M41T256Y

The M41T256Y Serial TIMEKEEPER^® SRAM is a low power 256 Kbit static CMOS SRAM organized as 32K words by 8 bits. A built-in 32.768 kHz oscillator (external crystal controlled) and 8 bytes of the SRAM (see Figure 9, page 17) are used for the clock/calendar function and are configured in binary coded decimal (BCD) format.

Addresses and data are transferred serially via a two line, bi-directional I²C interface. The built-in address register is incremented automatically after each WRITE or READ data byte.

The M41T256Y has a built-in power sense circuit which detects power failures and automatically switches to the battery supply when a power failure occurs. The energy needed to sustain the SRAM and clock operations can be supplied by a lithium button-cell supply when a power failure occurs. Functions available to the user include a non-volatile, time-of-day clock/calendar, and Power- on Reset. The eight clock address locations contain the year, month, date, day, hour, minute, second, and tenths/hundredths of seconds in 24- hour BCD format. Corrections for 28, 29 (leap year - valid until year 2100), 30 and 31 day months are made automatically. The first clock address location (7FF8h) stores the clock software calibration settings as well as the Write Clock Bit.

The M41T256Y is supplied in a 44-lead SOIC SNAPHAT^® package (MH - which integrates both crystal and battery in a single SNAPHAT top) or a 44-pin "hatless" SOIC (MT). The 44-pin, 330mil SOIC provides sockets with gold-plated contacts at both ends for direct connection to a separate SNAPHAT housing containing the battery and crystal. The unique design allows the SNAPHAT battery/crystal package to be mounted on top of the SOIC package after the completion of the surface- mount process.

Insertion of the SNAPHAT housing after reflow prevents potential battery and crystal damage due to the high temperatures required for device surface- mounting. The SNAPHAT housing is also keyed to prevent reverse insertion. The 44-pin SOIC and crystal/battery packages are shipped separately in plastic, anti-static tubes or in Tape & Reel form. For the 44-lead SOIC, the battery/crystal package (e.g., SNAPHAT) part number is "M4Txx-BR12SH" (see Table 15, page 24).

Caution: Do not place the SNAPHAT battery/crystal top in conductive foam, as this will drain the lithium, button-cell battery.