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1339RR-2

I2C Serial Real-Time Clock

Maxim Integrated 

DS1339 I2C Serial Real-Time Clock

DETAILED DESCRIPTION

The DS1339 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 DS1339 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 DS1339 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 block diagram in

Figure 3 shows the main elements of the serial real-time clock.

OSCILLATOR CIRCUIT

The DS1339 uses an external 32.768kHz crystal. The oscillator circuit does not require any external resistors or

capacitors to operate. Table 1 specifies several crystal parameters for the external crystal. Figure 4 shows a

functional schematic of the oscillator circuit. The startup time is usually less than 1 second when using a crystal

with the specified characteristics.

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 1).

Table 1. Power Control

SUPPLY CONDITION

VCC < VPF, VCC < VBACKUP

VCC < VPF, VCC > VBACKUP

VCC > VPF, VCC < VBACKUP

VCC > VPF, VCC > VBACKUP

READ/WRITE

ACCESS

No

No

Yes

Yes

POWERED

BY

VBACKUP

VCC

VCC

VCC

OSCILLATOR CIRCUIT

The DS1339 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. Figure 3 shows a

functional schematic of the oscillator circuit. The startup time is usually less than 1 second when using a crystal

with the specified characteristics.

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