AN320 Migrating from FM25256B to FM25W256 Author: Girija Chougala Associated Project: No Associated Part Family: FM25256B, FM25W256 Software Version: None Related Documents: For a complete list, click here AN320 discusses the key differences that need to be considered when migrating from FM25256B to FM25W256. FM25256B is now obsolete and this application note explains how FM25W256 is a replacement for FM25256B. Table 1 shows the compatibility chart of FM25256B and FM25W256. For a detailed comparison, see Table 3. Introduction FM25W256, a 256-Kbit SPI F-RAM™, is a replacement device for FM25256B, which is now obsolete. The two devices are identical in terms of pinout, package composition and dimensions, read/write functionality, Write Protect operation, and address pin functionality. This application note discusses the key differences between the two devices that need to be considered when migrating from FM25256B to FM25W256. Drop-In Replacement or Not? From a software point of view, the two devices are identical. From a hardware point of view the key differences are the lower active and standby currents in FM25W256. There is a potential issue when replacing an FM25256B in a 5 V system with an FM25W256. The FM25W256 operates down to 2.7 V and will provide read and write accesses down to this voltage. If the system controller does unpredictable things at VDD levels below 4 V (which is the FM25256B VDD(min) spec) and has relied on the undocumented lockout feature that the FM25256B provides, then the system designer must investigate this as a potential problem. Otherwise the controller could potentially overwrite data unintentionally in the FM25W256 at voltages below the VDD(min) specification of FM25256B. www.cypress.com Table 1. Compatibility Chart FM25256B Feature or Spec Is FM25W256 compatible? Package Yes Pinout Yes Temperature Range Yes Operating Voltage Yes Operating Current Yes Standby Current Yes Read / Write Function Yes Timing / Frequency Yes Data Retention Yes Endurance Yes Document No. 001-86843 Rev. *B 1 Migrating from FM25256B to FM25W256 Ordering Part Numbers Table 2 gives the recommended FM25W256 ordering part numbers that correspond to the now obsolete FM25256B ordering part numbers. Table 2. Recommended Ordering Part Numbers for Migration FM25256B Ordering Part Number FM25256B -G FM25W256 Status Comments Ordering Part Number Obsolete In production FM25W256 -G FM25256B -GTR Status FM25W256 -GTR No hardware or software change is required Comparison of FM25256B and FM25W256 Table 3 gives a detailed comparison of the two devices. Table 3. Detailed Comparison Package Types Package Outlines Pinout Temperature Range Operating Voltage Range Active Supply Current Standby Current Read / Write Function Clock Frequency FM25256B FM25W256 -G -G SOIC-8 SOIC-8 Comments Identical “green” SOIC package Identical outline and board footprint - - Identical –40 ºC to +85 ºC –40 ºC to +85 ºC Identical 4.0 V to 5.5 V 2.7 V to 5.5 V 750 µA @ 1 MHz 250 µA @ 1 MHz 15 mA @ 20 MHz 2 mA @ 20 MHz 150 µA 30 µA - - 20 MHz 20 MHz FM25W256 has a wider operating voltage range FM25W256 offers lower active current at all frequencies FM25W256 has lower standby current Identical 2-byte addressing, Identical op-codes, Identical status register Identical 10 years (+85 ºC) Data Retention 10 years (+85 ºC) 38 years (+75 ºC) Identical 151 years (+65 ºC) Endurance (Write/Read Cycles) VDD Power-Up Ramp Rate (tVR) VDD Power-Down Ramp Rate (tVF) Power-Up to First Access (tPU) 1E+14 1E+14 50 µs / V 30 µs / V Improved power-up ramp rate in FM25W256 30 µs / V Improved power-down ramp rate in FM25W256 50 µs / V (VDD ≥ 2 V) 1 ms / V (VDD < 2 V) 10 ms 1 ms Identical Faster first access in FM25W256 Critical Considerations You should consider all the parameter differences mentioned in Table 3 during the migration to FM25W256. This section discusses the critical differences. From the table, it can be inferred that FM25W256 is better than FM25256B in all parameters. Note that the FM25W256 allows read / write operation up to 2.7 V while the FM25256B is guaranteed to work up to 4 V. System designers should also review the datasheet when migrating to the new part. www.cypress.com Document No. 001-86843 Rev. *B 2 Migrating from FM25256B to FM25W256 Summary AN320 discussed the differences between FM25256B and FM25W256 that need to be considered during migration to the FM25W256. Related Documents Datasheet FM25W256: 256-Kbit (32 K × 8) Serial (SPI) F-RAM datasheet Application Note AN304 – SPI GUIDE FOR F-RAM www.cypress.com Document No. 001-86843 Rev. *B 3 Migrating from FM25256B to FM25W256 Document History Document Title: Migrating from FM25256B to FM25W256 - AN320 Document Number: 001-86843 Revision ECN Orig. of Change Submission Date Description of Change ** 3944550 GVCH 03/26/2013 New Spec. *A 4279018 MEDU 02/12/2014 Updated to Cypress Template. Updated the “VDD Power-Down Ramp Rate” for FM25W256 from 100 µs / V to 30 µs / V. *B 4498653 GVCH 09/22/2014 Changed title from “Differences between FM25256B and FM25W256” to “Migrating from FM25256B to FM25W256.” Updated abstract. Added “Ordering Part Numbers” section. Added title for Table 3. Added “Related Documents” section. www.cypress.com Document No. 001-86843 Rev. *B 4 Migrating from FM25256B to FM25W256 Worldwide Sales and Design Support Cypress maintains a worldwide network of offices, solution centers, manufacturer’s representatives, and distributors. To find the office closest to you, visit us at Cypress Locations. PSoC® Solutions Products Automotive cypress.com/go/automotive psoc.cypress.com/solutions Clocks & Buffers cypress.com/go/clocks PSoC 1 | PSoC 3 | PSoC 4 | PSoC 5LP Interface cypress.com/go/interface Lighting & Power Control cypress.com/go/powerpsoc cypress.com/go/plc Memory cypress.com/go/memory PSoC cypress.com/go/psoc Touch Sensing cypress.com/go/touch USB Controllers cypress.com/go/usb Wireless/RF cypress.com/go/wireless Cypress Developer Community Community | Forums | Blogs | Video | Training Technical Support cypress.com/go/support PSoC is a registered trademark of Cypress Semiconductor Corp. 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