ArduinoISP on Uno requires 10uF cap -- WHY?

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from http://arduino.cc/en/tutorial/arduinoisp:

quote
5.wire arduino board target shown in diagram below. (note arduino uno: you'll need add 10 uf capacitor between reset , ground.)


can explain electronically why needed , why works?  uno in particular causes problem?  why 10uf , not 0.1uf or something?

yes, tried searching, couldn't find explanations.  just, "do this, works."

when avrdude connects serial port dtr signal asserted, has side effect of triggering auto reset.  avrdude finds talking bootloader rather isp sketch.  capacitor absorbs auto-reset pulse , allows avrdude establish proper communication arduinoisp.

more recent versions of uno bootloader detect corrupt serial communication (the bootloader , isp sketch use different baud rates) , quit immediately.  allows avrdude start working without need suppression of auto-reset.


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