Auto update timestamp in postgres

update.txt

[bpm@dev000] c:/opt>cat sql
BEGIN;
CREATE OR REPLACE FUNCTION update_timestamp()
RETURNS TRIGGER AS $$
BEGIN
      NEW.updated = now(); 
      RETURN NEW;
END;
$$ language 'plpgsql';
COMMIT;

BEGIN;
CREATE TABLE account(
  id serial not null PRIMARY KEY,
  email VARCHAR(128) NOT NULL UNIQUE,
  username VARCHAR(30) NOT NULL UNIQUE,
  password VARCHAR(128) NOT NULL,
  updated timestamp default CURRENT_TIMESTAMP,
  inserted timestamp default CURRENT_TIMESTAMP
);

CREATE TRIGGER user_timestamp BEFORE INSERT OR UPDATE ON account
FOR EACH ROW EXECUTE PROCEDURE update_timestamp();

COMMIT;

[bpm@dev000] c:/opt>sudo -u postgres psql db
psql (8.4.20)
Type "help" for help.

db=# insert into account (email, username, password) values ('a@a.com', 'a', 'abc');
INSERT 0 1
db=# select * from account;
 id |  email  | username | password |          updated           |          inserted          
----+---------+----------+----------+----------------------------+----------------------------
  3 | a@a.com | a        | abc      | 2014-11-07 09:06:46.526592 | 2014-11-07 09:06:46.526592
(1 row)

db=# update account set username = 'b' where id = 3; 
UPDATE 1
db=# select * from account;
 id |  email  | username | password |          updated           |          inserted          
----+---------+----------+----------+----------------------------+----------------------------
  3 | a@a.com | b        | abc      | 2014-11-07 09:06:53.671612 | 2014-11-07 09:06:46.526592
(1 row)

db=#

This is not good. it will loop cycle