The NOP instruction will always be executed correctly also right after a frequency change. Thus, the next 8 instructions after the change should be NOP instructions. To ensure this, follow this procedure:
1.Clear the I bit in the SREG Register.
2.Set the new pre-scaling factor in XDIV register.
3.Execute 8 NOP instructions
4.Set the I bit in SREG
.....
Assembly Code Example:
CLI ; clear global interrupt enable
OUT XDIV, temp ; set new prescale value
NOP ; no operation
NOP ; no operation
NOP ; no operation
NOP ; no operation
NOP ; no operation
NOP ; no operation
NOP ; no operation
NOP ; no operation
SEI ; set global interrupt enable