AN2540 - Application note

Embedded Flash memory vs. EEPROM: main differences

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1

Embedded Flash memory vs. EEPROM: main

differences

Before describing the proposed concept for EEPROM emulation, it is important to

remember the main differences between the embedded Flash memory of a microcontroller

and serial external EEPROMs. These differences are the same for any microcontroller (that

is they are not specific to STR91xFxx products). They are summarized in Table 1.

1.1

Difference in write access time

As the Flash memory has a shorter write access time, critical parameters can be stored

faster into the emulated EEPROM than into an external serial EEPROM. The use of the

Flash memory therefore improves the system robustness.

1.2

Difference in writing method

One of the major differences between external 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 a power supply failure can interrupt the writing

process, so properly sizing the decoupling capacitors can secure the write process to a

standalone EEPROM.

●

Emulated EEPROM from an embedded Flash memory: once started by the CPU, the

writing 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) in their

applications, and to determine a proper method to handle them.

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: equivalent to a hundred µs per

word (5 to 10 ms per page)

A few µs

(e.g.: 20 µs per 16-bit word)

Erase time

N/A

seconds (e.g.: 1.5 s)

Write method

Once started, is not CPU-dependent,

needs only proper supply.

Once started, is CPU-dependent: a CPU

reset will stop the write process even if the

supply stays within specifications.

Read access

Serial: a hundred µs

random word: 92 µs

page: 22.5 µs/byte

Parallel: a hundred ns

very few CPU cycles per 16-bit word.