UPSD3312D datasheet(173/272 Pages) STMICROELECTRONICS

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UPSD33xx

PSD module

Doc ID 9685 Rev 7

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Figure 53.

PSDsoft Express memory mapping

27.2.4

EEPROM emulation

EEPROM emulation is needed if it is desired to repeatedly change only a small number of

bytes of data in Flash memory. In this case EEPROM emulation is needed because

although Flash memory can be written byte-by-byte, it must be erased sector-by-sector, it is

not erasable byte-by-byte (unlike EEPROM which is written AND erased byte-by-byte). So

changing one or two bytes in Flash memory typically requires erasing an entire sector each

time only one byte is changed within that sector.

However, two of the 8 Kbyte sectors of secondary Flash memory may be used to emulate

EEPROM by using a linked-list software technique to create a small data set that is

maintained by alternating between the two Flash sectors. For example, a data set of 128

bytes is written and maintained by software in a distributed fashion across one 8 Kbyte

sector of secondary Flash memory until it becomes full. Then the writing continues on the

other 8 Kbyte sector while erasing the first 8 Kbyte sector. This process repeats

continuously, bouncing back and forth between the two 8 Kbyte sectors. This creates a

wear-leveling effect, which increases the effective number of erase cycles for a data set of

128 bytes to many times more than the base 100K erase cycles of the Flash memory.

EEPROM emulation in Flash memory is typically faster than writing to actual EEPROM

memory, and more reliable because the last known value in a data set is maintained even if

a WRITE cycle is corrupted by a power outage. The EEPROM emulation function can be

called by the firmware, making it appear that the user is writing a single byte, or data record,

thus hiding all of the data management that occurs within the two 8 Kbyte Flash sectors.

EEPROM emulation firmware for the UPSD33xx is available from www.st.com/mcu.

UPSD3312D Datasheet(PDF) 173 Page - STMicroelectronics