FLC FAILED TO OBTAIN A HARD COPY OF ATC CLRNC PRIOR TO DEP.

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UPON TKOF FROM ATL TO CLT; DEP CTL WAS CONTACTED AT APPROX 1000 FT AGL. AFTER INITIAL CONTACT ATC REPLIED RADAR CONTACT. IN ABOUT 10 SECONDS LATER; ATC SAID SQUAWK XXXX PLEASE. I LOOKED DOWN AND FOUND THAT 0000 HAD BEEN SET IN THE XPONDER; SO I RESET THE REQUESTED SQUAWK. I ALSO 'ZERO OUT' THE XPONDER ON EACH LNDG AND DON'T RESET IT UNTIL A CLRNC FOR THE NEXT FLT IS RECEIVED. I IMMEDIATELY THOUGHT WE HAD NOT RECEIVED A CLRNC AND ASKED THE FO. WE CHKED THE MESSAGE FILE ON OUR ACARS MACHINE TO FIND WE HAD NO CLRNC STORED. WE IMMEDIATELY REQUESTED ONE ON ACARS AND RECEIVED IT ON ACARS WITHIN ABOUT 10 SECONDS. IT HAD THE SAME CLRNC AS FILED AND THE SAME SQUAWK AS ATC HAD REQUESTED US TO SQUAWK. WE HAD TAXIED OUT AND TAKEN OFF WITHOUT ANYONE IN ATC NOTICING OUR MISTAKE. I HAD ALSO BRIEFED THE DEP WITHOUT ACTUALLY HAVING ONE AS WE GET SO USED TO 'THE SAME THING.' REMEDIES: 1) DON'T RUN 'BEFORE START' CHKLISTS AT GATE UNTIL PDC IS ACTUALLY OBTAINED AND DISCUSSED IN DEP BRIEFING. 2) HAVE ATC CONFIRM SQUAWK ON CLRNC BEFORE PUSHBACK (AS LGA ATC DOES) WHEN RECEIVING PDC ON ACARS. CAUSES: BEFORE START CHKLIST WAS RUN ALMOST 30 MINS BEFORE PUSH (TOO SOON) BECAUSE FO WAS HOLDING CHKLIST IN HAND; I FELT NEED TO RUN IT.