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AN2594 Application note EEPROM emulation in STM32F101xx and STM32F103xx microcontrollers Introduction Many applications require EEPROM (electrically erasable programmable read-only memory) for non-volatile data storage. For low-cost purposes, the STM32F101xx and STM32F103xx devices do not use EEPROM. Instead, they implement EEPROM emulation using the embedded Flash memory. This application note explains the differences between external EEPROM and embedded Flash memory, and it describes a software method for emulating EEPROM using the on- chip Flash memory of the STM32F101xx and STM32F103xx devices. This document also focuses on some embedded aspects in emulated EEPROM data storage, that the reader is assumed to know. Glossary Medium-density devices are STM32F101xx and STM32F103xx microcontrollers where the Flash memory density ranges between 32 and 128 Kbytes. High-density devices are STM32F101xx and STM32F103xx microcontrollers where the Flash memory density ranges between 256 and 512 Kbytes. June 2008 Rev 2 1/16 www.st.com

Contents Contents AN2594 1 2 3 Embedded Flash memory versus EEPROM: main differences . . . . . . . 5 Difference in write access time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1 1.2 Difference in writing method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Difference in erase time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.3 Implementing EEPROM emulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1 Application example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.1.1 2.1.2 EEPROM software description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Embedded application aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Data granularity management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.1 Programming on a word-by-word basis . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.1.1 Programming on a byte-by-byte basis . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.1.2 3.2 Wear leveling: Flash memory endurance improvement . . . . . . . . . . . . . . 11 3.2.1 Wear-leveling implementation example . . . . . . . . . . . . . . . . . . . . . . . . . 11 Page header recovery in case of power loss . . . . . . . . . . . . . . . . . . . . . . 12 Cycling capability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.3 3.4 4 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2/16

AN2594 List of tables List of tables Table 1. Table 2. Table 3. Table 4. Differences between embedded Flash memory and EEPROM . . . . . . . . . . . . . . . . . . . . . . 5 Status combinations and actions to be taken . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Maximum number of variables stored in emulated EEPROM (with 10 000 cycles) . . . . . . 14 Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3/16

List of figures List of figures AN2594 Header status switching between page0 and page1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 1. EEPROM variable format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 2. Figure 3. Data update flow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 4. WriteVariable flowchart. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Page swap scheme with four pages (wear leveling). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 5. 4/16

AN2594 1 Embedded Flash memory versus EEPROM: main differences Embedded Flash memory versus EEPROM: main differences Electrically erasable and programmable read-only memory (EEPROM) is a key component of many embedded applications that require non-volatile storage of data updated with a byte or word granularity during run time. On the other hand, the microcontrollers used in those systems are each time more often based on embedded Flash memory. To eliminate components, save silicon space and reduce system cost, the STM32F10xxx Flash memory may be used instead of EEPROM for simultaneous code and data storage. Unlike Flash memory, however, external EEPROM does not require an erase operation to free up space before data can be rewritten. Hence a special software management is required to store data into embedded Flash memory. Obviously the emulation software scheme depends on many factors, including the EEPROM reliability, the architecture of the Flash memory used, and the product requirements. The main differences between embedded Flash memory and external serial EEPROM are generic to any microcontroller that use the same Flash memory technology (it is not specific to the STM32F10xxx family products). The major differences are summarized in Table 1. Table 1. Differences between embedded Flash memory and EEPROM Feature External EEPROM Emulated EEPROM using on-chip Flash memory Write time – a few ms – random byte: 5 to 10 ms – page: a hundred µs per word (5 to 10 ms per page) Word program time: 20 µs Erase time N/A Page/Mass Erase time: 20 ms Write method – once started, is not CPU-dependent – only needs proper supply. once started, is CPU-dependent: a CPU reset will stop the write process even if the supplied power stays within specifications. Read access – serial: a hundred µs – random word: 92 µs – page: 22.5 µs per byte – parallel: a hundred ns – very few CPU cycles per word. – Access time: 35 ns Write/Erase cycles – from 10 kilocycles to 1 000 kilocycles – from 10 kilocycles to 100 kilocycles (the use of many on-chip Flash memory pages is equivalent to increasing the number of write cycles) see Section 3.4: Cycling capability 1.1 Difference in write access time As Flash memories have a shorter write access time, critical parameters can be stored faster in the emulated EEPROM than in an external serial EEPROM, thereby improving data storage. 5/16

Embedded Flash memory versus EEPROM: main differences AN2594 1.2 1.3 Difference in writing method One of the major differences between external EEPROM and emulated EEPROM for embedded applications is the writing method. ● Standalone external EEPROM: once started by the CPU, the writing of a word cannot be interrupted by a CPU reset. Only supply failure will interrupt the write process, so properly sizing the decoupling capacitors can secure the complete writing process inside a standalone EEPROM. Emulated EEPROM using an embedded Flash memory: once started by the CPU, the write process can be interrupted by a power failure and by a CPU reset. This difference should be analyzed by system designers to understand the possible impact(s) on their applications and to determine a proper handling method. ● Difference in erase time The difference in erase time is the other major difference between a standalone EEPROM and emulated EEPROM using embedded Flash memory. Unlike Flash memories, EEPROMs do not require an erase operation to free up space before writing to them. This means that some form of software management is required to store data in Flash memory. Moreover, as the erase process of a block in the Flash memory takes a few milliseconds, power shut-down and other spurious events that might interrupt the erase process (for example a reset) should be considered when designing the Flash memory management software. To design a robust Flash memory management software it is necessary to have a deep understanding of the Flash memory erase process. 6/16

AN2594 2 2.1 Implementing EEPROM emulation Implementing EEPROM emulation Principle EEPROM emulation is performed in various ways by considering the Flash memory limitations and product requirements. The approach detailed below requires at least two Flash memory pages of identical size allocated to non-volatile data. One that is initially erased, and offers byte-by-byte programmability; the other that is ready to take over when the former page needs to be garbage-collected. A header field that occupies the first 16-bit half word of each page indicates the page status. The header field is located at the base address of each page and gives the page status information. Each page has three possible states: ● ● ● ERASED: the page is empty. RECEIVE_DATA: the page is receiving data from the other full page. VALID_PAGE: the page contains valid data and this state does not change until all valid data are completely transferred to the erased page. Figure 1 shows how the page statuses change with respect to each other. Figure 1. Header status switching between page0 and page1 Page0 Valid Page1 Erased Write Page0 data Page0 Full Page0 Valid Page1 Receive Copy Page0 data -> Page1 Erase Page0 Page0 Erased Page1 Valid Write Page1 data Page1 Full Page0 Receive Page1 Valid Copy Page1 data -> Page0 Erase Page1 ai14606 Generally, when using this method, the user does not know in advance the variable update frequency. The software and implementation described in this document use two Flash memory pages to emulate EEPROM. Each variable element is defined by a virtual address and a value to be stored in Flash memory for subsequent retrieval or update (in the implemented software both virtual address and data are 16 bits long). When data is modified, the modified data associated with the earlier virtual address is stored into a new Flash memory location. Data retrieval returns the modified data in the latest Flash memory location. 7/16

Implementing EEPROM emulation AN2594 Figure 2. EEPROM variable format Variable data (16 bits) Variable virtual address (16 bits) EEPROM variable element = 32-bit word 256 elements (1 Kbyte page) for Medium-density devices or 512 elements (2 Kbyte page) for High-density devices page0 page1 ai14608b 2.1.1 Application example The following example shows the software management of three EEPROM variables (Var1, Var2 and Var3) with the following virtual addresses: Var1: 5555h, Var2: 6666h and Var3: 7777h Figure 3. Data update flow Page0 FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF Page1 FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF Add Var3 =1232h Page0 12 32 77 77 FF FF FF FF FF FF FF FF FF FF FF FF Page1 FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF Add Var3 =1245h Page0 12 32 77 77 12 45 77 77 FF FF FF FF FF FF FF FF Page1 FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF Add Var1 =BCBCh Page0 12 32 77 77 12 45 77 77 BC BC 55 55 FF FF FF FF Page1 FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF Active Page = Page0 FF FF FF FF Active Page = Page0 FF FF FF FF Active Page = Page0 FF FF FF FF Active Page = Page0 Add var2 =3434h Page0 12 32 77 77 12 45 77 77 BC BC 55 55 34 34 66 66 Page1 FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF Active Page = Page0 Page0 12 32 77 77 32 32 77 77 22 45 66 66 BD BD 55 55 54 54 77 77 Page1 FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF Add Var2 =6464h Page0 12 32 77 77 32 32 77 77 22 45 66 66 BD BD 55 55 54 54 77 77 Page1 64 64 66 66 54 54 77 77 BD BD 55 55 FF FF FF FF FF FF Active Page = Page0 Active Page = Page1 Erase Page0 Page0 FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF Page1 64 64 66 66 54 54 77 77 BD BD 55 55 FF FF FF FF FF FF Active Page = Page1 2.1.2 EEPROM software description This section describes the driver implemented for EEPROM emulation using the STM32F10xxx Flash memory driver provided by STMicroelectronics. A sample demonstration program is also supplied to demonstrate and test the EEPROM emulation driver using the three variables Var1, Var2 and Var3 defined in the VirtAddVarTab table declared in the software main.c file. ai14609 8/16

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