clock/calendar registers. Ten

registers contain the time, calendar, and alarm data,

and four bytes are used for control and status. All regis-

ters can be directly written or read except for the follow-

ing:

1. Registers C and D are read–only.

2. Bit 7 of Register A is read–only.

3. The high order bit of the seconds byte is read–only.

030598 5/32

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