Reading the Fuse and Lock Bits from Software

It is possible to read both the Fuse and Lock bits (LB) from software. To read the Lock bits, load the Z-pointer with 0x0001 and set the BLBSET and SPMEN bits in SPMCSR (SPMCSR.BLBSET and SPMCSR.SPMEN). When an LPM instruction is executed within three CPU cycles after the BLBSET and SPMEN bits are set in SPMCSR (SPMCSR.BLBSET and SPMCSR.SPMEN), the value of the Lock bits will be loaded in the destination register. The SPMCSR.BLBSET and SPMCSR.SPMEN 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 SPMCSR.BLBSET and SPMCSR.SPMEN are cleared, LPM will work as described in the Instruction set Manual.

The algorithm for reading the Fuse Low byte (FLB) is similar to the one described above for reading the Lock bits. To read the Fuse Low byte, load the Z-pointer with 0x0000 and set the BLBSET and SPMEN bits in SPMCSR (SPMCSR.BLBSET and SPMCSR.SPMEN). When an LPM instruction is executed within three cycles after the SPMCSR.BLBSET and SPMCSR.SPMEN are set, the value of the Fuse Low byte (FLB) will be loaded into the destination register as shown below.

Similarly, when reading the Fuse High byte (FHB), load 0x0003 in the Z-pointer. When an LPM instruction is executed within three cycles after the SPMCSR.BLBSET and SPMCSR.SPMEN are set, the value of the Fuse High byte (FHB) will be loaded into the destination register as shown below.

When reading the Extended Fuse byte (EFB), load 0x0002 in the Z-pointer. When an LPM instruction is executed within three cycles after the SPMCSR.BLBSET and SPMCSR.SPMEN are set, the value of the Extended Fuse byte (EFB) will be loaded into the destination register as shown below.

Fuse and Lock bits that are programmed read as '0'. Fuse and Lock bits that are unprogrammed, will read as '1'.