The 脡lan鈩C300 and 脡lanSC310 microcontrollers (referred to in this document as 脡lanSC300)
have a few issues that need to be addressed when using a back-up battery as a secondary power
source for Micro Power Off mode. The three issues covered in this application note are the basic
function of the 脡lanSC300 microcontroller鈥檚 Micro Power Off mode, the difference between the
脡lanSC300 microcontroller鈥檚 RTC and a 146818A-compatible RTC, and using a back-up battery in
an 脡lanSC300 microcontroller-based system.
鈩?/div>
SC300 microcontroller includes a built-in
146818A-compatible real-time clock (RTC) with
114 bytes of static random access memory (SRAM).
The RTC SRAM is designed to hold configuration data
and to maintain the accurate time and date when the
rest of the system is powered down. This state is called
the Micro Power Off mode.
Micro Power Off mode allows the system to conserve
battery power by removing all power to all system com-
ponents and the 脡lanSC300 microcontroller except for
the AVCC and VCC pins. Maintaining power on these
pins allows the RTC to remain powered up, preventing
the system from losing its date, time, and system con-
figuration data. This feature allows an AT-compatible
system to be implemented without using an external
RTC device.
input is derived from the main system power source,
and when active will reset all internal registers in the
脡lanSC300 microcontroller. This causes an issue with
the power-loss bit (VRT), Index 0DH, bit 7 of the RTC
map. The VRT bit is intended to provide a method of
determining when the RTC core voltage supply has
dropped below an acceptable level.
On a 146818A-compatible device, anything below
2.4 V will cause a low-battery condition and will cause
the power-loss bit to go Low. On the 脡lanSC300 micro-
controller the 32-KHz clock used by the RTC to main-
tain time stops oscillating before the VRT bit or RAM
contents get cleared because the VRT bit will only get
cleared when the RESIN pin is asserted Low. Thus, the
RTC time will be inaccurate even though the RAM con-
tents are valid and the VRT bit is still set.
Real-Time Clock
The 脡lanSC300 microcontroller鈥檚 RTC is designed to
operate properly while in Micro Power Off mode at volt-
ages down to 2.4 V, with the power consumption typi-
cally around 30 碌A(chǔ). (For register settings that will affect
power consumption during Micro Power Off mode, see
the power consumption data in the
脡lan
鈩?/div>
SC300 Mi-
crocontroller Data Sheet
, order #18514 and the
脡lan
鈩?/div>
SC310 Microcontroller Data Sheet,
order #20668.) Any source below 2.4 V will not guaran-
tee proper functionality, which could mean the loss of
date, time, and system configuration data.
There are a few differences between the 脡lanSC300
microcontroller鈥檚 RTC and a 146818A-compatible de-
vice. The 脡lanSC300 microcontroller鈥檚 RTC relies on
the VCC and AVCC planes to remain powered, rather
than on a single V
CC
powering an external RTC device.
A 146818A-compatible device has a dedicated reset
pin that is used mainly to monitor the 146818A V
CC
and
will reset only the RTC when a low-power condition is
detected. The 脡lanSC300 microcontroller鈥檚 RTC uses
the RESIN input pin for resetting the RTC. The RESIN
Note:
Although the 32-KHz clock stops oscillating be-
fore the power-loss bit is cleared, this event occurs well
below the 2.4-V specification for proper 脡lanSC300 mi-
crocontroller functionality.
This condition presents a problem because the BIOS
will usually perform a checksum of the RAM contents
or look at the VRT bit in order to determine the validity
of the configuration data and the RTC time and date
data, then notify you that they are no longer valid.
Suggested Workaround:
s
Use the system to manually determine that the
clock has stopped by comparing the current time to
the DOS time.
s
Implement a comparator circuit with the output con-
nected to the RESIN input of the 脡lanSC300 micro-
controller to allow comparison of the battery voltage
(at the VCC Core and AVCC pins of the 脡lanSC300
microcontroller) to a reference voltage (2.4 V) only
when the system is powered up by the main power
source. Driving a pulse on RESIN will clear the VRT
bit, thus allowing the BIOS to detect any subse-
quent low-battery condition by reading Index Regis-
ter 0DH, bit 7 (VRT).
This document contains information on a product under development at Advanced Micro Devices. The information
is intended to help you evaluate this product. AMD reserves the right to change or discontinue work on this proposed
product without notice.
Publication#
20746
Rev:
A
Amendment/0
Issue Date:
June 1997
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