Lexington, Mass.--July 6, 1998--The latest member of the Timekeeper family of non-volatile Static RAMs fromSTmicroelectronics (Lexington) is the M48T559Y Snaphat.
The SRAM array utilizes low standby power, 4-transistor SRAM cells while the integral power-failcircuit monitors the Vcc line for an out of tolerance condition,switching automatically to the lithium energy source if this conditionoccurs. At the same time, the chip is switched to its write-protected mode,eliminating the possibility of spurious write cycles. To maximize itsoperating lifetime, the lithium cell is electrically disconnected untilpower is applied for the first time. The technique allows non-volatile data retention to be achieved withunlimited write cycles, a minimum data retention of ten years, and a standardSRAM access.
The M48T559Y reduces the quantity of processor I/O ports required foraddress/data by incorporating a multiplexed address/address/data I/Oarchitecture. That approach eliminates the need for latches required tointerface to a multiplexed I/O processor.
The M48T559Y also contains a watchdog input and two debounced reset inputs.The watchdog timer can check for an out of control processor by periodicallyupdating the watchdog registers or it can monitor for a hung bus conditionby using the watchdog input pin. One of the reset input pins has a debouncecycle optimized for a hardware input such as a push button, while the otherpin is optimized for a one shot pulse such as a soft reboot from a keyboard.The reset output goes valid for either of these two input conditions orwhenever Vcc falls below the write protect voltage threshold and is validover all voltage conditions.
The Snaphat package consists of two portions: the SOICbase and a removable Snaphat top housing the crystal and battery, which aretherefore protected from the high temperatures experienced during solderreflow. The removable Snaphat battery/crystal also allows for replacement ofexpended lithium cells, which can then be disposed of in an environmentallysafe manner.
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