Section 6 Lithium/Battery Backup - Mitsubishi DS907x SIP User Manual

USER'S GUIDE
SECTION 6: LITHIUM/BATTERY BACKUP
Soft Microcontroller devices are lithium backed for data
retention in the absence of V
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 (V ) begins to fail, the processor gener-
CC
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
(V ) is switched from V
CCO
subsequently ignored, except for comparators that
monitor its level. Lithium backed chip enables are main-
050396 55/173
. In the Soft Microcon-
CC
to the lithium cell. V
CC CC
tained at a logic high state with lithium power, but non–
backed chip enables follow V
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 V
age threshold and is still above the lithium voltage. If the
power supply slews between these threshold voltages
faster than 40 s (130 s 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
is lithium backup. Figure 6–1 illustrates the power supply
conditions that should be met.
56
down. Individual prod-
CC
is falling below the reset volt-
CC