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AN1470 APPLICATION NOTE
Changing from the M24256-A to the M24256-B In Your Application
This document is written for users of the M24256-A and M24256-AW. These devices are now designated as "Not for New Design", and will eventually be withdrawn from production. For the vast majority of applications, the M24256-B and M24256-BW are direct replacements and can be treated as pin compatible, and functionally equivalent. Table 1. Table of Direct or Indirect Replacements
Old Device M24256-A BN M24256-A W BN < 3.6V M24256-A MN M24256-A W MN < 3.6V M24256-A MW M24256-A W MW < 3.6V M24256-A DL M24256-A W DL < 3.6V M24256-B W DL or M24256-B V DL M24256-B W MW or M24256-B V MW M24256-B DL M24256-B W DL TSSOP14 M24256-B W MN or M24256-B V MN M24256-B MW M24256-B W MW SO8 (200mil) M24256-B W BN or M24256-B V BN M24256-B MN M24256-B W MN SO8 (150mil) VCC(max) M24256-B BN M24256-B W BN PDIP8 Replacement Device Package
WHAT ARE THE DIFFERENCES? The difference between the A and B versions of the M24256 is the E2 input. - the M24256-B and M24256-BW have three chip enable lines (E0, E1, E2) - the M24256-A and M24256-AW have two chip enable lines (E0, E1). On the M24256-B and M24256-BW, the E2 signal appears: - on pin 3, for PDIP8 and SO8 packages, and - on pin 6, for the TSSOP14 package. On the M24256-A and M24256-AW, these pins are specified as "Not Connected".
November 2001
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AN1470 - APPLICATION NOTE
The user might, therefore, have adopted any one of the following options the pin (NC of the old device, E2 of the replacement): 1. it might have been left unconnected (without any connection on the PCB) 2. it might be tied to V SS (ground) 3. it might be tied to V CC (supply voltage) 4. it might be tied to some other level 5. it might be tied to a logic signal that the M24256-A ignores The first two are the most likely options to have been adopted by the user. For these, the M24256-B and M24256-BW are direct replacements for the M24256-A and M24256-AW (respectively). The third option is less likely, and requires a change in the user's software when the replacement is made. The last two are the least likely options to have been adopted by the user, and are somewhat more problematic. OPTIONS 1 AND 2: M24256-B AND M24256-BW ARE DIRECT REPLACEMENTS FOR THE M24256-A AND M24256-AW As specified, in their respective data sheets, both versions A and B of the M24256 device have an internal pull-down resistor on each chip enable input. Therefore, regardless of whether E2 of the replacement device is left unconnected in the application, or is explicitly connected to VSS, it is read as 0. (However, the case of the unconnected E2 pin only applies if it is not connected to a PCB track that can serve as an antenna, and pick up signals parasitically). As specified in the data sheet for the M24256-A and M24256-AW, the b3 bit of the Device Select Code must be set to zero. When this software is run, unchanged, on the M24256-B and M24256-BW, it is interpretted as a "chip address" in which E2 is 0. The replacement device automatically has the same "chip address" as the user's software was already setting for the old device. OPTION 3: M24256-B AND M24256-BW DOES NOT DIRECTLY REPLACE THE M24256-A AND M24256-AW If the NC pin of the old device, and hence the E2 pin of its replacement, has been tied to VCC (supply voltage), a change needs to be made to the hardware or the software. So that the user's software specifies the same "chip address" as that now being interpretted by the replacement device, either the hardware needs to be modified so that E2 is left floating or pulled to VSS, or the software needs to be modified so that the Device Select Code has its b3 bit set to 1. OPTIONS 4 AND 5: M24256-B AND M24256-BW DOES NOT READILY REPLACE THE M24256-A AND M24256-AW If the NC pin of the old device is being driven to some other voltage level, or is carrying a logic signal that the M24256-A ignores, a change needs to be made to the hardware. The hardware needs to be modified so that E2 is left unconnected to the PCB, or pulled to VSS. In case of any problems, or questions, users should contact their nearest ST sales office, or email "apps.eeprom@st.com"
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For current information on ST products, please consult our pages on the world wide web: www.st.com If you have any questions or suggestions concerning the matters raised in this document, please send them to the following electronic mail addresses:
apps.eeprom@st.com ask.memory@st.com
(for application support) (for general enquiries)
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 (c) November 2001 STMicroelectronics - All Rights Reserved STMicroelectronics group of companies Austalia - Brazil - Canada - China - Finland - France - Germany - Hong Kong India - Israel - Italy - Japan - Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States. www.st.com
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