ENTITY serial2parallel IS
  GENERIC (bps : INTEGER);
  PORT (serial, received : IN BIT;  dataready : BUFFER BIT;  
        overrun, frame_error : OUT BIT; 
        parallel_out : BUFFER BIT_VECTOR (7 DOWNTO 0));
END serial2parallel;
--
ARCHITECTURE waiting OF serial2parallel IS
BEGIN
  collect : PROCESS  
    VARIABLE buff : BIT_VECTOR (7 DOWNTO 0);
    CONSTANT half_bit : TIME := (1000000.0/REAL(bps))/2.0 * 1 US; 
    CONSTANT full_bit : TIME := (1000000.0/REAL(bps)) * 1 US;
  BEGIN
  --Look for Start, start collecting data
  --If Stop not found, issue frame error
  END PROCESS collect;
  --
  too_fast : PROCESS
  BEGIN
  -- While collecting, check for overrun
  END PROCESS too_fast;
END waiting;

ARCHITECTURE waiting OF serial2parallel IS
BEGIN
  collect : PROCESS  
    VARIABLE buff : BIT_VECTOR (7 DOWNTO 0);
    CONSTANT half_bit : TIME := (1000000.0/REAL(bps))/2.0 * 1 US;  
    CONSTANT full_bit : TIME := (1000000.0/REAL(bps)) * 1 US;
  BEGIN
    WAIT UNTIL serial = '0';
    WAIT FOR half_bit; 
    FOR count IN 0 TO 7 LOOP
      WAIT FOR full_bit;
      buff (count) := serial;
    END LOOP;
    WAIT FOR full_bit;
    IF serial = '0' THEN
      frame_error <= '1';
      WAIT UNTIL serial = '1';
    ELSE
      frame_error <= '0';
      dataready <= '1';
      parallel_out <= buff;
      WAIT UNTIL received = '1';
      WAIT UNTIL received = '0';
      dataready <= '0';
    END IF;
  END PROCESS collect;
  --
  too_fast : PROCESS
  BEGIN
  -- While collecting, check for overrun
  END PROCESS too_fast;
END waiting;

ARCHITECTURE waiting OF serial2parallel IS
BEGIN
  collect : PROCESS  
    VARIABLE buff : BIT_VECTOR (7 DOWNTO 0);
    CONSTANT half_bit : TIME := (1000000.0/REAL(bps))/2.0 * 1 US;  
    CONSTANT full_bit : TIME := (1000000.0/REAL(bps)) * 1 US;
BEGIN
  --Look for Start, start collecting data
  --If Stop not found, issue frame error
  END PROCESS collect;
  --
  too_fast : PROCESS
  BEGIN
    IF dataready = '1' THEN
      WAIT UNTIL serial = '0';
      IF dataready = '1' THEN
        overrun <= '1';
      END IF;
    ELSE
      overrun <= '0';
    END IF;
    WAIT ON dataready;
  END PROCESS too_fast;
END waiting;

PACKAGE waiting IS
  TYPE wait_array IS ARRAY (NATURAL RANGE <>) OF NATURAL;
END waiting;
--
USE WORK.waiting.ALL;  
ENTITY arbitrator IS
  GENERIC (wait_states : wait_array := (0, 1, 2, 3); 
           clock_period : TIME := 1 US);
  PORT (request : IN BIT_VECTOR (3 DOWNTO 0); 
        grant : BUFFER BIT_VECTOR (3 DOWNTO 0); clock : IN BIT);  
END arbitrator;
--
ARCHITECTURE behavioral OF arbitrator IS
BEGIN
  wait_cycle: PROCESS 
  BEGIN
    IF clock = '0' THEN
      WAIT FOR 20 NS;
      FOR i IN request'RANGE LOOP
        IF request(i) = '1' THEN
          IF grant = "0000" THEN 
            WAIT FOR wait_states (i) * clock_period;
            grant (i) <= '1', '0' AFTER clock_period;  
          END IF;
        END IF; 
      END LOOP;
    END IF;
    WAIT ON clock;
  END PROCESS wait_cycle;
END behavioral;