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

[colin]

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.

Colin Stanley

-----Original Message-----
From: address@hidden
[mailto:address@hidden On Behalf
Of Kevin Neff
Sent: Wednesday, March 09, 2005 11:17 AM
To: address@hidden
Subject: [avr-chat] Timer1 Frequency Doubling in ATMEGA128 App



I am in the midst of designing a real time embedded system that uses the
ATMEGA128.  Time keeping is handled by Timer1, a 16-bit timer. Every
second, an LED on the board is toggled or or off to make it easy to see
if the system crashes.

Depending on which power supply I use, I get the light blinking at 1 or
2 Hz.  The STK500 supply produces a reliable 1 Hz, but a 12 VDC supply
through a 7805 5VDC linear regulator produces 2 Hz.  It's also
interesting to know that if I reset (pull RESET\ low) the uC, it goes
back to 1 Hz.

Does anyone have a guess as to what's happening here?

--Kevin Neff





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