DS80C320-MCG Datasheet, PDF (163/175 Pages) Dallas Semiconductor – High-Speed Microcontroller User Guide

High-Speed Microcontroller User’s Guide
SECTION 15: BATTERY BACKUP
The DS87C530 incorporates a feature which can maintain timekeeping and on-chip SRAM contents in
the absence of V_CC. An external energy source such as a lithium battery or 0.47 F 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 will automatically enter 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 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_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 will be above V_BAT , so no problem will
occur. Suppose that a loss of power occurs, and V_CC begins to drop. Under normal circumstances, the
device will continue to operate until it reaches V_RST (4.0V to 4.25V), at which time device operation will
halt. If V_BAT is higher than V_RST , however, RTC and SRAM access will be 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.
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
MANUFACTURER MODEL NUMBER
TYPE
NOMINAL
VOLTAGE
Panasonic
CR1620
Lithium/Manganese Dioxide
3V
Phone:
+1-201 348–5266
CR1616
CR1220
BR1616
Lithium/Manganese Dioxide
3V
Lithium/Manganese Dioxide
3V
Lithium/poly–carbon monofluoride
3V
BR1225
Lithium/poly–carbon monofluoride
3V
CAPACITY
70 mAh
50 mAh
35 mAh
48 mAh
38 mAh
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.
Backup current is a function of temperature, and therefore battery life is dependent on the operating
environment.
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