ATmega48/88/168
When reading the Extended Fuse byte, load 0x0002 in the Z-pointer. When an LPM
instruction is executed within three cycles after the BLBSET and SELFPRGEN 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 118 on page 272 for detailed
description and mapping of the Extended Fuse byte.
Bit
Rd
7
6
5
4
3
2
1
0
–
–
–
–
EFB3
EFB2
EFB1
EFB0
Fuse and Lock bits that are programmed, will be read as zero. Fuse and Lock bits that
are unprogrammed, will be read as one.
Preventing Flash Corruption
During periods of low VCC, the Flash program can be corrupted because the supply volt-
age 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 cor-
rectly. 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 voltage matches the detection level. If not, an external low VCC
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.
3. Keep the AVR core in Power-down sleep mode during periods of low VCC. This
will prevent the CPU from attempting to decode and execute instructions, effec-
tively protecting the SPMCSR Register and thus the Flash from unintentional
writes.
Programming Time for Flash
when Using SPM
The calibrated RC Oscillator is used to time Flash accesses. Table 108 shows the typi-
cal programming time for Flash accesses from the CPU.
Table 108. SPM Programming Time
Symbol
Min Programming Time Max Programming Time
3.7 ms 4.5 ms
Flash write (Page Erase, Page Write,
and write Lock bits by SPM)
265
2545D–AVR–07/04
ATMEL [ ATMEL ]
