ST AN968 Application note

AN968

APPLICATION NOTE

VMEbus TIMEKEEPER Requirements

Surface-mount TIMEKEEPER devices, from STMicroelectronics, are designed explicitly for inclusion on
CPU boards. A typical arrangement is shown in Figure 1.

Figure 1. CP U mo dule with a TIME K E E PE R D e vi ce

Local Microprocessor

Bus Interface

Shared Memory

PCI Bus

Bridge

ISA Bus

TIMEKEEPER

VMEbus

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AN968 - APPLICATION NOTE

Each member of the TIMEKEEPER family provid e s su ppo r t for the follo w ing function s:

■ General timekeeping (i.e. seconds, hour, date, year,...)

■ Time stamping (data collection correlated against time)

■ Event sequencing during the manufacturing proces s

■ Watchdog (to monitor for processes in the system that are out of control)

■ RST: reset signal available for the CPU

■ Non-volatile RAM (to store start-up parameters, ethernet address, etc.)

Most of the members of the TIMEKEEPER family (M48T02, M48T18, M48T559, M48T5X and M48T3X)
are already being used by manufacturers of VME (Versa Module Eurocard) boards. However, the ones

that are best suited for new ‘design-in’ boards are the M48T59, M48T58 and the M48T559. These have
the following features:

■ Automatic chip deselect and write protect when the power fails (as described in Application Note

AN1012

■ BCD coded year, month, day, date, hours, minutes, and seconds (as described in Application Note

AN923

■ Software calibration for increased clock accuracy (as described in Application Note

■ 8K x 8 non-volatile static RAM (as described in Application Note

■ Microprocessor power-on reset (devices with 16-Byte register maps, only) (as described in Application

Note

■ Battery low warning software bit (devices with 16-Byte register maps, only)

■ WDF interrupt function (devices with 16-Byte register maps, only) (as described in Application Note

AN924

■ Periodic alarms (devices with 16-Byte register maps, only)

■ Industry’s first fully surface mountable TIMEKEEPER solution (as described in Application Note

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■ Industry’s smallest non-volatile TIMEKEEPER footprint (as shown in Application Note

■ Address/Address/Data Multiplex (M48T559 only) prevents the need for additional latch circuitry (as

described in Application Note

However, the significant feature is that each is available in ST’s revolutionary SNAPHAT package: a
standard SOIC that is surface mounted in the conventional way, and then is fitted with a SNAPHAT top,
as shown in Figure 2, containing the battery and crystal.

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Figure 2. SOIC and SNAPHAT Package

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