236
7799D–AVR–11/10
ATmega8U2/16U2/32U2
High byte.
When reading the Extended Fuse byte, load 0x0002 in the Z-pointer. When an (E)LPM instruc-
tion is executed within three cycles after the BLBSET and SPMEN bits are set in the SPMCSR,
the value of the Extended Fuse byte (EFB) will be loaded in the destination register as shown
Fuse byte.
Fuse and Lock bits that are programmed, will be read as zero. Fuse and Lock bits that are
unprogrammed, will be read as one.
23.8.10
Reading the Signature Row from Software
To read the Signature Row from software, load the Z-pointer with the signature byte address
LPM instruction is executed within three CPU cycles after the SIGRD and SPMEN bits are set in
SPMCSR, the signature byte value will be loaded in the destination register. The SIGRD and
SPMEN bits will auto-clear upon completion of reading the Signature Row Lock bits or if no LPM
instruction is executed within three CPU cycles. When SIGRD and SPMEN are cleared, LPM will
work as described in the Instruction set Manual
ATmega8U2/16U2/32U2 includes a unique 10 bytes serial number located in the signature row.
This unique serial number can be used as a USB serial number in the device enumeration pro-
cess. The pointer addresses to access this unique serial number are given in
Table 23-6 onNote:
All other addresses are reserved for future use.
23.8.11
Preventing Flash Corruption
During periods of low VCC, the Flash program can be corrupted because the supply voltage is
too low for the CPU and the Flash to operate properly. These issues are the same as for board
level systems using the Flash, and the same design solutions should be applied.
A Flash program corruption can be caused by two situations when the voltage is too low. First, a
regular write sequence to the Flash requires a minimum voltage to operate correctly. Secondly,
the CPU itself can execute instructions incorrectly, if the supply voltage for executing instructions
is too low.
Bit
76
54
32
10
Rd
FHB7
FHB6
FHB5
FHB4
FHB3
FHB2
FHB1
FHB0
Bit
76
54
32
10
Rd
–
EFB2
EFB1
EFB0
Table 23-6.
Signature Row Addressing
Signature Byte
Z-Pointer Address
Device Signature Byte 1
0x0000
Device Signature Byte 2
0x0002
Device Signature Byte 3
0x0004
RC Oscillator Calibration Byte
0x0001
Unique Serial Number
From 0x000E to 0x0018