Mitsubishi DS907x SIP, DS5000TK LITHIUM/BATTERY Backup, Data Retention, Battery Backed Circuits

USER'S GUIDE

SECTION 6: LITHIUM/BATTERY BACKUP

Soft Microcontroller devices are lithium backed for data retention in the absence of VCC. In the Soft Microcon- troller the state of the microcontroller is also maintained, unlike a conventional processor system using an exter- nal NV RAM. This section is a comprehensive discus- sion of the lithium back up feature. It covers system design, battery attach procedure, I/O pin restrictions, lifetime calculations, and battery/RAM size tradeoffs. Some of the information is unnecessary to module users but will provide background information for proper handling and system design. Each section will highlight both chip and module considerations when there are dif- ferences.

When properly used, lithium backed microcontrollers provide better than 10 years of data retention in the absence of power. This means that a total of over 10 years in the absence of power at room temperature is guaranteed. Elevated temperatures cause higher than normal data retention current to be drawn by a RAM. However, these remarks are only relevant to a system that is powered down. While +5V is applied to the device, the lithium cell is isolated from any loading. Therefore, data retention must be viewed in the context of the power supply duty cycle. For example, if a system is rated for 10 years of data retention, but will have power applied for 12 hours per day, the expected life- time is greater than 20 years.

DATA RETENTION

The Secure Microcontroller family provides nonvolatile storage in ordinary SRAM. It accomplishes this by bat- tery±backing the memory in the absence of power. When power (VCC) begins to fail, the processor gener- ates an internal power±fail reset condition as discussed in the next chapter. At this time, SRAM chip enables are taken to a logic high inactive state. Also, I/O port pins also go to a logic high state. If power continues to fall and crosses below the lithium threshold, the microprocessor enters the data retention state, and power is drawn from the lithium cell. The power supply output to the SRAM

(VCCO) is switched from VCC to the lithium cell. VCC is subsequently ignored, except for comparators that

monitor its level. Lithium backed chip enables are main-

tained at a logic high state with lithium power, but non± backed chip enables follow VCC down. Individual prod- uct differences should be observed. Maintaining chip enables at an inactive level and lowering the power sup- ply to approximately +3V causes the NV RAM to enter a data retention state. Thus the combination retains data for a long period as the circuits draw a very small current from the lithium cell. Modules easily attain better than 10 years of data retention. Chip solutions can be designed to achieve a much greater lifetime depending on the user's needs.

BATTERY BACKED CIRCUITS

The Secure Microcontroller is the only computer that is completely lithium backed. This means that both inter- nal configuration and data are preserved when power is removed. However, unlike a simple NV RAM, the micro- processor is an extremely complex circuit that must be fully prepared for lithium backup. Once prepared, the microprocessor is guaranteed to draw less than 75 nA from its backup source. This number is typically 5 nA. The user's selection of RAM will determine the total loading on the lithium cell. In the case of a module, Dal- las has screened the RAM to make certain that the total loading guarantees better than 10 years of data reten- tion for the selected lithium cell at room temperature.

In order to achieve this ultra±low power state, special logic in the microprocessor places all internal nodes in a predictable (low power) state. This occurs during sys- tem power down while VCC is falling below the reset volt- age threshold and is still above the lithium voltage. If the power supply slews between these threshold voltages faster than 40 μs (130 μs for DS5001/2), the circuits may not complete the backup procedure and the micropro- cessor backup current could be substantially greater than 75 nA, and/or program/data corruption could occur. Fortunately, a modest amount of system capacitance is enough to prevent fast slewing. The actual value will depend on the total system loading. This slew rate must be met for either a chip or module solution. In either case, the microprocessor must have time to prepare for lithium backup. Figure 6±1 illustrates the power supply conditions that should be met.

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