M48T201Y_09 Datasheet, PDF (10/37 Pages) STMicroelectronics – 5.0 or 3.3 V TIMEKEEPER® supervisor

Operation
2
Operation
M48T201Y, M48T201V
Automatic backup and write protection for an external SRAM is provided through V_OUT,
E_CON, and G_CON pins. (Users are urged to insure that voltage specifications, for both the
supervisor chip and external SRAM chosen, are similar.) The SNAPHAT^® containing the
lithium energy source is used to retain the RTC and RAM data in the absence of V_CC power
through the V_OUT pin. The chip enable output to RAM (E_CON) and the output enable output
to RAM (G_CON) are controlled during power transients to prevent data corruption. The date
is automatically adjusted for months with less than 31 days and corrects for leap years (valid
until 2100). The internal watchdog timer provides programmable alarm windows.
The nine clock bytes (7FFFFh-7FFF9h and 7FFF1h) are not the actual clock counters, they
are memory locations consisting of BiPORT™ READ/WRITE memory cells within the static
RAM array. Clock circuitry updates the clock bytes with current information once per second.
The information can be accessed by the user in the same manner as any other location in
the static memory array. Byte 7FFF8h is the clock control register. This byte controls user
access to the clock information and also stores the clock calibration setting.
Byte 7FFF7h contains the watchdog timer setting. The watchdog timer can generate either a
reset or an interrupt, depending on the state of the watchdog steering bit (WDS). Bytes
7FFF6h-7FFF2h include bits that, when programmed, provide for clock alarm functionality.
Alarms are activated when the register content matches the month, date, hours, minutes,
and seconds of the clock registers. Byte 7FFF1h contains century information. Byte 7FFF0h
contains additional flag information pertaining to the watchdog timer, the alarm condition,
the battery status and square wave output operation. 4 bits are included within this register
(RS0-RS3) that are used to program the square wave output frequency (see Table 7 on
page 21). The M48T201Y/V also has its own power-fail detect circuit. This control circuitry
constantly monitors the supply voltage for an out of tolerance condition. When V_CC is out of
tolerance, the circuit write protects the TIMEKEEPER^® register data and external SRAM,
providing data security in the midst of unpredictable system operation. As V_CC falls below
the battery backup switchover voltage (V_SO), the control circuitry automatically switches to
the battery, maintaining data and clock operation until valid power is restored.
2.1
Address decoding
The M48T201Y/V accommodates 19 address lines (A0-A18) which allow direct connection
of up to 512 K bytes of static RAM. Regardless of SRAM density used, timekeeping,
watchdog, alarm, century, flag, and control registers are located in the upper RAM locations.
All TIMEKEEPER registers reside in the upper RAM locations without conflict by inhibiting
the G_CON (output enable RAM) signal during clock access. The RAM's physical locations
are transparent to the user and the memory map looks continuous from the first clock
address to the upper most attached RAM addresses.
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