AN2014 datasheet(42/65 Pages) STMICROELECTRONICS

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Software considerations

AN2014

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DocID10701 Rev 10

5.3.2

Hardware write protection

By default the hardware write protection feature should be set and the time during which the

device is left unprotected should be limited to the time required to issue and execute Write

instructions. The hardware write protection is very effective against parasitic or hazardous

instructions transiting on the interface bus.

Also use the hardware write protection features during Power-up, Power-down and normal

operation (Refer to the Hardware considerations).

•

The WC pin in I²C products protects the entire array.

•

The W pin protects the entire array by resetting the Write Enable Latch (WEL) bit in 1

Kbit to 4 Kbit SPI products whereas it protects the non-volatile bits of the Status

It is recommended to change the state of the Write Protect pin only if no data transfer or

program cycle is in progress. The Write Protect and Write Control pins should be controlled

with very conservative timings:

•

1 clock cycle time clearance (with no data transfer) before the select (or start) event.

•

1 clock cycle time clearance (with no data transfer) after the deselect (or stop) event.

•

Wait for the write cycle (tW) completion before changing the protection.

This conservative sequence will not affect the communication speed but will ensure the safe

operation of the products (see Figure 36 and Figure 37).

The Write Protect signal (W for SPI, WC for I²C) is glitch sensitive and a short (parasitic)

pulse could cause a write request to be aborted. This feature can also be of great help in

emergency situations like power loss or Master reset. See the Power supply loss and

application reset section for details.

Figure 36. .Recommended use of the WC pin in I²C products

Figure 37. Recommended use of the W pin in SPI products

Clearance

1 Clock Period

Clearance

1 Clock Period

ai10921

START + WRITE +ADDRESS + DATA + STOP

tW < 5ms

SDA

Clearance

1 Clock Period

Clearance

1 Clock Period

ai10922

WRITE +ADDRESS + DATA

tW < 5ms

SDA

WREN

Polling

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