AVR Hardware Design Considerations [APPLICATION NOTE]
Atmel-2521M-AVR-Hardware-Design-Considerations_ApplicationNote_092014
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Figure 1-2.
Decoupling with Series Inductor
Generally, the Atmel AVR devices where power and ground lines are placed close together (like the Atmel
ATmega8535) will get better decoupling than devices with industry standard pin-out (like the Atmel
ATmega8515), where the power and ground pins are placed in opposite corners of the DIP package. This
disadvantage can be overcome by using for example a TQFP package, which allows decoupling capacitors to be
placed very close to the die. For devices with multiple pairs of power and ground pins, it is essential that every
pair of pins get its own decoupling capacitor.
The main supply should also have a tantalum or ceramic capacitor of some µF to stabilize it.
1.2 Analog Supply
The AVR devices that have built-in ADC have a separate analog supply voltage pin, AVcc. This separate voltage
supply is provided to make the analog circuits less prone to the digital noise originating from the switching of the
digital circuits.
To be able to obtain good accuracy with the ADC the analog supply voltage must be decoupled separately, in the
same manner as the digital supply voltage. AREF must also be decoupled; a typical value for the capacitor is
100nF. If a separate analog ground (AGND) is present, the analog ground should be separated from the digital
ground, - so that the analog and digital grounds are only connected at one point, - the origins of the GND i.e. at
the power supply GND.
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Connection of RESET Pin on AVRs
The RESET pin on the AVR is active LOW, and setting the pin LOW externally will thus result in a reset of the
AVR. The RESET has two purposes:
1.
To release all lines by tri-stating all pins (except XTAL pins), initialize all I/O registers and set program
counter to zero.
2.
To enter programming mode (for some parts also the PEN line is used to enter programming mode).
Furthermore it is possible to enter high-voltage/parallel programming mode by drawing the RESET pin
“very” high, where very high means 11.5V – 12.5V (refer to the datasheet of the device for more
information).
The reset line has an internal pull-up resistor, but if the environment is noisy it can be insufficient and reset can
therefore occur sporadically. Refer to datasheet for value of pull-up resistor on 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 applying a pull-up resistor to the RESET line. This pull-up resistor makes sure that reset does
