AN2014 datasheet(47/65 Pages) STMICROELECTRONICS

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AN2014

Software considerations

Note:

The intrinsic(b) cycling limits measured on the CMOS F8H automotive devices are much

higher than the safe conditions suggested in Figure 39.

5.5.4

Defining the application cycling strategy

ST EEPROM products have no built-in function to limit cycling and therefore application

designers have to evaluate the number of write cycles that will be executed during the life of

the end application. To ensure the safest conditions, it is strongly recommended to define a

temperature profile of the write cycles performed by the EEPROM.

•

Define the main temperature stages at which the EEPROM is operating in the end

application.

•

For each temperature, estimate the number of write cycles executed for each data

block.

•

For each data block (with different cycling profiles), calculate the cumulated cycling

effect using Table 10.

If the total percentage of cumulated cycles (last row in Table 10) is lower than 100%, the

data stored in the EEPROM are safely cycled.

If the total percentage of cumulated cycles is above 100%, the intrinsic safe margin for

cycling is exceeded and a data relocation strategy must be defined.

Cycling on each EEPROM cell is not infinite (as shown in Figure 39), it is therefore wise to

define a data relocation strategy by distributing the total number of cycles over several

memory locations. To do this:

•

Define a cycling limit for each data block according to the application needs and

product performance (see Table 10).

•

Count the numbers of cycles executed on each data block (counter value can be stored

in the EEPROM).

•

When the counter exceeds the defined limit, the cycled data block must be relocated to

another physically independent memory address. The software developer should not

move it to a location in the same page (when possible, not in the same column either)

as the reference column/page. This means that the 2 addresses should differ by at

Intrinsic = belonging to the essential nature or constitution of the EEPROM die (extrinsic = originating from a random

event).

Table 10. Application cycling profile evaluation(1)

1. The table can be adapted according to the temperature profile by taking care of putting down the maximum

cycling for each temperature (using Figure 39).

Temperature

Number of cycles(2)

2. w, x, y and z are the forecast number of cycles for a specific data block.

% of specification max

25 °C

(w/1M) × 100 = a%

55 °C

(x/600K) × 100 = b%

85 °C

(y/300K) × 100 = c%

125 °C

(z/150K) × 100 = d%

Total

w + x + y + z

a + b + c + d%

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