ATmega128 Rev. I
• First Analog Comparator conversion may be delayed
• Interrupts may be lost when writing the timer registers in the asynchronous timer
• Stabilizing time needed when changing XDIV Register
• Stabilizing time needed when changing OSCCAL Register
• IDCODE masks data from TDI input
1. First Analog Comparator conversion may be delayed
If the device is powered by a slow rising VCC, the first Analog Comparator conver-
sion will take longer than expected on some devices.
Problem Fix/Workaround
When the device has been powered or reset, disable then enable theAnalog Com-
parator before the first conversion.
2. Interrupts may be lost when writing the timer registers in the asynchronous
timer
If one of the timer registers which is synchronized to the asynchronous timer2 clock
is written in the cycle before a overflow interrupt occurs, the interrupt may be lost.
Problem Fix/Workaround
Always check that the Timer2 Timer/Counter register, TCNT2, does not have the
value 0xFF before writing the Timer2 Control Register, TCCR2, or Output Compare
Register, OCR2
3. Stabilizing time needed when changing XDIV Register
After increasing the source clock frequency more than 2% with settings in the XDIV
register, the device may execute some of the subsequent instructions incorrectly.
Problem Fix / Workaround
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
This will ensure that all subsequent instructions will execute correctly.
Assembly Code Example:
CLI
; clear global interrupt enable
; set new prescale value
; no operation
OUT XDIV, temp
NOP
NOP
NOP
NOP
NOP
NOP
NOP
NOP
SEI
; no operation
; no operation
; no operation
; no operation
; no operation
; no operation
; no operation
; clear global interrupt enable
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ATmega128
2467OS–AVR–10/06
ATMEL [ ATMEL ]
