PDF《Application Report》

SPRAB69 C September

2009 Submit Documentation Feedback EEPROM Emulation With the TMS320F28xxx DSCs

1 Copyright ? 2009, Texas Instruments Incorporated Application Report SPRAB69CSeptember

2009 EEPROM Emulation With the TMS320F28xxx DSCs Tim Love and Pradeep Shinde ABSTRACT Many applications require storing small quantities of system related data (e.

g., calibration values, device configuration) in a non-volatile memory, so that it can be used or modified and reused even after power cycling the system. EEPROMs are primarily used for this purpose. EEPROMs have the ability to erase and write individual bytes of memory many times over and the programmed locations retain the data over a long period even when the system is powered down. This application report and the associated code help to define one sector of onboard Flash memory as the emulated electrically erasable programmable read-only memory (EEPROM) are transparently used by the application program for writing, reading and modifying the data. Project collateral and source code discussed in this application report can be downloaded from the following URL: http://www.ti.com/lit/zip/SPRAB69.

1 Introduction The F28xxx parts come with different configurations of Flash memory that is arranged in multiple sectors. Unfortunately, the CMOS process technology used for the on-chip Flash memory does not allow adding a traditional EEPROM on the chip. Some designers use the external EEPROM part for such non-volatile storage. As such, Flash memory is a specific type of EEPROM. The good news is all the F28xxx parts have in-circuit programming capability for the Flash memory. This application report makes use of this facility and allows using one sector of on-chip Flash as EEPROM by emulating the EEPROM functionality within the limitations of the Flash memory. Note that one Flash sector is entirely used as an emulated EEPROM;

therefore, it is not available for the application code.

2 Difference Between EEPROM and On-Chip Flash EEPROMs are available in different capacities and connect with the host microcontroller via a serial and sometimes parallel interface. The serial inter-integrated circuit (I2C) and serial peripheral interface (SPI) are quite popular due to the minimal number of pins/traces. EEPROMs can be programmed and erased electrically and most of the serial EEPROMs allow byte-by-byte program or erase operations. Compared to EEPROM, Flash memories have higher density, which allow larger memory arrays (sectors) implemented on-chip. Flash erase and write cycles are performed by applying time-controlled voltages to each cell. In the erase condition, each cell (bit) reads logical 1. Therefore, every Flash location of an F28xxx part reads 0xFFFF when erased. Through programming, the cell can be changed to logical 0. Any word can be overwritten to change a bit from logical

1 to 0;

but not the other way around. The on-chip Flash memory on F28xxx parts require TI-supplied specific algorithms (Flash API) for erase and write operations. The major difference between EEPROM and Flash operations is seen in the write and erase timings. A typical Flash write time is

50 ?s/16-bit word;

whereas, EEPROM typically requires

5 to

10 ms. The EEPROM does not require a page (sector) erase operation. One can erase a particular byte requiring the specified time. Flash erase time runs in seconds for a page. For F28xxx, the typical value for erase time is

10 seconds/8K sector. The Flash power supply must be steady during write/erase operations. Code Composer Studio is a trademark of Texas Instruments. All other trademarks are the property of their respective owners. Implementation Scheme www.ti.com Copyright ? 2009, Texas Instruments Incorporated There are challenges involved in emulating EEPROM with the Flash, due to their different characteristics. NOTE: For the Flash erase/program/read times, see the Flash Timing section in Electrical Characteristics of the device-specific data manual.