DS1343_1112 Datasheet, PDF (10/20 Pages) Maxim Integrated Products – Low-Current SPI/3-Wire RTCs

Low-Current SPI/3-Wire RTCs
Detailed Description
The DS1343/DS1344 low-current real-time clocks (RTCs)
are timekeeping devices that consume an extremely low
timekeeping current and also support high-ESR crystals,
broadening the pool of usable crystals for the device.
The devices provide a full binary-coded decimal (BCD)
clock calendar that is accessed by a simple serial inter-
face. The clock/calendar 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 through 2099. The clock operates in either
a 24-hour or 12-hour format with an AM/PM indicator. In
addition, 96 bytes of NV RAM are provided for data stor-
age. The devices maintain the time and date, provided
that the oscillator is enabled, as long as at least one sup-
ply is at a valid level.
Both devices provide two programmable time-of-day
alarms. Each alarm can generate an interrupt on a pro-
grammable combination of seconds, minutes, hours, and
day. Don’t-care states can be inserted into one or more
fields if it is desired for them to be ignored for the alarm
condition. The time-of day alarms can be programmed
to assert two different interrupt outputs or to assert one
common interrupt output. Both interrupt outputs operate
when the device is powered by VCC or VBAT.
The devices support a direct interface to SPI serial-data
ports or standard 3-wire interface. A straight-forward
address and data format is implemented in which data
transfers can occur one byte at a time or in multiple-byte
burst mode.
The devices have a built-in temperature-compensated
power-sense circuit that detects power failures and
automatically switches to the backup supply. The VBAT
pin can be configured to provide trickle charging of a
rechargeable voltage source, with selectable charging
resistance and diode-voltage drops.
I/O and Power-Switching Operation
The devices operate as slave devices on a 3-wire or SPI
serial bus. Access is obtained by selecting the part by
the CE pin and clocking data into/out of the part using
the SCLK and SDI/SDO pins. Multiple byte transfers
are supported within one CE high period; see the Serial
Peripheral Interface (SPI) section for more information.
The devices are fully accessible and data can be writ-
ten and read when VCC is greater than VPF. However,
when VCC falls below VPF, the internal clock registers
are blocked from any access, and the device power is
switched from VCC to VBAT.
If VPF is less than the voltage on the backup supply, the
device power is switched from VCC to the backup sup-
ply when VCC drops below VPF. If VPF is greater than the
backup supply, the device power is switched from VCC
to the backup supply when VCC drops below the backup
supply. The registers are maintained from the backup
supply source until V_CC is returned to nominal levels.
The Functional Diagram illustrates the main elements.
Freshness Seal Mode
When a battery is first attached to the device, the device
does not immediately provide battery-backup power to
the RTC or internal circuitry. After VCC exceeds VPF,
the devices leave the freshness seal mode and provide
battery-backup power whenever VCC subsequently falls
below VBAT. This mode allows attachment of the battery
during product manufacturing, but no battery capacity is
consumed until after the system has been activated for
the first time. As a result, minimum battery energy is used
during storage and shipping.
Oscillator Circuit
The devices use an external 32.768kHz crystal. The
oscillator circuit does not require any external resistors or
capacitors to operate. The DS1343 includes integrated
capacitive loading for a 6pF CL crystal, and the DS1344
includes integrated capacitive loading for a 12.5pF CL
crystal. See the Crystal Parameters table for the external
crystal parameters. The Functional Diagram shows a
simplified schematic of the oscillator circuit. The startup
time is usually less than one second when using a crystal
with the specified characteristics.
Clock Accuracy
When running from the internal oscillator, the accuracy of
the clock is dependent upon the accuracy of the crystal
and the accuracy of the match between the capacitive
load of the oscillator circuit and the capacitive load for
which the crystal was trimmed. Additional error is added
by crystal frequency drift caused by temperature shifts.
External circuit noise coupled into the oscillator circuit
can result in the clock running fast. Figure 1 shows a
typical PCB layout for isolation of the crystal and oscil-
lator from noise. Refer to Application Note 58: Crystal
Considerations with Dallas Real-Time Clocks for detailed
information.
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