ems metaboard

Posted on Feb 6, 2014

Any microcontroller in the ATmegaXX8 family should work, though if you`re going to go to the trouble of building this, spending a few tens of cents on extra flash memory seems like a prudent idea. A DIP socket is, strictly speaking, optional, but not having it is a genuinely bad idea. (Update: a 28-pin DIP socket is a bit of an oddball size. If

ems metaboard
Click here to download the full size of the above Circuit.

you don`t have one handy, you can substitute a 20-pin socket plus an 8-pin socket. ) The 20pF caps are for loading the crystal. The cheap dipped ceramic kind work fine. You might be able to get away with 22pF if that`s what you have on hand (but make sure they match!). If you pick a different crystal speed, check the specs to find the right loading capacitor value. The crystal can be 12, 15, 16 or 20MHz. (Make sure you get one with at least five significant digits, e. g. 12. 000. ) You might also be able to substitute a three-pin crystal oscillator for the crystal and loading caps. The Zener diodes are there to clamp the USB D+ and D- voltages to 3. 6V (rather than the native 5V) when the AVR is driving the data lines. Most modern USB hosts deal with 5V on the data lines just fine, but some older ones do not. Not just any 3. 6V Zener diode will do; the high-current ones switch too slowly and distort the USB waveform too much. Get the 1N5227B rated for 20mA Zener current. The pushbutton switch is for reset. It`s totally optional, but convenient. Use any junk-box normally-open switch you like. (I salvaged mine from a broken stereo receiver. ) The breakaway header should fit the kind of jumper you see on computer motherboards and hard disks. The jumper is there to let you select if the bootloader or application runs when power is first applied. You could use an SPST toggle switch instead if you like. The least expensive...

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