AN2014 datasheet(54/65 Pages) STMICROELECTRONICS

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Power supply loss and application reset

AN2014

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DocID10701 Rev 10

Figure 43. EEPROM power backup capacitor

1. Sometimes filtering capacitors placed after voltage regulators, are big enough to allow the EEPROM

device to finish the write operation. In this case, backup capacitors are no longer necessary.

For a complete and detailed calculation, the discharge current through the MCU protection

diodes must be taken into account. The EEPROM inputs/outputs do not draw any current

during write operations and only the pull-up resistors connected to the EEPROM VCC pin

will discharge the backup capacitor through the MCU connection.

Optimum robustness is obtained by adding a discharge path to ground. At power-down, the

EEPROM is usually in the standby mode, where it draws little current (no more than a few

µA) and the backup capacitor takes a long time to discharge. The system designer must

either:

•

take into consideration the long discharge time to allow the EEPROM VCC supply to

reach ground level (0 V) before setting the application supply voltage active, or

•

add a discharge path to ground to accelerate the discharge if the system may or must

be re-started after a short time

When applying this recommendation, please read also Section 3.3.1: Power-up and power-

on-reset sequence.

6.2.3

Interruption of an EEPROM request

When the Power loss occurs while the Master is still sending a command, it is strongly

recommended to have an emergency software procedure able to interrupt safely the

request being sent to the EEPROM. The set of below recommendations is adapted to each

product family.

I²C Products

Emergency procedure to interrupt an I2C request:

•

Drive the WC pin High. One SCL clock pulse will inhibit the current write request.

•

Send a START condition followed by a STOP condition (a START condition resets the

device and a STOP condition sets it in standby mode). The re-synchronization

sequence described in AN1471 can also be used. See Figure 44: Emergency

sequence I2C products.

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