Datasheet

286
2490R–AVR–02/2013
ATmega64(L)
Reading the Fuse and
Lock Bits from
Software
It is possible to read both the Fuse and Lock bits from software. To read the Lock bits, load the
Z-pointer with 0x0001 and set the BLBSET and SPMEN bits in SPMCSR. When an LPM instruc-
tion is executed within three CPU cycles after the BLBSET and SPMEN bits are set in SPMCSR,
the value of the Lock bits will be loaded in the destination register. The BLBSET and SPMEN
bits will auto-clear upon completion of reading the Lock bits or if no LPM instruction is executed
within three CPU cycles or no SPM instruction is executed within four CPU cycles. When BLB-
SET and SPMEN are cleared, LPM will work as described in the AVR Instruction Set Reference
Manual.
The algorithm for reading the Fuse Low bits is similar to the one described above for reading the
Lock bits. To read the Fuse Low bits, load the Z-pointer with 0x0000 and set the BLBSET and
SPMEN bits in SPMCSR. When an LPM instruction is executed within three cycles after the
BLBSET and SPMEN bits are set in the SPMCSR, the value of the Fuse Low bits (FLB) will be
loaded in the destination register as shown below. Refer to Table 119 on page 292 for a detailed
description and mapping of the Fuse Low bits.
Similarly, when reading the Fuse High bits, load 0x0003 in the Z-pointer. When an LPM instruc-
tion is executed within three cycles after the BLBSET and SPMEN bits are set in the SPMCSR,
the value of the Fuse High bits (FHB) will be loaded in the destination register as shown below.
Refer to Table 118 on page 292 for detailed description and mapping of the Fuse High bits.
When reading the Extended Fuse bits, load 0x0002 in the Z-pointer. When an LPM instruction is
executed within three cycles after the BLBSET and SPMEN bits are set in the SPMCSR, the
value of the Extended Fuse bits (EFB) will be loaded in the destination register as shown below.
Refer to Table 117 on page 291 for detailed description and mapping of the Fuse High bits.
Fuse and Lock bits that are programmed will be read as zero. Fuse and Lock bits that are unpro-
grammed will be read as one.
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. Second,
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
Bit 76543210
Rd – – BLB12 BLB11 BLB02 BLB01 LB2 LB1
Bit 76543210
Rd FLB7 FLB6 FLB5 FLB4 FLB3 FLB2 FLB1 FLB0
Bit 76543210
Rd FHB7 FHB6 FHB5 FHB4 FHB3 FHB2 FHB1 FHB0
Bit 76543210
Rd ––––––EFB1EFB0