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avrdude: verification error, first mismatch at byte 0x0000
         0x0c != 0xff

30.12.3NVM Commands
The NVM commands that can be used for accessing the NVM memories from external programming are listed in Table 30-5. This is a super-set of the commands available for self-programming.

     if(
      /*      (FUSEBYTE0 == SP_ReadFuseByte( 0 ) ) && Manual doesn't match reality, we don't use JTAGID */
      (FUSEBYTE1 == SP_ReadFuseByte( 1 )) &&
      (FUSEBYTE2 == SP_ReadFuseByte( 2 )) &&
      /* There is no FUSEBYTE3 */
      (FUSEBYTE4 == SP_ReadFuseByte( 4 )) &&
      (FUSEBYTE5 == SP_ReadFuseByte( 5 )) &&
      (LOCK_BITS == NVM_LOCKBITS)
     )
    )
    {
      selftest_u16 &= ((uint16_t) ~SELFTEST_FUSES); /*  Passed */
    }

#include <avr/io.h>
#define NVM_CMD_READ_FUSES_gc (0x07<<0)   // Read fuse byte

.extern SP_CommonCMD

; ---
; This routine reads the fuse byte given by the index in R24.
;
; Input:
;     R24 - Byte index.
;
; Returns:
;     R24 - Fuse byte.
; ---

.section .text
.global SP_ReadFuseByte

SP_ReadFuseByte:
   sts   NVM_ADDR0, r24              ; Load fuse byte index into NVM Address Register 0.
   clr   r24                         ; Prepare a zero.
   sts   NVM_ADDR1, r24              ; Load zero into NVM Address Register 1.
   sts   NVM_ADDR2, r24              ; Load zero into NVM Address Register 2.
   ldi   r20, NVM_CMD_READ_FUSES_gc  ; Prepare NVM command in R20.
   rcall   SP_CommonCMD                ; Jump to common NVM Action code.
   movw   r24, r22                    ; Move low byte to 1 byte return address.
   ret

#include <avr/io.h>
#define NVM_CMD_NO_OPERATION_gc (0x00<<0)   // NoOp/Ordinary LPM
#define CCP_IOREG_gc (0xD8<<0)   // IO Register Protection

; ---
; This routine is called by several other routines, and contains common code
; for executing an NVM command, including the return statement itself.
;
; If the operation (NVM command) requires the NVM Address registers to be
; prepared, this must be done before jumping to this routine.
;
; Note that R25:R24:R23:R22 is used for returning results, even if the
; C-domain calling function only expects a single byte or even void.
;
; Input:
;     R20 - NVM Command code.
;
; Returns:
;     R25:R24:R23:R22 - 32-bit result from NVM operation.
; ---

.section .text
.global SP_CommonCMD

SP_CommonCMD:
   sts   NVM_CMD, r20        ; Load command into NVM Command register.
   ldi   r18, CCP_IOREG_gc   ; Prepare Protect IO-register signature in R18.
   ldi   r19, NVM_CMDEX_bm   ; Prepare bitmask for setting NVM Command Execute bit into R19.
   sts   CCP, r18            ; Enable IO-register operation (this disables interrupts for 4 cycles).
   sts   NVM_CTRLA, r19      ; Load bitmask into NVM Control Register A, which executes the command.
   lds   r22, NVM_DATA0      ; Load NVM Data Register 0 into R22.
   lds   r23, NVM_DATA1      ; Load NVM Data Register 1 into R23.
   lds   r24, NVM_DATA2      ; Load NVM Data Register 2 into R24.

   ldi   r25, NVM_CMD_NO_OPERATION_gc ; Prepare NVM command code
   sts   NVM_CMD, r25                 ; Set NVM command to No Operation for clean operation to follow

        clr   r25                 ; Clear R25 in order to return a clean 32-bit value.
   ret