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[bug #56584] Arduino double reset before programming (TIOCM_DTR clear/se

From: David Grayson
Subject: [bug #56584] Arduino double reset before programming (TIOCM_DTR clear/set swapped)
Date: Tue, 3 Dec 2019 14:16:02 -0500 (EST)
User-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/78.0.3904.108 Safari/537.36

Follow-up Comment #2, bug #56584 (project avrdude):

Hello.  The existing code in AVRDUDE is correct.  We need to set DTR to 0 for
some time to ensure that the the DTR pin is high and that the capacitor
discharges.  (Both sides of the capacitor will be high after it discharges.) 
Then we need to set DTR to 1, which causes a falling edge on the DTR pin,
which sends a low  pulse to the AVR's reset line.  The reset line gradually
rises back to a high level as the capacitor charges.

This code is probably redundant on some operating systems, like macOS, where
DTR line automatically gets set high when you connect to a serial port.  But
it's needed on Windows with usbser.sys, and in general.


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