Rainbow Electronics ATmega64C1 - Manual (Page 283)

283
7647A–AVR–02/08
ATmega32/64/M1/C1
value of the Extended Fuse byte (EFB) will be loaded in the destination register as shown below.
Refer to
Table 24-4 on page 290 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.
23.7.10 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 23-5 on page 283 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.
23.7.11 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):
1. If there is no need for a Boot Loader update in the system, program the Boot Loader
Lock bits to prevent any Boot Loader software updates.
2. Keep the AVR RESET active (low) during periods of insufficient power supply voltage.
This can be done by enabling the internal Brown-out Detector (BOD) if the operating volt-
age matches the detection level. If not, an external low V
CC
reset protection circuit can be
used. If a reset occurs while a write operation is in progress, the write operation will be
completed provided that the power supply voltage is sufficient.
Bit 7 6 5 4 3 2 1 0
Rd – – – – EFB3 EFB2 EFB1 EFB0
Table 23-5. 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
TSOFFSET Temp Sensor Offset 0x0003
TSGAIN Temp Sensor Gain 0x0005