ATmega16U4 datasheet(343/408 Pages) ATMEL

343 / 408 page

343

7766D–AVR–01/09

ATmega16U4/ATmega32U4

Similarly, when reading the Fuse High byte, load 0x0003 in the Z-pointer. When an (E)LPM

instruction is executed within three cycles after the BLBSET and SPMEN bits are set in the

SPMCSR, the value of the Fuse High byte (FHB) will be loaded in the destination register as

shown below. Refer to Table 28-4 on page 350 for detailed description and mapping of the Fuse

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

below. Refer to Table 28-3 on page 349 for detailed description and mapping of the Extended

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.

27.7.10

Reading the Signature Row from Software

To read the Signature Row from software, load the Z-pointer with the signature byte address

given in Table 27-6 on page 343 and set the SIGRD and SPMEN bits in SPMCSR. When an

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.

Note:

All other addresses are reserved for future use.

27.7.11

Preventing Flash Corruption

During periods of low V

CC, 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.

Flash corruption can easily be avoided by following these design recommendations (one is

sufficient):

Bit

FHB7

FHB6

FHB5

FHB4

FHB3

FHB2

FHB1

FHB0

Bit

–

EFB2

EFB1

EFB0

Table 27-6.

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

ATmega16U4 Datasheet(PDF) 343 Page - ATMEL Corporation

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