-- *****************************************************************************
-- uc_interface.vhd
-- 
-- Created:  9/15/00 ALS
--      This file provides the 8051 external data memory bus interface 
--      for the SPI Master.
--
-- Revised:  12/12/02 JRH
-- *****************************************************************************

library IEEE;
use IEEE.std_logic_1164.all;


entity uC_interface is
    generic (UC_ADDRESS : std_logic_vector(15 downto 8):= "00000000"  );
    port(
            -- 8051  bus interface
            clk         : in  std_logic;
            reset       : in  std_logic;     
        
            addr_data   : inout std_logic_vector (7 downto 0);  -- multiplexed address/data bus
            addr        : in    std_logic_vector (7 downto 0);  -- high byte of address 
            ale_n       : in    std_logic;                      -- address latch enable, active low 
            psen_n      : in    std_logic;                      -- program store enable, active low
        
            -- directional pins
            rd_n        : in    std_logic;  -- active low read strobe 
            wr_n        : in    std_logic;  -- active low write strobe
            int_n       : inout std_logic;  -- active low interrupt request
    
            -- internal spi signals
            -- interface to spi_control_sm
            spien       : inout std_logic;  -- enables the spi interface logic
            start       : inout std_logic;  -- start transfer
            done        : in    std_logic;  -- byte transfer is complete
            rcv_load    : in    std_logic;  -- load control signal to spi receive register
            spissr      : inout std_logic_vector(7 downto 0);   -- uc slave select register
            ss_n        : in    std_logic;  -- internal version of ss_n being output
            ss_in_int   : in    std_logic;  -- internal sampled version of ss_in_n needed by
                                -- uc to generate an interrupt
            xmit_empty  : in    std_logic;  -- flag indicating that spitr is empty
            xmit_empty_reset: inout std_logic;  -- xmit empty flag reset when spitr is written
            rcv_full    : in    std_logic;  -- flag indicating that spirr has new data
            rcv_full_reset  : inout std_logic;  -- rcv full flag reset when spirr is read

            -- interface to sck_logic;
            clkdiv      : inout std_logic_vector(1 downto 0);   -- sets the clock divisor for sck clock
            cpha        : inout std_logic;  -- sets clock phase for output sck clock
            cpol        : inout std_logic;  -- sets clock polarity for output sck clock
         
            -- interface to receive and transmit shift registers 
            spitr       : inout std_logic_vector (7 downto 0);  -- data to transmit on spi bus
            rcv_cpol    : inout std_logic;          -- clock polarity for incoming data 
            receive_data: in    std_logic_vector (7 downto 0)   -- data received from SPI bus
        
        );
        

end uC_interface;




architecture BEHAVIOUR of uC_interface is

--**************************** Constants ***************************************
constant RESET_ACTIVE : STD_LOGIC := '0';

-- Base Address for SPI Module (addr_bus[15:8])
constant BASE_ADDR  : STD_LOGIC_VECTOR(15 downto 8) := UC_ADDRESS;

-- Register Addresses (5 Total):
-- Status Register (BASE + 80h)
constant SPISR_ADDR     : STD_LOGIC_VECTOR(7 downto 0) := "10000000";

-- Control Register (BASE + 84h)
constant SPICR_ADDR     : STD_LOGIC_VECTOR(7 downto 0) := "10000100";

-- Slave Select Register (BASE + 88h)
constant SPISSR_ADDR    : STD_LOGIC_VECTOR(7 downto 0) := "10001000";

-- Transmit Data Register (BASE + 8Ah)
constant SPITR_ADDR     : STD_LOGIC_VECTOR(7 downto 0) := "10001010";

-- Receive Data Register (BASE + 8Eh)
constant SPIRR_ADDR : STD_LOGIC_VECTOR(7 downto 0) := "10001110";


--**************************** Signal Definitions ***************************************

-- Internal handshaking lines for microprocessor
signal data_out : STD_LOGIC_VECTOR(7 downto 0); -- holds the data to be output on the data bus
signal data_in  : STD_LOGIC_VECTOR(7 downto 0); -- holds the data to be input to the chip
signal data_oe  : STD_LOGIC;                    -- allows data to be output on the data bus

-- State signals for target state machine
type STATE_TYPE is (IDLE, ADDR_DECODE, DATA_TRS, END_CYCLE);
signal prs_state, next_state : STATE_TYPE;                  

-- Address match
signal address_match    : std_logic;

-- Register Enable Lines
signal cntrl_en     : std_logic;    -- control register is addressed
signal stat_en      : std_logic;    -- status register is addressed
signal xmit_en      : std_logic;    -- transmit data register is addressed  
signal rcv_en       : std_logic;    -- receive data register is addressed
signal ssel_en      : std_logic;    -- slave select register is addressed

-- Register reset lines
signal spierr_reset : STD_LOGIC;    -- writing 0 this bit in the status register 
                                    -- generates a reset to the bit
signal int_reset    : STD_LOGIC;    -- writing 0 this bit in the status register 
                                    -- generates a reset to the bit

-- low byte address lines
signal address_low  : STD_LOGIC_VECTOR(7 downto 0);

-- receive data register
signal spirr        : STD_LOGIC_VECTOR(7 downto 0); -- data received from SPI bus

-- control register signals
signal inten        : STD_LOGIC;    -- interrupt enable

-- status register signals
signal dt       : STD_LOGIC;    -- data transferring bit
signal spierr       : STD_LOGIC;    -- spi error bit
signal bb       : STD_LOGIC;    -- bus busy bit



begin

--************************** Bi-directional Data Bus **********************************

-- Bi-directional Data bus
addr_data <= data_out when (data_oe = '1') else (others => 'Z');
data_in <= addr_data when wr_n = '0' else (others => '0');


--************************** uC Interface State Machine *******************************
-- Register process registers next state signals
-- Return to IDLE state whenever RESET is asserted  

UC_SM_REGS: process (clk, reset)
begin
        if reset = RESET_ACTIVE then
            prs_state <= IDLE;
            
        elsif clk'event and clk = '1' then
            prs_state <= next_state;
            
        end if;
    
end process;

-- Combinatorial process determines next state logic
COMBINATIONAL: process (prs_state, ale_n, rd_n, wr_n, address_match, psen_n)
    
begin
    
next_state <= prs_state;
data_oe <= '0';
    
    
    case prs_state is
    
        --****************** IDLE State  *********************
        when IDLE =>

                -- Wait for falling edge of ALE_N with PSEN_N negated
            if ale_n = '0' and psen_n = '1' then
                -- falling edge of ALE_N
                next_state <= ADDR_DECODE;
            end if;

        
        --****************** ADDR_DECODE State  *****************
        when ADDR_DECODE =>
            
            -- Check that this module is being address
            if address_match = '1' then
                    -- Wait for rd_n or wr_n to be asserted
                    if rd_n = '0' or wr_n = '0' then
                        next_state <= DATA_TRS;
                end if;
            else
                -- this module is not being addressed
                  next_state <= IDLE;
            end if;
    
            
        --****************** DATA_TRS State  *********************
        when DATA_TRS =>

                -- Read or write from enabled register (see uC_regs process)
                -- if read cycle, assert the data output enable
                if rd_n = '0' then
                    data_oe <= '1';
                end if;
                
                -- wait until rd_n and wr_n negates before ending cycle
                if rd_n = '1' and wr_n = '1' then
                    next_state <= END_CYCLE;
                end if;

                        
        --****************** END_CYCLE State ********************
        when END_CYCLE =>
                
            
            -- Wait for negation of ale_n
            if (ale_n = '1') then
              next_state <= IDLE;
            end if;
        
             end case;
        
end process;

--************************** Address Registers **********************************
-- This process registers the low byte of address from the multiplexed address/data bus
-- on the falling edge of ale

address_regs: process(reset, ale_n)
begin
        if reset = RESET_ACTIVE then
        
            address_low <= (others => '0');
            
        elsif ale_n'event and ale_n = '0' then
            
            address_low <= addr_data;
        
        end if;
    
end process;

--************************** Address Decode **********************************
-- This process decodes the address and sets enables for the registers
address_decode: process (reset, clk)
begin
    if reset = RESET_ACTIVE then
        
        address_match   <= '0'; -- signal indicating that base address matches
        
        xmit_en     <= '0'; -- xmit data register enable
        rcv_en      <= '0'; -- receive data register enable
        cntrl_en    <= '0'; -- control register enable
        stat_en     <= '0'; -- status register enable
        ssel_en     <= '0'; -- slave select register enable
          
          
        -- Synchronize with rising edge of clock
        elsif clk'event and (clk = '1') then
        
          if ale_n = '0' and addr = BASE_ADDR and psen_n = '1' then
            -- base address matches and address is stable   
            address_match <= '1';
            
                -- Check appropriate register address
                case address_low(7 downto 0) is
              
                    when SPISR_ADDR =>  -- status register has been addressed