AVR Hardware Design Considerations [APPLICATION NOTE]
Atmel-2521M-AVR-Hardware-Design-Considerations_ApplicationNote_092014
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not go low unintended. The pull-up resistor can in theory be of any size, but if the Atmel AVR should be
programmed from e.g. STK
®
500/AVRISP the pull-up should not be so strong that the programmer cannot
activate RESET by draw the RESET line low. The recommended pull-up resistor is 4.7kΩ or larger when using
STK500 for programming. For DebugWIRE to function properly, the pull-up must not be smaller than 10kΩ.
To protect the RESET line further from noise, it is an advantage to connect a capacitor from the RESET pin to
ground. This is not directly required since the AVR internally have a low-pass filter to eliminate spikes and noise
that could cause reset. Applying an extra capacitor is thus an additional protection. However, note that this
capacitor cannot be present if DebugWIRE or PDI is used.
If not using High Voltage Programming it is recommended to add an ESD protecting diode from RESET to Vcc,
since this is not internally provided due to High Voltage Programming. Alternatively, or in addition, a Zener diode
can be used to limit the RESET voltage relative to GND. The Zener diode is highly recommended in noisy
environments. The components should be located physically close to the RESET pin of the AVR.
shows the recommended circuit on the RESET line.
2.1 External RESET Switch
If an external switch is connected to the RESET pin it is important to add a series resistance. Whenever the
switch is pressed it will short the capacitor, the current through the switch can have high peak values. This will
cause the switch to bounce and give steep spikes in 2ms - 10ms periods until the capacitor is discharged. The
PCB tracks and the switch metal will introduce a small inductance, this inductance can generate high voltages
according to Vi = L * di/dt. These voltages are most likely outside the specification of the RESET pin. By adding a
series resistor between the switch and the capacitor the peak currents generated will be significantly reduced,
these currents will not be large enough to generate high voltages at the RESET pin. An example connection is
shown in
Figure 2-1.
Recommended Reset Pin Connection
