3 Connection of RESET Pin on AVR Devices

The RESET pin on the AVR device is active-low, and setting the pin low externally will reset the device. The Reset has two purposes:
  1. To release all the lines by tri-stating all pins (except XTAL pins), initialize all I/O registers and set the Program Counter (PC) to zero.
  2. To enter Programming mode (for some parts, the PEN line is also used to enter Programming mode). It is also possible to enter High-Voltage/Parallel Programming (HVPP) mode by drawing the RESET pin very high (11.5V – 12.5V). Refer to the respective device data sheet for more specific information about the RESET pin and its functionality.

The Reset line has an internal pull-up resistor. If the environment is noisy, it can be insufficient and Reset may occur sporadically. Refer to the device data sheet for the value of the pull-up resistor that must be used for specific devices.

Connecting the Reset so that it is possible to enter both high-voltage programming and ordinary low-level Reset can be achieved by using a pull-up resistor to the Reset line. This pull-up resistor avoids any unintended low signal that will trigger a Reset. Theoretically, the pull-up resistor can be of any value, but if the AVR device should be programmed using an external programmer, the pull-up should not be in such a high state that the programmer is not able to activate Reset by drawing the Reset line low. The recommended pull-up resistor value is 4.7 kΩ or larger when using STK®600 for programming. For DebugWIRE to function properly, the pull-up must not be less than 10 kΩ.

To protect the Reset line from further noise, connect a capacitor from the RESET pin to ground. This is not directly required since AVR devices internally have a low-pass filter to eliminate spikes and noise that could cause reset. Using an extra capacitor is an additional protection. However, such extra capacitor cannot be used when DebugWIRE or PDI is used.

ESD protection diode is not provided internally from Reset to VCC in order to allow HVPP. If HVPP is not used, it is recommended to add an ESD protection diode externally from Reset to VCC. Alternatively, a Zener diode can be used to limit the Reset voltage relative to GND. A Zener diode is highly recommended in noisy environments. The components should be located physically close to the RESET pin of the AVR device. A recommended circuit of a Reset line is shown in the following circuit diagram.

Figure 3-1. Recommended Reset Pin Connection
Note: The values of resistor R and capacitor C are typical values used for the RESET pin. For specific design requirements of an application, these values must be changed accordingly.