CAPT OF AN AIRBUS A320 OVERSHOT ASSIGNED ALT ON A STAR ARR DUE TO CAPT'S FMS PROGRAM ERROR.

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WE WERE APCHING OUR DEST; ORD. OUR AIRPLANE WAS AN AIRBUS A320. I WAS THE PF. WE HAD BEEN GIVEN A DSCNT CLRNC TO FL250. AS WE WERE DSNDING AND APPROX 175 NM FROM ORD (BTWN KEOKK INTXN AND BDF) THE FO WENT OFF THE #1 RADIO TO GET THE ATIS ON #2 AND CALL IN-RANGE TO OPS. I WAS TALKING TO ZAU ON FREQ 135.15. AS I WAS APCHING FL250; CTR GAVE ME FURTHER DSCNT TO FL240. I ENTERED THE ALT INTO THE ALT SELECTOR ON THE FCU AND INITIATED A MANAGED DSCNT. THEN I WENT TO THE MCDU AND ENTERED A NEW CRUISE ALT OF FL240 ON THE PROGRESS PAGE. AS I WAS APCHING FL240; ZAU HANDED ME OFF TO ANOTHER SECTOR ON FREQ 124.55. WHEN I CHKED IN WITH THE NEW CTLR; I SAID '...LEVELING AT 240; AHH; DSNDING THROUGH FL240 FOR FL230.' I HAD GLANCED AT THE FCU ALT WINDOW AND IT SHOWED FL230. THIS IS WHAT CAUSED ME TO REVISE MY STATEMENT TO THE CTR CTLR. THE CTLR ACKNOWLEDGED MY XMISSION. I CONTINUED THE DSCNT TOWARDS FL230 AND ENTERED THE NEW CRUISE ALT INTO THE MCDU PROGRESS PAGE. AT ABOUT FL233; THE CTR CTLR TOLD ME TO 'GET BACK UP TO FL240.' I CLBED BACK UP TO FL240. AT THIS TIME; THE FO CAME BACK ON TO THE #1 RADIO. HAVING REALIZED MY MISTAKE; I NOTIFIED MY FO THAT I HAD BUSTED AN ALT.