power looks reasonably ok, though that reset looks incredibly ugly!! the arduino designers should have placed a diode from RESET to +5V so as to clamp any overshoot on the pin, preventing it from rising more than 0.6v above the supply rail. does the RESET line look the same on the R1 board?
i'd be betting my money on the overshoot of the reset pulse causing the problem. the overshoot, in turn, being caused by the 1k pulldown resistor charging up the 0.1uf series capacitor (by default, the 8U2 pins are open-circuit until configured).a pulse that goes above the supply rail will do unpredictable things to the 328p, and the presence of your 4k7 pullup merely enables a set of conditions that direct the over-voltage pulse towards a lock-up condition.
now the interesting question will be... will the arduino team recall all the R2 boards?
:-) these are the moments that i live for - i'm a test engineer by trade, though a slightly unemployed one right now.
a pulse that goes above the supply rail will do unpredictable things to the 328p, and the presence of your 4k7 pullup merely enables a set of conditions that direct the over-voltage pulse towards a lock-up condition.
try placing a diode between RESET and +5V on the POWER connector, with the cathode (end with the bar) towards the +5V.
[...]Maybe not so unpredictable. Isn't a higher then Vcc voltage to the reset pin the method that puts the chip into it's high voltage programming mode? As I understand it the reset pin does not have an internal positive clamping diode to Vcc (unlike the normal I/O pins, just because of the higher then Vcc on reset method to enter HV programming mode.[...]Lefty
Would this then prevent the arduino board from being able to be programmed via the 'high voltage' method? Granted that would represent a very small population of users, however I've seen at least one shield based high voltage programmer avalible for sale. It allows those that have managed to 'brick' their processor by errors in fuse settings, i.e. change reset pin to be a I/O pin, etc.
If it does, it's easy for new design to add two via around the diode to shunt it easily using a basic wire, or even add an open pad to be shunt with solder.
QuoteIf it does, it's easy for new design to add two via around the diode to shunt it easily using a basic wire, or even add an open pad to be shunt with solder.But wouldn't that also shunt out the reset's pull-up resistor to Vcc, which would then cause direct connection from Vcc to the high voltage pulse/level used in HV parallel programming mode?Lefty
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