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--------------------------------------------------------------------

--!

--! PDP-8 Processor

--!

--! \brief

--!      LS8E Serial Printer

--!

--! \details

--!      This device provides a LS8E register set compatible

--!      interface to a serial printer.

--!

--!

--! \note

--!      The design information for this device is taken from

--!      the LS8E Engineering Specificaton number LS8-E-2

--!

--! \file

--!      ls8e.vhd

--!

--! \author

--!      Rob Doyle - doyle (at) cox (dot) net

--!

--------------------------------------------------------------------

--

--  Copyright (C) 2009, 2010, 2011, 2012 Rob Doyle

--

-- This source file may be used and distributed without

-- restriction provided that this copyright statement is not

-- removed from the file and that any derivative work contains

-- the original copyright notice and the associated disclaimer.

--

-- This source file is free software; you can redistribute it

-- and/or modify it under the terms of the GNU Lesser General

-- Public License as published by the Free Software Foundation;

-- version 2.1 of the License.

--

-- This source is distributed in the hope that it will be

-- useful, but WITHOUT ANY WARRANTY; without even the implied

-- warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR

-- PURPOSE. See the GNU Lesser General Public License for more

-- details.

--

-- You should have received a copy of the GNU Lesser General

-- Public License along with this source; if not, download it

-- from http://www.gnu.org/licenses/lgpl.txt

--

--------------------------------------------------------------------

--

-- Comments are formatted for doxygen

--

library ieee;                                   --! IEEE Library

use ieee.std_logic_1164.all;                    --! IEEE 1164

use work.uart_types.all;                        --! UART Types

use work.ls8e_types.all;                        --! PR8E Types

use work.cpu_types.all;                         --! CPU Types

--

--! LS8E Serial Printer Entity

--

entity eLS8E is port (

    sys    : in  sys_t;                         --! Clock/Reset

    uartBR : in  uartBR_t;                      --! Baud Rate Select

    uartHS : in  uartHS_t;                      --! Handshaking Select

    devNUM : in  devNUM_t;                      --! IOT Device

    cpu    : in  cpu_t;                         --! CPU Input

    dev    : out dev_t;                         --! Device Output

    dtr    : in  std_logic;                     --! Data Terminal Ready

    dsr    : out std_logic;                     --! Data Set Ready

    rxd    : in  std_logic;                     --! Serial Data In

    txd    : out std_logic                      --! Serial Data Out

);

end eLS8E;

--

--! LS8E Serial Printer RTL

--

architecture rtl of eLS8E is

    -- State

    type     state_t   is (stateIdle, stateBuffered, stateWaiting);

    signal   state     : state_t;               --! Transmitter State

    signal   clkBR     : std_logic;             --! 16x Baud Rate Clock Enable

    -- PR8E Registers

    signal   ieSET     : std_logic;             --! Set/Clear Interrupt Enable Register

    signal   ieREG     : std_logic;             --! Interrupt Enable Register

    signal   flagCLR   : std_logic;             --! Clear Flag Register

    signal   flagSET   : std_logic;             --! Set Flag Register

    signal   flagREG   : std_logic;             --! Flag REG

    signal   busyREG   : std_logic;             --! Printer status

    -- RX signals

    signal   rxIntr    : std_logic;             --! UART Receiver has data

    signal   rxData    : ascii_t;               --! UART Receiver Buffer

    -- TX signals

    signal   txIntr    : std_logic;             --! UART Transmitter has sent data

    signal   txData    : ascii_t;               --! UART Transmitter Buffer

    -- Misc

    signal   start     : std_logic;             --! Start

    signal   loadUART  : std_logic;             --! Load Transmitter UART

    signal   DTRd      : std_logic_vector(0 to 1);  --! DTR demet

begin

    --

    --! LE8E Bus Interface

    --!

    --! \details

    --!     The Bus Interface decodes the individual LS8E IOT instructions.

    --!     The various operations: Enable Interrupts, Disable Interrupts,

    --!     Set Flag, and Clear Flag are decoded and provided to the

    --!     synchronous process that maintains the PR8E register state.

    --!

    --! \note

    --!     The Bus Interface is totally asynchronous.  The dev.ack,

    --!     dev.skip, and dev.devc signals are combinationally derived from

    --!     CPU output bus signals.  These signals will be sampled by the

    --!     CPU on the device bus input on the next clock cycle.

    --

    LS8E_BUSINTF : process(cpu.buss, devNUM, flagREG, ieREG)

    begin

        dev     <= nulldev;

        ieSET   <= '0';

        flagSET <= '0';

        flagCLR <= '0';

        start   <= '0';

        if cpu.buss.addr(0 to 2) = opIOT and cpu.buss.addr(3 to 8) = devNUM and cpu.buss.lxdar = '1' then

            --

            -- Printer IOTs

            --

            case cpu.buss.addr(9 to 11) is

                --

                -- IOT 6xx0: Set Printer Flag

                --  Sets the Printer interrupt enable flip-flop

                --  so that an interrupt request will be generated

                --  when the printer flag is set.

                --

                when opLS0 =>

                    dev.ack  <= '1';

                    dev.devc <= devWR;

                    dev.skip <= '0';

                    flagSET  <= '1';

                --

                -- IOT 6xx1: Skip on Printer Flag

                --  This senses the printer flag.  If the flag

                --  is set, then the next instructon is skipped.

                --

                when opLS1 =>

                    dev.ack  <= '1';

                    dev.devc <= devWR;

                    dev.skip <= flagREG;

                --

                -- IOT 6xx2: Clear Printer Flag

                --

                when opLS2 =>

                    dev.ack  <= '1';

                    dev.devc <= devWR;

                    dev.skip <= '0';

                    flagCLR  <= '1';

                --

                -- IOT 6xx4: Load Printer Buffer

                --  Transfers AC(5:11) to the output buffer and

                --  later outputs the buffer content to the

                --  printer.  The Printer Flag is not cleared.

                --

                when opLS4 =>

                    dev.ack  <= '1';

                    dev.devc <= devWR;

                    dev.skip <= '0';

                    start    <= '1';

                --

                -- IOT 6xx5: Set/Clear Interrupt Enable

                --  Enable interrupts if AC(11) is set.

                --  Disable interrupts if AC(11) is cleared.

                --

                when opLS5 =>

                    dev.ack  <= '1';

                    dev.devc <= devWR;

                    dev.skip <= '0';

                    ieSET    <= '1';

                --

                -- IOT 6xx6: Load Printer Buffer Sequence

                --  Transfers AC(5:11) to the output buffer and

                --  later outputs the buffer content to the

                --  printer.  The Printer Flag is cleared.

                --

                when opLS6 =>

                    dev.ack  <= '1';

                    dev.devc <= devWR;

                    dev.skip <= '0';

                    flagCLR  <= '1';

                    start    <= '1';

                --

                -- Everything else

                --

                when others =>

                    null;

            end case;

        end if;

        dev.intr <= flagREG and ieREG;

        dev.data <= o"0000";

    end process;

    --

    --! This process maintains the LS8E Registers

    --!

    --! -# The Printer Flag is cleared by initialize or CAF (IOCLR)

    --! -# The Interupt Enable is set by initialize or CAF (IOCLR)

    --

    REG_LS8E : process(sys)

    begin

        if sys.rst = '1' then

            ieREG    <= '1';

            flagREG  <= '0';

            loadUART <= '0';

            txData   <= (others => '0');

            state    <= stateIdle;

        elsif rising_edge(sys.clk) then

            if cpu.buss.ioclr = '1' then

                ieREG   <= '1';

                flagREG <= '0';

            else

                --

                -- Update Interrupt Enable Register

                --

                if ieSET = '1' then

                    ieREG <= cpu.buss.data(11);

                end if;

                --

                -- Update Flag Register

                --

                if flagCLR = '1' then

                    flagREG <= '0';

                elsif flagSET = '1' or txIntr = '1' then

                    flagREG <= '1';

                end if;

                --

                -- Send character

                --

                case state is

                    --

                    -- stateIdle:

                    -- Wait for print IOT.  Buffer character to print.

                    --

                    when stateIdle =>

                        if start = '1' then

                            txData <= cpu.buss.data(4 to 11);

                            state  <= stateBuffered;

                        end if;

                    --

                    -- stateBuffered:

                    -- Wait for printer to be not busy.  Load UART

                    --

                    when stateBuffered =>

                        if busyREG = '0' then

                            loadUART <= '1';

                            state    <= stateWaiting;

                        end if;

                    --

                    -- stateWaiting:

                    -- Wait for IOT to finish.

                    --

                    when stateWaiting =>

                        if start = '0' then

                            state <= stateIdle;

                        end if;

                    --

                    -- Everything Else

                    --

                    when others =>

                        null;

                end case;

            end if;

        end if;

    end process REG_LS8E;

    --

    --! This process keeps track of the printer status.

    --! The printer status can be controlled by hardware (DTR)

    --! signal or by software (XON/XOFF) protocol.

    --

    REG_BUSY : process(sys)

    begin

        if sys.rst = '1' then

            busyREG <= '0';

            DTRd    <= (others => '0');

        elsif rising_edge(sys.clk) then

            if cpu.buss.ioclr = '1' then

                busyREG   <= '0';

            else

                --

                -- Synchronize DTR

                --

                DTRd(0) <= dtr;

                DTRd(1) <= DTRd(0);

                case uartHS is

                    --

                    -- No Handskaking

                    --

                    when uartHSnone =>

                        busyREG <= '0';

                    --

                    -- Hardware Handshaking

                    --

                    when uartHShw =>

                        busyREG <= DTRd(1);

                    --

                    -- Software (XON/XOFF) handshaking

                    --

                    when uartHSsw =>

                        if rxIntr = '1' then

                            if rxdata = xon then

                                busyREG <= '1';

                            elsif rxdata = xoff then

                                busyREG <= '0';

                            end if;

                        end if;

                    --

                    -- Everythig Else

                    --

                    when others =>

                        null;

                end case;

            end if;

        end if;

    end process REG_BUSY;

    --

    --! UART Baud Rate Generator

    --

    iUART_BRG : entity work.eUART_BRG port map (

        sys    => sys,

        uartBR => uartBR,

        clkBR  => clkBR

    );

    --

    --! The UART Receiver is used for ACK/NAK handshaking

    --

    iUART_RX : entity work.eUART_RX port map (

        sys   => sys,

        clkBR => clkBR,

        rxd   => rxd,

        intr  => rxIntr,

        data  => rxData

    );

    --

    --! The UART Transmitter sends data to the printer

    --

    iUART_TX : entity work.eUART_TX port map (

        sys   => sys,

        clkBR => clkBR,

        data  => txData,

        load  => loadUART,

        intr  => txIntr,

        txd   => txd

    );

end rtl;