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DS1343E-18+ Datasheet(PDF) 10 Page - Maxim Integrated Products |
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DS1343E-18+ Datasheet(HTML) 10 Page - Maxim Integrated Products |
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10 / 20 page  Low-Current SPI/3-Wire RTCs 10 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 VCC 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. |
Similar Part No. - DS1343E-18+ |
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Similar Description - DS1343E-18+ |
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