The revision letter in this section refers to the revision of the ATmega64A device.
9.1
ATmega64A, rev. D
• 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
• Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt request
1. First Analog Comparator conversion may be delayed
If the device is powered by a slow rising VCC, the first Analog Comparator conversion will take longer than
expected on some devices.
Problem Fix/Workaround
When the device has been powered or reset, disable then enable theAnalog Comparator before the first
conversion.
2. Interrupts may be lost when writing the timer registers in the asynchronous timer
The interrupt will be lost if a timer register that is synchronous timer clock is written when the asynchronous
Timer/Counter register (TCNTx) is 0x00.
Problem Fix / Workaround
Always check that the asynchronous Timer/Counter register neither have the value 0xFF nor 0x00 before writ-
ing to the asynchronous Timer Control Register (TCCRx), asynchronous Timer Counter Register (TCNTx), or
asynchronous Output Compare Register (OCRx).
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
ATmega64A [DATASHEET]
11
8160DS–AVR–02/2013
ATMEL [ ATMEL ]
