ST AN1021 Application note
AN1021
APPLICATION NOTE
Using the M40Z300W SUPERVISOR to Gain x16
NVRAM Functionality from Two 128K x 8 SRAMs
INTRODUCTION
The M40Z300W from STMicroelectronics, Inc., can be used to make SRAM devices behave like non-volatile RAM (NVRAM). When the supply voltage, V
protects the RAM, switches to its internal battery supply, and asserts the reset line. Figure 1 shows the
arrangement for controlling two “128K x 8” devices now arranged as 128K x 16.
As soon as V
is found to be below the appropriate threshold value, the M40Z300W performs three vital
CC
functions:
1. It switches the SRAM devices to being write protected;
2. It switches the SRAM devices to being powered by the battery; and
3. It drives the reset line, RST
It also drives the BL
line low if the internal battery voltage becomes less than 2.5 volts. This is monitored
, low.
during every power-up and every 24-hour interval (while V
Figure 1. Block diagram for 128K x16 battery-backed SRAM Configuration
V
CC
, falls below the preset threshold level, the device write
CC
is valid).
CC
0.1µF
1N5817
V
CC
M40Z300W
A
B
E
THS
V
SS
E1
E2
E3
E4
V
OUT
CON
CON
CON
CON
RST
BL
0.1µF
0.1µF
To Microprocessor
To Battery Monitor Circuit
V
CC
CMOS
SRAM
(128K x 8)
CE
V
CC
CMOS
SRAM
(128K x 8)
CE
DQ0 - DQ7
DQ8 - DQ16
AI02400
1/2October 2001
AN1021 - APPLICATION NOTE
CONTACT INFORMATION
If you have any questions or suggestions concerning the matters raised in this document, please send
them to the following electronic mail addresses:
apps.nvram@st.com
ask.memory@st.com
(for application support)
(for general inquiries)
Please remember to include your name, company, location, telephone num ber, and fax number.
Information furnished is believed to be accurate and reliable. However, STMicroelec tronics as sumes no responsibility for the consequences
of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted
by implic ation or o th erwise u nder any pat ent or pat ent righ ts of STMicroelectron i cs. Specifications m entioned i n this publication are s ubject
to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not
authorized for use as c ritical components in li f e support dev i ces or systems without express writ t en approval of STMicroel ectronics.
The ST log o i s registered trademark of STMic roelectronics
All other names are the property of their respective owners.
© 2001 STMicroelectronics - All Rights Reserved
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