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.
The only safe time to write to any of the Timer2 registers in asynchronous mode is in a com-
pare interrupt routine where the compare register is not 0xFF, or if the compare register is
0xFF, after a delay of at least one asynchronous clock cycle from the start of the interrupt.
34.1.4
Rev A
• Part may hang in reset
• Wrong values read after Erase Only operation
• Watchdog Timer Interrupt disabled
• Start-up time with Crystal Oscillator is higher than expected
• High Power Consumption in Power-down with External Clock
• Asynchronous Oscillator does not stop in Power-down
• Interrupts may be lost when writing the timer registers in the asynchronous timer
1. Part may hang in reset
Some parts may get stuck in a reset state when a reset signal is applied when the internal
reset state-machine is in a specific state. The internal reset state-machine is in this state for
approximately 10 ns immediately before the part wakes up after a reset, and in a 10 ns win-
dow when altering the system clock prescaler. The problem is most often seen during In-
System Programming of the device. There are theoretical possibilities of this happening also
in run-mode. The following three cases can trigger the device to get stuck in a reset-state:
- Two succeeding resets are applied where the second reset occurs in the 10ns window
before the device is out of the reset-state caused by the first reset.
- A reset is applied in a 10 ns window while the system clock prescaler value is updated by
software.
- Leaving SPI-programming mode generates an internal reset signal that can trigger this
case.
The two first cases can occur during normal operating mode, while the last case occurs only
during programming of the device.
Problem Fix/Workaround
The first case can be avoided during run-mode by ensuring that only one reset source is
active. If an external reset push button is used, the reset start-up time should be selected
such that the reset line is fully debounced during the start-up time.
The second case can be avoided by not using the system clock prescaler.
The third case occurs during In-System programming only. It is most frequently seen when
using the internal RC at maximum frequency.
If the device gets stuck in the reset-state, turn power off, then on again to get the device out
of this state.
2. Wrong values read after Erase Only operation
At supply voltages below 2.7 V, an EEPROM location that is erased by the Erase Only oper-
ation may read as programmed (0x00).
Problem Fix/Workaround
358
ATmega48/88/168
2545M–AVR–09/07
ATMEL [ ATMEL ]
