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DS1339A

Low-Current, I2C, Serial Real-Time Clock

Maxim Integrated

Detailed Description

The DS1339A serial real-time clock (RTC) is a low-

power clock/date device with two programmable time-

of-day alarms and a programmable square-wave output.

Address and data are transferred serially through an I2C

bus. The clock/date provides seconds, minutes, hours,

day, date, month, and year information. The date at the

end of the month is automatically adjusted for months

with fewer than 31 days, including corrections for leap

year. The clock operates in either the 24-hour or 12-hour

format with AM/PM indicator. The DS1339A has a built-

in power-sense circuit that detects power failures and

automatically switches to the backup supply, maintaining

time, date, and alarm operation.

Operation

The DS1339A operates as a slave device on the serial

bus. Access is obtained by implementing a START

condition and providing a device identification code

followed by data. Subsequent registers can be accessed

sequentially until a STOP condition is executed. The

device is fully accessible and data can be written and

read when VCC is greater than VPF. However, when VCC

falls below VPF, the internal clock registers are blocked

from any access. If VPF is less than VBACKUP, the device

power is switched from VCC to VBACKUP when VCC

drops below VPF. If VPF is greater than VBACKUP, the

device power is switched from VCC to VBACKUP when

VCC drops below VBACKUP. The registers are maintained

from the VBACKUP source until VCC is returned to nominal

levels. The Functional Diagram shows the main elements

of the serial real-time clock.

Power Control

The power-control function is provided by a precise,

temperature-compensated voltage reference and a

comparator circuit that monitors the VCC level. The

device is fully accessible and data can be written and

read when VCC is greater than VPF. However, when VCC

falls below VPF, the internal clock registers are blocked

from any access. If VPF is less than VBACKUP, the device

power is switched from VCC to VBACKUP when VCC

drops below VPF. If VPF is greater than VBACKUP, the

device power is switched from VCC to VBACKUP when

VCC drops below VBACKUP. The registers are maintained

from the VBACKUP source until VCC is returned to nominal

levels (Table 1). After VCC returns above VPF, read and

write access is allowed after tREC (Figure 2). On the first

application of power to the device the time and date

registers are reset to 01/01/00 01 00:00:00 (DD/MM/YY

DOW HH:MM:SS).

Oscillator Circuit

The DS1339A uses an external 32.768kHz crystal. The

oscillator circuit does not require any external resistors

or capacitors to operate. Table 2 specifies several

crystal parameters for the external crystal. The Functional

Diagram shows a basic schematic of the oscillator circuit.

The startup time is usually less than 1 second when using

a crystal with the specified characteristics.

Table 1. Power Control

Table 2. Crystal Specifications*

*The crystal, traces, and crystal input pins should be isolated

from RF generating signals. Refer to Application Note 58:

Crystal Considerations for Dallas Real-Time Clocks for addi-

tional specifications.

SUPPLY CONDITION

READ/

WRITE

ACCESS

POWERED

VCC < VPF, VCC < VBACKUP

VBACKUP

VCC < VPF, VCC > VBACKUP

VCC

VCC > VPF, VCC < VBACKUP

Yes

VCC

VCC > VPF, VCC > VBACKUP

Yes

VCC

PARAMETER

SYMBOL MIN

TYP

MAX UNITS

Nominal

Frequency

32.768

kHz

Series Resistance

ESR

Load

Capacitance

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