AVR-Chat

On Mar 31, 2006, at 4:56 AM, Massimo Banzi wrote:
>
> I'm having some weird issues with an Atmega8 based board where the
> processor won't start the program unless the
> avrisp dongle is connected.
>
>
> You can see the circuit here http://www.potemkin.org/uploads/Pid/
> arduino_rs232_v2.png
>
> the rs232 interface part is not installed.
>
> I've checked and the fuses are programmed correctly (there are about
> 1500 usb boards that we sold with the same fuse settings)
>
> it also happened a few times with some usb boards that when they are
> powered via the external power supply they have troubles starting up
> while when powered by the USB they work correctly.
>
>
I took a look at the power-up on a USB one configured for external  
power. This board has a 47uF aluminum cap before and after the  
regulator, rather than the 100uF cap shown in the schematic.

Starting it up with a 12V/500mA brick by connecting the brick and  
turning on the switch on the power strip I got the attached trace at  
the output of the regulator. Notice the large burst of trash. At  
times (especially when turning the supply off) it was a bit in excess  
of 5V!

Your input cap of 47uF is probably just fine.

You might want to reduce the output cap size.

[ See Linear Tech's app note AN88 on the overvoltage transient  
problems caused by using ceramic input capacitors: http:// 
www.linear.com/pc/downloadDocument.do?navId=D4125 ]