2013-2016 Microchip Technology Inc. DS20005215C-page 13
24AA256UID
9.0 PRE-PROGRAMMED SERIAL
NUMBER AND NODE
ADDRESSES
The 24AA256UID is programmed at the factory with
globally unique EUI-48 and EUI-64 node addresses,
and a 32-bit serial number stored in the upper eighth of
the array and permanently write-protected. The
remaining 229,376 bits are available for application
use.
FIGURE 9-1: MEMORY ORGANIZATION
9.1 32-Bit Serial Number
The 24AA256UID features a unique 32-bit serial
number stored in array locations 0x7FFC through
0x7FFF, as shown in Figure 9-2.
9.1.1 MANUFACTURER AND DEVICE
IDENTIFIERS
In addition to the serial number, a manufacturer code is
stored at location 0x7FFA and a device identifier is
stored at 0x7FFB. The manufacturer code is fixed
as 0x29. For the 24AA256UID, the device identifier
is 0x48. The ‘4’ indicates the I
2
C family and the ‘8’
indicates a 256 Kbit memory density.
9.1.2 EXTENDING THE 32-BIT SERIAL
NUMBER
For applications that require serial numbers larger than
32 bits, additional data bytes can be used to pad the
provided serial number to meet the required length.
Any data byte values can be used for padding as the
32-bit serial number ensures the extended serial
number remains unique.
The padding can be performed in two ways. The first
method is to pad the data in software by combining the
32-bit serial number from the 24AA256UID with fixed
data. The second method is to extend the number of
bytes read from the 24AA256UID to meet the required
length. Table 9-1 shows example address ranges and
their corresponding serial number lengths.
TABLE 9-1: EXTENDED READ EXAMPLES
Standard EEPROM
0000h
6FFFh
7000h
7F79h
7F7Ah
7F7Fh
7FB8h
7FBFh
7FFAh
7FFFh
Unused (Read as 0xFF)
32-Bit Serial Number and Identifiers
Unused (Read as 0xFF)
EUI-48™ Node Address
Unused (Read as 0xFF)
EUI-64™ Node Address
(User – Writable)
Note: Shaded region is permanently write-protected
Note: The 32-bit serial number is unique across
all Microchip UID-family serial EEPROM
devices.
Start Address End Address
Serial Number
Length
0x7FFC 0x7FFF 32 bits
0x7FFA 0x7FFF 48 bits
0x7FF8 0x7FFF 64 bits
0x7FF0 0x7FFF 128 bits
0x7FE0 0x7FFF 256 bits
