Maxim Integrated High-Speed Microcontroller - Manual (Page 161)

High-Speed Microcontroller User’s Guide
Rev: 062210 161 of 176
15. BATTERY BACKUP
The DS87C530 incorporates a feature that can maintain timekeeping and on-chip SRAM contents in the
absence of V
CC
. An external energy source such as a lithium battery or 0.47F super cap can be connected
to the V
BAT
pin. The nominal battery voltage should be 3V. For proper operation, the battery voltage must
always be at least a diode drop (0.7V) below V
CC
, and is recommended to be below V
RST
.
The DS87C530 automatically enters data retention mode when V
CC
< V
BAT
. When in data retention
mode, the RTC and SRAM contents are powered from the energy source connected to the V
BAT
pin and
electrically isolated from the rest of the device. This means that writes to battery-backed SFRs and
SRAM are ignored and reads return erroneous data while in data retention mode. The DS87C530 data
sheet contains a functional diagram of the internal battery switching circuitry.
The data retention switch voltage, the point at which the device switches into data retention mode, is a
function of the battery voltage, not an absolute reference. Care must be taken when selecting a battery so
that its voltage stays below V
CC
during normal operation to prevent an unplanned lockout of the RTC and
SRAM. Although it is unlikely that such a situation would occur, it could become an issue if a relatively
high-voltage battery is used. For example, suppose a 4.5V battery is used with a device operating at a V
CC
of 5.0V. During normal operation, V
CC
is above V
BAT
, so no problem occurs. Suppose that a loss of
power occurs, and V
CC
begins to drop. Under normal circumstances, the device continues to operate until
it reaches V
RST
(4.0V to 4.25V), at which time device operation halts. If V
BAT
is higher than V
RST
,
however, RTC and SRAM access are prohibited before the device enters reset. This means that there may
be a short period of time before reset when the device is operating but could read erroneous data from the
RTC or SRAM or fail to write to them. One solution would be to use the power-fail interrupt to halt reads
or writes to the RTC or SRAM when V
CC
is dropping. The best approach is to carefully select battery
voltages to avoid the problem entirely.
15.1 Selecting a Battery
There are a number of battery chemistries and brands that are suitable for use with battery-backed
members of the high-speed microcontroller family. The use of lithium chemistry batteries, such as
Lithium Manganese Dioxide, is preferred as their nominal voltage is approximately 3.0V. Coin cells are
particularly suited for use with the high-speed microcontroller family because of their capacity, low
profile, and small diameter. Many are available with PC mount tabs attached for automated assembly.
Table 15-A shows a list of some common batteries and their capacities. This list is by no means
exhaustive, and the inclusion or exclusion of any vendor from this list is in no way a comment on the
suitability of a specific battery in a customer’s application.
Table 15-A. Suggested Batteries for the DS87C530
MANUFACTURER MODEL NUMBER TYPE
NOMINAL
VOLTAGE (V)
CAPACITY
(mAh)
CR1620 Lithium/Manganese Dioxide 3 70
CR1616 Lithium/Manganese Dioxide 3 50
CR1220 Lithium/Manganese Dioxide 3 35
BR1616 Lithium/Polycarbon Monofluoride 3 48
Panasonic
BR1225 Lithium/Polycarbon Monofluoride 3 38
Battery life can be calculated by dividing the rated battery capacity by the I
BAT
current specified on the
device specific data sheet. Note that this determines the minimum battery life; while V
CC
is applied to the
device, it draws negligible current from the battery, and so battery life will be lengthened accordingly.