Atmega startup(power-up) problem

[agreatstar]

I have a zigbee connected to atmega16. I am using 12V lead-acid battery to power the controller. Sometimes when I power up, the Zigbee TX line is contnuously turned on and xbee doesnt work. But if I remove the battery and replug it, everything works fine. Problem occurs randomly, almost 4 times in 10 runs. One thing that I dout is Atmega reset coz on my board, reset is directly connected to VCC (no external power-up reset circuit is present and brown-out detection is disabled). Can that be the problem?

 

[tgootee]

Do you have proper decoupling and bypass capacitors from all power pins to ground?

 

[jayanth.devarayanadurga]

Turn ON uC first and then XBee. Let uC control the power to XBee.