Mitsubishi DS5000TK, DS907x SIP manual Lithium Battery Usage, Freshness Seal

USER'S GUIDE

LITHIUM BATTERY USAGE

In the vast majority of applications, lithium batteries pro- vide a reliable means of backing up data and configura- tion. The voltage varies only slightly over its useful life, so it is difficult to measure capacity. A CR chemistry will begin life at 3.3V and drop to 2.9V near the end of life. As a consequence, some users choose to incorporate battery clips so that lithium cells are easily replaced. This is not recommended since such clips are suscepti- ble to shock and vibration. It is possible that the connec- tion to a lithium cell would be momentarily lost during such a shock, resulting in a potential loss of data. There- fore, soldered battery tabs are recommended. If a user elects to use a battery clip with a capacitor (to support momentary disconnect), the leakage of the capacitor should be considered in the lifetime calculations.

FRESHNESS SEAL

The Secure Microcontroller family is designed to maxi- mize the lifetime of a lithium backup source. The circuits described above contribute to a long life. There is one further provision that will benefit users that intend to store their systems in an unpowered state, but that do not require it to retain data during this period. An exam- ple might be a completed system stored in inventory. Since data retention is not required, there is no benefit to using even the modest lithium current that will normally be drawn. For this reason, Secure Microcontrollers

incorporate the Freshness Seal. The Freshness Seal electrically isolates the lithium cell from any external loading. Thus even in the absence of power, the SRAM and Real±Time Clock leakage currents will not be drawn from the lithium cell for as long as the Freshness Seal is applied.

This feature is available to module users of the DS5000 series [DS5000(T), DS2250T] and all users of the DS5001/2 series [DS5001FP, DS5002FP, DS2251T, DS2252T]. In the case of DS5000 and DS2250 mod- ules, the factory ships these with the Freshness Seal applied. In the case of a DS5001, DS5002 series device, the Freshness Seal can be applied via the Boot- strap Loader at any time. Thus if the Freshness Seal is not removed, the time that a Secure Microcontroller based system is stored in inventory will not reduce the data retention lifetime since the lithium cell is unloaded.

To clear the Freshness Seal, simply apply VCC. On a DS5000 series device, the Freshness Seal can not be restored by the user. Therefore, if Freshness Seal is desired for storage, the part should not be powered up when received or installed. Since a DS5001/DS5002 series device can invoke the Freshness Seal via the Loader, this restriction does not apply. To invoke the Freshness Seal on a DS5001, DS5002 series device, the ªNº command should be issued to the Bootstrap Loader.

IMPORTANT APPLICATION NOTE

The pins on a Secure Microcontroller chip or module are generally as resilient as other CMOS circuits. They have no unusual susceptibility to electrostatic discharge (ESD) or other electrical transients. However, no pin on a Soft Microcontroller chip or module should ever be taken to a voltage below ground. Negative voltages on any pin can turn on internal parasitic diodes that draw current directly from the battery. If a device pin is connected to the ªoutside worldº where it may be handled or come in contact with electrical noise, protection should be added to pre- vent the device pin from going below ±0.3V. It is also common for power supplies to give a small undershoot on power up, which should be prevented. Application Note 93, Design Guidelines for Microcontrollers Incorporating NV RAM, discusses how to protect devices against these conditions.

050396 58/173

59