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 CC, falls below the preset threshold level, the device write 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 CC is found to be below the appropriate threshold value, the M40Z300W performs three vital 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, low. It also drives the BL line low if the internal battery voltage becomes less than 2.5 volts. This is monitored during every power-up and every 24-hour interval (while V CC is valid). Figure 1. Block diagram for 128K x16 battery-backed SRAM Configuration
VCC 1N5817 VCC 0.1F M40Z300W VOUT 0.1F VCC CMOS SRAM (128K x 8) CE
DQ0 - DQ7
A B
E1CON E2CON E3CON E4CON 0.1F
VCC CMOS SRAM (128K x 8) DQ8 - DQ16
E THS VSS RST BL To Microprocessor
CE
To Battery Monitor Circuit
AI02400
October 2001
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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 number, and fax number.
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes 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 implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is registered trademark of STMicroelectronics All other names are the property of their respective owners. 2001 STMicroelectronics - All Rights Reserved STMicroelectronics GROUP OF COMPANIES Australia - Brazil - China - Finland - France - Germany - Hong Kong - India - Italy - Japan - Malaysia - Malta - Morocco Singapore - Spain - Sweden - Switzerland - United Kingdom - U.S.A. www.st.com
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