L9300 datasheet(89/178 Pages) STMICROELECTRONICS

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Figure 59. Answer check generation algorithms block diagram

Seed

-Seed

Answer high

Answer low

GADG0408171328PS

The answer is a 16-bit wide word checked against a 16-bit word composed by two bytes, Answer_Low and

Answer_High, generated from the seed sent.

Answer_Low is the logical 2’s complement of the seed while Answer_High is a replica of the seed sent.

14.2

Q&A watchdog SPI registers

When a Read operation is performed, a new seed is sent and the read is treated as a new Seed request. When

an Answer write is not treated as a new Seed request and the seed related to that answer is sent back.

14.2.1

All the SPI parameters can be programmed before the WD mechanism starts. After the first seed request, the

registers are locked to prevent from unexpected modification by the µC.

To modify the register, µC must send an unlock key. After the key is received a register can be modified. After the

first SPI write access (to any address) received after the Unlock key, registers are automatically locked.

Unlock_Cmd = 20’h0f0f0

Example 1 : Example of unlock usage: To change the WD Req Timings and Answ Timing while WD is working

WD is running → All the WD config registers are locked

Unlock_Cmd is received → Lock is removed

Write Access to WD Req Timing register → WD Req Timing updated

→ All the WD config registers are automatically locked

Unlock_Cmd is received → Lock is removed

Write Access to WD Answ Timing register → WD Ans Timing updated

→ WD config registers are automatically locked

L9300

Q&A Watchdog

DS12258 - Rev 5

page 89/178

L9300 Fiches technique(PDF) 89 Page - STMicroelectronics