Re: [avr-chat] Timer1 Frequency Doubling in ATMEGA128 App

[Erik Christiansen]

On Wed, Mar 09, 2005 at 12:55:47PM -0800, colin wrote:
> Kevin:
> 
> You probaly need to add some more decoupling on the power line. Assuming
> the firmware is the same in both systems, Its probable that when the led
> lights, the extra current is feeding back into the clock or reset lines.
> There is an Atmel app note suggesting adding a 4.7nF cap on the reset
> line for noisy circuits. A 7805 should have a filter cap on the 5V side.

   Decoupling, adjacent to the 7805 pins as suggested, has another
benefit. The 5v regulator is liable to oscillate badly if not bypassed.
(In my early days I had 'em radiating strongly at 30 MHz. ;-) If that
doesn't make the cpu behave oddly, little will.

   If the LED current is sufficient to drop the supply, then the
electrolytic on the regulator input is probably inadequate.

Erik