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Topic: Can someone check our Eagle schematic for a homebrew board (Read 1 time) previous topic - next topic

Hi all,

I have been playing around with the idea of having a basic board with only a 7805, 16 mhz crystal, the atmega chip and the pin headers for outputs/inputs and the FTDI cable connection. Can someone please verify that this schematic would, infact work?

I am a beginner in electronics, please be gentle :)

Thanks in advance.

ps:Used an Atmega8 on the schematic, I believe it has the same pinout with 168 and 328.

Jack Christensen

If JP1 is an FTDI header, I'd connect pin 2 to ground as well. Also I believe RX and TX are crossed. A 100nF capacitor should go between JP1 pin 6 and the ATmega pin1. Maybe a reset button from pin 1 to GND.

Connect a 10K resistor from the ATmega pin 1 to +5V.
Connect AVcc to +5V.
Connect a 100nF ceramic capacitor from AREF to GND.
Connect a 100nF ceramic from GND to either pin 7 or to pin 20 on the ATmega. Some people might use one on each pin, but I've found that one is sufficient.

I also like to add 100nF ceramics in parallel with the electrolytics on the input and output of the regulator. Kinda like chicken soup, it can't hurt.

I'd avoid the diagonal lines on the schematic, makes it hard to read. Also, some signals that should be connected don't have dots where they meet, not sure if that's a problem. What was the drawing made with?

Thanks a bunch Jack Christensen! That was very helpful.
I have revised the schematic according to your suggestions as best as I can.

The schematic is made with Eagle, which I have also attached.

Why do we connect pin1 to 5V? is it not the rest pin and should it not be connected to the DTR on the FTDI end? Or is it actually always fed 5V and the DTR actually resets the chip by cutting the current?

Thank you very many :)

Jack Christensen

Why do we connect pin1 to 5V? is it not the rest pin and should it not be connected to the DTR on the FTDI end? Or is it actually always fed 5V and the DTR actually resets the chip by cutting the current?

Pin 1 is not connected directly to 5V, it's connected through a 10K pullup resistor. The resistor helps to ensure that reset is held high and that no unintended resets occur. It can still be pulled low with a button, or the FTDI interface does it through a 100nF capacitor, this generates enough of a negative-going pulse to reset the MCU.

By the way, the component labeled 10K2 is not a resistor, that is the symbol for a capacitor.

One of the non-obvious things with Eagle at first is that components are not connected with wires! Use NETs instead. If the toolbar is docked on the left side of the window, the NET icon is down a few from WIRE and on the right. So bad news, all the wires need to be changed to NETs. That way, the dots will automatically appear showing connections.

Another nice thing with NETs is that they can be named. When there are two (or more) separate NETs on a schematic, but they are given the same name, then Eagle understands that they are connected together. Vcc and GND are common examples. I'm attaching my basic ATmega328 circuit. Note that pin 1 hooks to a NET named /RESET, and that there is another NET named /RESET above the reset switch. Eagle understands these two to be electrically connected. When using this technique, it's strongly suggested to LABEL the NETs so that it's clear when looking at the drawing that they're connected. In fact, if you don't, I believe Eagle's electrical rule check will complain about it. This is a feature some people love to hate, and while a person can certainly go too far with it, OTOH it does have its place.

One other feature I like to include is a diode on the power input to prevent damage if reverse-polarity power is accidentally connected. Good insurance for a few pennies.

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