DS80C320-ECD DALLAS [Dallas Semiconductor], DS80C320-ECD Datasheet - Page 163

• Manufacturer Part Number: DS80C320-ECD
• Description: High-Speed Microcontroller User Guide
• Manufacturer: DALLAS [Dallas Semiconductor]

SECTION 15: BATTERY BACKUP
The DS87C530 incorporates a feature which can maintain timekeeping and on-chip SRAM contents in
the absence of V
connected to the V
voltage must always be at least a diode drop (0.7V) below V
The DS87C530 will automatically enter data retention mode when V
mode, the RTC and SRAM contents are powered from the energy source connected to the V
electrically isolated from the rest of the device. This means that writes to battery backed SFRs and
SRAM are ignored and reads will 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 will stay below V
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
occur. Suppose that a loss of power occurs, and V
device will continue to operate until it reaches V
halt. If V
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
dropping. The best approach is to carefully select battery voltages to avoid the problem entirely.
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 15-1 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.
SUGGESTED BATTERIES FOR USE WITH DS87C530 Table 15-1
Battery life can be calculated by dividing the rated battery capacity by the I
device specific data sheet. Note that this determines the minimum battery life; while V
device, it draws negligible current from the battery, and so battery life will be lengthened accordingly.
Backup current is a function of temperature, and therefore battery life is dependent on the operating
environment.
MANUFACTURER
[phone]
Panasonic
Phone:
BAT
is higher than V
CC
CC
BAT
. An external energy source such as a lithium battery or 0.47 F super cap can be
of 5.0V. During normal operation, V
pin. The nominal battery voltage should be 3V. For proper operation, the battery
MODEL NUMBER
CR1620
CR1616
CR1220
BR1616
BR1225
RST
CC
, however, RTC and SRAM access will be prohibited before the device
during normal operation to prevent an unplanned lockout of the RTC
Lithium/poly–carbon monofluoride
Lithium/poly–carbon monofluoride
163 of 175
Lithium/Manganese Dioxide
Lithium/Manganese Dioxide
Lithium/Manganese Dioxide
RST
CC
(4.0V to 4.25V), at which time device operation will
begins to drop. Under normal circumstances, the
TYPE
CC
CC
, and is recommended to be below V
will be above V
CC
High-Speed Microcontroller User’s Guide
< V
BAT
BAT
NOMINAL
VOLTAGE
. When in data retention
BAT
current specified on the
3 V
3 V
3 V
3 V
3 V
, so no problem will
CC
is applied to the
CAPACITY
BAT
70 mAh
50 mAh
35 mAh
48 mAh
38 mAh
pin and
RST
CC
.
is