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1. DESCRIPTION
The DS1643-100+ is an 8K x 8 nonvolatile static RAM with a full function Real Time Clock (RTC) that are both accessible in a byte-wide format. The nonvolatile timekeeping RAM is functionally equivalent to any JEDEC standard 8K x 8 SRAM. The device can also be easily substituted in ROM, EPROM and EEPROM sockets providing read/write nonvolatility and the addition of the real time clock function. The real time clock information resides in the eight uppermost RAM locations. The RTC registers contain year, month, date, day, hours, minutes, and seconds data in 24-hour BCD format. Corrections for the day of the month and leap year are made automatically. The RTC clock registers are double-buffered to avoid access of incorrect data that can occur during clock update cycles. The double-buffered system also prevents time loss as the timekeeping countdown continues unabated by access to time register data. The DS1643 also contains its own power-fail circuitry, which deselects the device when the VCC supply is in an out of tolerance condition. This feature prevents loss of data from unpredictable system operation brought on by low VCC as errant access and update cycles are avoided.
2. FEATURES
1. Integrated NV SRAM, Real-Time Clock, Crystal, Power-Fail Control Circuit and Lithium Energy Source
2. Clock Registers are Accessed Identically to the Static RAM. These Registers Reside in the Eight Top RAM Locations.
3. Totally Nonvolatile with Over 10 Years of Operation in the Absence of Power
4. Access Times of 85ns and 100ns
5. BCD-Coded Year, Month, Date, Day, Hours, Minutes, and Seconds with Leap Year Compensation Valid Up to 2100
6. Power-Fail Write Protection Allows for ±10% VCC Power Supply Tolerance
7. Lithium Energy Source is Electrically Disconnected to Retain Freshness Until Power is Applied for the First Time
8. DS1643 Only (DIP Module) Standard JEDEC Byte-Wide 8K x 8 RAM Pinout
9. UL Recognized
10. DS1643P Only (PowerCap Module Board) Surface Mountable Package for Direct Connection to PowerCap Containing Battery and Crystal Replaceable Battery (PowerCap)
11. Power-Fail Output Pin-for-Pin Compatible with Other Densities of DS164XP Timekeeping RAM
3. PIN CONFIGURATIONS
4. PIN DESCRIPTION
5. CLOCK OPERATIONS—READING THE CLOCK
While the double-buffered register structure reduces the chance of reading incorrect data, internal updates to the DS1643 clock registers should be halted before clock data is read to prevent reading of data in transition. However, halting the internal clock register updating process does not affect clock accuracy. Updating is halted when a one is written into the read bit, the seventh most significant bit in the control register. As long as a 1 remains in that position, updating is halted. After a halt is issued, the registers reflect the count, that is day, date, and time that was current at the moment the halt command was issued. However, the internal clock registers of the double-buffered system continue to update so that the clock accuracy is not affected by the access of data. All of the DS1643 registers are updated simultaneously after the clock status is reset. Updating is within a second after the read bit is written to 0.
6. BATTERY LONGEVITY
The DS1643 has a lithium power source that is designed to provide energy for clock activity, and clock and RAM data retention when the VCC supply is not present. The capability of this internal power supply is sufficient to power the DS1643 continuously for the life of the equipment in which it is installed. For specification purposes, the life expectancy is 10 years at 25C with the internal clock oscillator running in the absence of VCC power. Each DS1643 is shipped from Dallas Semiconductor with its lithium energy source disconnected, guaranteeing full energy capacity. When VCC is first applied at a level greater than VPF, the lithium energy source is enabled for battery backup operation. Actual life expectancy of the Ds1643 will be much longer than 10 years since no lithium battery energy is consumed when VCC is present.
