Maxim Integrated Secure Microcontroller - Manual (Page 61)

Secure Microcontroller User’s Guide
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is conservative. This gives a total data retention current of 2475nA. In this system, a Rayovac BR2325
with a capacity of 180mAh is used.
Note that these ratings are for continuous data retention so V
CC
is assumed absent for the entire period.
The lifetime will increase based on the ratio of time when V
CC
is applied vs. data retention time.
Using Lithium Cells
In the vast majority of soft/secure microcontroller applications, lithium cells are the preferred battery.
Their voltage varies only slightly over its useful life; a CR chemistry begins life at +3.3V and drops to
+2.9V near the end of life. Although some users choose to incorporate battery clips so that lithium cells
can be replaced, this is not recommended since such clips are susceptible to shock and vibration and
could result in a corruption of program or data memory. Therefore, 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 maximize the lifetime of the backup battery. These
devices incorporate a solid-state freshness seal that electrically isolates the battery from any loading when
systems do not require data retention, such as a completely assembled but unprogrammed system stored
in inventory. Since data retention is not required, there is no need to draw any current from the battery.
Thus even in the absence of power, the SRAM and RTC leakage currents are not drawn from the battery
while 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]. All secure microcontroller are shipped
with the freshness seal applied. In the case of a DS5001/DS5002 series device, the freshness seal can be
reapplied via the bootstrap loader at any time. To invoke the freshness seal on a DS5001, DS5002 series
device, the “N” command should be issued to the bootstrap loader.
To clear the freshness seal, simply apply V
CC
. On a DS5000 series device, the user cannot restore the
freshness seal. Therefore, if freshness seal is desired for storage, the part should not be powered up when
received or installed.
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 activate internal parasitic diodes that draw current directly from
the battery. If a device pin is connected to the “outside world” where it can be handled or come in contact
with electrical noise, protection should be added to prevent 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.
180 x 10
-3
= 180 x 10
-3
(2400 + 75) x 10 -9 x 24 x 365) = 21.68 x 10 -3
= 8.3 years