HSSEC15.PDF

(16 KB) Pobierz
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
Panasonic
Phone:
+1-201 348–5266
MODEL NUMBER
CR1620
CR1616
CR1220
BR1616
BR1225
TYPE
Lithium/Manganese Dioxide
Lithium/Manganese Dioxide
Lithium/Manganese Dioxide
Lithium/poly–carbon monofluoride
Lithium/poly–carbon monofluoride
NOMINAL
VOLTAGE
3V
3V
3V
3V
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.
163 of 175
High-Speed Microcontroller User’s Guide
The registers shown in Table 15-2 are battery-backed, and one or more bits will be indeterminate
following a no-battery reset. They should be initialized as part of a no-battery reset procedure.
BATTERY BACKED SFRS
Table 15-2
REGISTER NAME
TRIM
RTASS
RTAS
RTAM
RTAH
RTCC
RTCSS
RTCS
RTCM
RTCH
RTCD0
RTCD1
LOCATION
96h
F2h
F3h
F4h
F5h
F9h
FAh
FBh
FCh
FDh
FEh
FFh
LITHIUM BATTERY CONSIDERATIONS
Lithium primary (non-rechargeable) batteries can fail and/or rupture if subjected to reverse current from
the device they are powering. The battery-switching circuitry inside the DS87C530 was designed to
reduce or eliminate the need for external hardware required to meet battery safety regulations. As shown
in the DS87C530 Data Sheet, a current limiting resistor is always in series with a switching field-effect
transistor, regardless of whether the DS87C530 is drawing current from V
CC
or V
BAT
pins. This satisfies
the two mechanism requirement of most safety codes.
164 of 175

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika:

Zgłoś jeśli naruszono regulamin