MCP795W21 datasheet(39/54 Pages) MICROCHIP

39 / 54 page

 2011-2012 Microchip Technology Inc.

Preliminary

DS22280C-page 39

MCP795W1X/MCP795W2X

FIGURE 10-2:

WRITE SEQUENCE

10.1.4

WRITE TO THE UNLOCK REGISTER

The MCP795WXX contains a protected area of 64 bits

that can be used to hold a unique ID, such as a serial

number or MAC address code. To gain write access to

these locations, a specific sequence is required. Any

deviation from this sequence will reset the lock on

these locations. Once these locations have been

unlocked they have to be written to in the next

command by issuing the correct command. A write to

a different location will lock the ID locations and clear

the WEL bit.

The following is a list of strict conditions which have to

be followed before the unique locations can be written

to:

• EEWREN instruction successfully executed

• UNLOCK 0x55 instruction successfully executed

• UNLOCK 0xAA instruction successfully executed

To issue each UNLOCK instruction the Unlock

command is sent followed by 0x55. Then in a separate

command the Unlock command is issued followed by

0xAA. It is a requirement that each command be

separate, that is CS must toggle between each

command.

Information on how to read and write the ID locations

is detailed in Section 9.1.6, Unique ID Locations.

FIGURE 10-3:

UNLOCK SEQUENCE

SCK

23456789 10 11 12 13 14 15 16 17 18 19 20 21 22

A7 6

1A0 76543210

Instruction

Address Byte

Data Byte

High-Impedance

SCK

234567

89 10 11 12 13 14 15

Instruction

Data

High-Impedance

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