OOOPPPs
Joined: 04 Mar 2007
Posts: 1
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 13 Jul 2008 22:03 Basic Schematic Design (AVR) |
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Hi,
Im not sure if this is the right section, but i just want to ask for some information regarding the basic circuitry design. My Electrical Engineering skills are not flashy.
I want to essentially make a programmable microcontroler board (using ATmega64 series) which is connected to a bunch of IO devices (such as LCD/LED), but when I look at sample schematics, I find that there are resistors attached to certain ports of the microcontroller (ie. nearly all of the schematics have a Resistor connected to each pin of the I/O ports) and then some crazy capacitors at the reset & Vcc pins.
(http://www.blitzlogic.com/mat_90.gif)
Is there a way I could find out how I should connect this?
Yes im pretty much starting from a very very very basic platform, so any info is good info heheh.
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banjo
Joined: 24 Dec 2005
Posts: 455
Helped: 86
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| 14 Jul 2008 3:35 Basic Schematic Design (AVR) |
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The resistors are there to limit the current. The LEDs have a typical voltage drop of about 2V. The AVR is driving common cathode LEDs and each individual anode connection has a 120 ohm resistor. Use good old Ohms Law and assume that the drop across the AVR can be ignored. The LED current is (5-2)/120 or 25mA. Without these 120 ohm resistors, the AVR or LED would be damaged.
The 2.2K resistors are used to limit the base current to the transistors to a reasonable level.
The resistor and capacitor on the reset is a very common arrangement for micrcontrollers that use an active high reset. The large cap pulls a large current at powerup. This pulls the reset pin high due to the voltage drop across the 8.2K resistor, R2. Once C3 charges up, the voltage on the reset pin drops low and the AVR starts executing code.
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