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90-0107 Datasheet(PDF) 9 Page - Maxim Integrated Products

Part No. 90-0107
Description  I2C Serial Real-Time Clock
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Maker  MAXIM [Maxim Integrated Products]
Homepage  http://www.maxim-ic.com
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 9 page
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DS1339 I
2C Serial Real-Time Clock
9 of 20
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 I
2C 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 V
PF, 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.
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). On the first application of power to the device the
time and date registers are reset to 01/01/00 01 00:00:00 (MM/DD/YY DOW HH:MM:SS).
Table 1. Power Control
SUPPLY CONDITION
READ/WRITE
ACCESS
POWERED
BY
VCC < VPF, VCC < VBACKUP
No
VBACKUP
VCC < VPF, VCC > VBACKUP
No
VCC
VCC > VPF, VCC < VBACKUP
Yes
VCC
VCC > VPF, VCC > VBACKUP
Yes
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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