ATmega128 reset source

Reset the AVR

At reset, all I/O registers are set to their initial values ​​and the program starts executing from the reset vector. The instruction at the reset vector must be an absolute jump JMP instruction so that the program jumps to the reset handling routine. If the program never enables interrupts, the interrupt vector can be overwritten by normal program code. Figure 22 is a circuit diagram of the reset logic. Table 19 defines the electrical parameters of the reset circuit.

When the reset source takes effect, the I/O ports are immediately reset to their initial values ​​without any clock assistance.

When all reset signals disappear, the delay counter is activated, thus prolonging the internal reset and allowing the power supply to reach a stable level before the MCU works normally. The overflow time of the delay counter is set by the user through the fuse bit CKSEL. For the selection of the delay time, please refer to P 34 “Clock Source”.

Reset Sources
ATmega128 has 5 reset sources:
• Power-on reset. When the power supply voltage is lower than the power-on reset threshold (VPOT), the MCU is reset.
• External reset. The MCU is reset when the low level on the RESET pin lasts longer than the minimum pulse width.
• Watchdog reset. A reset occurs when the watchdog is enabled and the watchdog timer times out.
• Brownout reset. When the brownout reset function is enabled, the MCU is reset when the power supply voltage is lower than the brownout reset threshold (VBOT).
• JTAG AVR reset: The MCU is reset when the reset register is 1. See P 235 “IEEE 1149.1 (JTAG) Boundary Scan”.

 
Notes: 
1. During a voltage drop, a reset will only occur when the voltage is below VPOT. 
2. Some devices may have a VBOT lower than the nominal minimum operating voltage. These devices are tested during production testing with VCC = VBOT to ensure that a brownout detection reset occurs before VCC drops to a level that stops the processor from functioning properly. The test condition for the ATmega128L is BODLEVEL=1 and the test condition for the ATmega128 is BODLEVEL=0. BODLEVEL=1 is not applicable to the ATmega128.