SQL> create table ss(ss timestamp );
Table created.

SQL> insert into ss values(systimestamp);
1 row created.

CREATE OR REPLACE FUNCTION Fun_Ts (par_ts IN TIMESTAMP)
  RETURN CHAR
IS
  l_ts VARCHAR2(30);
BEGIN
  l_ts := TO_CHAR(par_ts, 'dd.mm.yyyy hh24:mi:ss:ff6');
  RETURN (l_ts);
END;
/

SELECT fun_ts(ss)
INTO :ts
FROM ss
WHERE rowid = :rowid;

FUNCTION Fun_Ts (par_ts IN TIMESTAMP DEFAULT NULL)

SELECT fun_ts(nvl(ss, systimestamp))

SQL> CREATE OR REPLACE FUNCTION Fun_Ts(par_ts IN TIMESTAMP default null)
  2    RETURN CHAR
  3  IS
  4    l_ts VARCHAR2(30);
  5  BEGIN
  6    l_ts := TO_CHAR(par_ts, 'dd.mm.yyyy hh24:mi:ss:ff6');
  7    RETURN (l_ts);
  8  END;
  9  /
Function created.

Begin
	:Control.Time_Stamp := TO_CHAR(SYSTIMESTAMP,'DD-MON-YYYY HH24:MI:SSxFF TZH:TZM');
End;