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-- HF-RISCV v1.3

-- Sergio Johann Filho, 2015 - 2016

--

-- *This is a quick and dirty organization of a 3-stage pipelined RISC-V microprocessor. All registers / memory

--  accesses are synchronized to the rising edge of clock. The same processor could be designed with only 2

--  pipeline stages, but this would require memories to be either asynchronous (as presented on comp arq text

--  books), double clocked or operating on the opposite edge. Pipeline stages are:

--

--  FETCH: instruction memory is accessed (address is PC), data becomes available in one cycle. PC is updated.

--  DECODE: an instruction is fed into the decoding / control logic and values are registered for the next

--  stage. pipeline stalls, as well as bubble insertion is performed in this stage.

--  EXECUTE: the register file is accessed and the ALU calculates the result. data access is performed (loads

--  and stores) or simply the result (or pc) is written to the register file (normal operations). branch target

--  and outcome are calculated.

--

-- *This design is a compromise between performance, area and complexity.

-- *Only the RV32I base instruction set is implemented. FENCE and SYSTEM instructions are missing. SYSTEM

--  instructions always trap the processor and can be handled in software.

-- *Memory is accessed in little endian mode.

-- *No co-processor is implemented and all peripherals are memory mapped.

-- *Loads and stores take 2/1 cycles with separated code/data memories and 3 cycles otherwise. This version is organized

--  as a Von Neumann machine, so there is only one memory interface that is shared betweeen code and data accesses.

-- *Branches have a 1 cycle delay (not taken) or 3 cycle dalay (taken), including two branch delay slots.

--  This is a side effect of the pipeline refill and memory access policy. All other instructions are single

--  cycle. No branch predictor is implemented (default branch target is 'not taken').

-- *Interrupts are handled using VECTOR, CAUSE, MASK, STATUS and EPC registers. The VECTOR register is used to hold

--  the address of the default (non-vectored) interrupt handler. The CAUSE register is read only and peripheral

--  interrupt lines are connected to this register. The MASK register is read/write and holds the interrupt mask

--  for the CAUSE register. The interrupt STATUS register is automatically cleared on interrupts, and is set by

--  software when returning from interrupts - this works as a global interrupt enable/disable flag. This register is

--  read and write capable, so it can also be cleared by software. Setting this register just before returning

--  from interrupts (enable is delayed in a few cycles) re-enables interrupts. The EPC register holds the program

--  counter when the processor is interrupted (we should re-execute the last instruction (EPC-4), as it was not

--  commited yet). EPC is a read only register, and is used to return from an interrupt using simple LW / JR

--  instructions. As an interrupt is accepted, the processor jumps to VECTOR address where the first level of irq

--  handling is done. A second level handler (in C) implements the interrupt priority mechanism and calls the

--  appropriate ISR for each interrupt.

-- *Built in peripherals: running counter (32 bit), two counter comparators (32 and 24 bit), I/O ports and UART. the

--  UART baud rate is defined in a 16 bit divisor register. Two counter bits (bits 18 and 16 and their complements) are

--  tied to interrupt lines, so are the two counter comparators and the UART.

--

-- Memory map:

--

-- ROM                                  0x00000000 - 0x1fffffff (512MB)

-- System                               0x20000000 - 0x3fffffff (512MB)

-- SRAM                                 0x40000000 - 0x5fffffff (512MB)

-- External RAM / device                0x60000000 - 0x9fffffff (1GB)

-- External RAM / device                0xa0000000 - 0xdfffffff (1GB)           (uncached)

-- External Peripheral                  0xe0000000 - 0xefffffff (256MB)         (uncached)

-- Peripheral (core)                    0xf0000000 - 0xf7ffffff (128MB)         (uncached)

-- Peripheral (extended)                0xf8000000 - 0xffffffff (128MB)         (uncached)

--

--   IRQ_VECTOR                 0xf0000000

--   IRQ_CAUSE                  0xf0000010

--   IRQ_MASK                   0xf0000020

--   IRQ_STATUS                 0xf0000030

--   IRQ_EPC                    0xf0000040

--   COUNTER                    0xf0000050

--   COMPARE                    0xf0000060

--   COMPARE2                   0xf0000070

--   EXTIO_IN                   0xf0000080

--   EXTIO_OUT                  0xf0000090

--   DEBUG                      0xf00000d0

--   UART_WRITE / UART_READ     0xf00000e0

--   UART_DIVISOR               0xf00000f0

--

-- Interrupt masks:

--

-- IRQ_COUNTER                  0x0001          (bit 18 of the counter is set)

-- IRQ_COUNTER_NOT              0x0002          (bit 18 of the counter is clear)

-- IRQ_COUNTER2                 0x0004          (bit 16 of the counter is set)

-- IRQ_COUNTER2_NOT             0x0008          (bit 16 of the counter is clear)

-- IRQ_COMPARE                  0x0010          (counter is equal to compare, clears irq when updated)

-- IRQ_COMPARE2                 0x0020          (counter bits 23 to 0 are equal to compare2, clears irq when updated)

-- IRQ_UART_READ_AVAILABLE      0x0040          (there is data available for reading on the UART)

-- IRQ_UART_WRITE_AVAILABLE     0x0080          (UART is not busy)

-- EXT_IRQ0                     0x0100          (external interrupts on extio_in, 'high' level triggered)

-- EXT_IRQ1                     0x0200

-- EXT_IRQ2                     0x0400

-- EXT_IRQ3                     0x0800

-- EXT_IRQ4                     0x1000

-- EXT_IRQ5                     0x2000

-- EXT_IRQ6                     0x4000

-- EXT_IRQ7                     0x8000

library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_unsigned.all;

use ieee.std_logic_arith.all;

entity busmux is

        generic(

                log_file: string := "UNUSED";                   -- options are "out.txt" and "UNUSED"

                uart_support: string := "no"                    -- options are "yes" and "no".

        );

        port (  clock:          in std_logic;

                reset:          in std_logic;

                stall:          in std_logic;

                stall_cpu:      out std_logic;

                busy_cpu:       out std_logic;

                irq_vector_cpu: out std_logic_vector(31 downto 0);

                irq_cpu:        out std_logic;

                irq_ack_cpu:    in std_logic;

                exception_cpu:  in std_logic;

                inst_addr_cpu:  in std_logic_vector(31 downto 0);

                inst_in_cpu:    out std_logic_vector(31 downto 0);

                data_addr_cpu:  in std_logic_vector(31 downto 0);

                data_in_cpu:    out std_logic_vector(31 downto 0);

                data_out_cpu:   in std_logic_vector(31 downto 0);

                data_w_cpu:     in std_logic_vector(3 downto 0);

                data_access_cpu:        in std_logic;

                addr_mem:       out std_logic_vector(31 downto 0);

                data_read_mem:  in std_logic_vector(31 downto 0);

                data_write_mem: out std_logic_vector(31 downto 0);

                data_we_mem:    out std_logic_vector(3 downto 0);

                extio_in:       in std_logic_vector(7 downto 0);

                extio_out:      out std_logic_vector(7 downto 0);

                uart_read:      in std_logic;

                uart_write:     out std_logic

        );

end busmux;

architecture arch of busmux is

        signal write_enable: std_logic;

        signal irq_cause, irq_mask_reg, uart_divisor: std_logic_vector(15 downto 0);

        signal irq_status_reg, extio_out_reg: std_logic_vector(7 downto 0);

        signal irq_vector_reg, irq_epc_reg, compare_reg, counter_reg: std_logic_vector(31 downto 0);

        signal compare2_reg: std_logic_vector(23 downto 0);

        signal interrupt, irq, irq_counter, irq_counter_not, irq_counter2, irq_counter2_not, irq_compare, irq_compare2, compare_trig, compare2_trig: std_logic;

        signal data_read_uart, data_write_uart: std_logic_vector(7 downto 0);

        signal enable_uart, enable_uart_read, enable_uart_write, uart_write_busy, uart_data_avail: std_logic;

        type pulse_state_type is (irq_idle, irq_int, irq_req, irq_ackn, irq_done);

        signal pulse_state: pulse_state_type;

        signal pulse_next_state: pulse_state_type;

        signal periph_access, periph_access_we, data_access_cpu_dly, data_access_cpu_dly2: std_logic;

        signal data_we_mem_s: std_logic_vector(3 downto 0);

begin

        -- address decoder, read from peripheral registers

        process(data_addr_cpu, irq_vector_reg, irq_cause, irq_mask_reg, irq_status_reg, irq_epc_reg, compare_reg, compare2_reg, counter_reg, data_read_uart, uart_divisor, data_read_mem, extio_in, extio_out_reg)

        begin

                case data_addr_cpu(31 downto 27) is

                        when "11110" =>                         -- Peripherals     (f000 0000 - f7ff ffff)

                                case data_addr_cpu(7 downto 4) is

                                        when "0000" =>          -- IRQ_VECTOR           (RW)

                                                data_in_cpu <= irq_vector_reg;

                                        when "0001" =>          -- IRQ_CAUSE            (RO)

                                                data_in_cpu <= irq_cause(7 downto 0) & irq_cause(15 downto 8) & x"0000";

                                        when "0010" =>          -- IRQ_MASK             (RW)

                                                data_in_cpu <= irq_mask_reg(7 downto 0) & irq_mask_reg(15 downto 8) & x"0000";

                                        when "0011" =>          -- IRQ_STATUS           (RW)

                                                data_in_cpu <= irq_status_reg & x"000000";

                                        when "0100" =>          -- IRQ_EPC              (RO)

                                                data_in_cpu <= irq_epc_reg(7 downto 0) & irq_epc_reg(15 downto 8) & irq_epc_reg(23 downto 16) & irq_epc_reg(31 downto 24);

                                        when "0101" =>          -- COUNTER              (RO)

                                                data_in_cpu <= counter_reg(7 downto 0) & counter_reg(15 downto 8) & counter_reg(23 downto 16) & counter_reg(31 downto 24);

                                        when "0110" =>          -- IRQ_COMPARE          (RW)

                                                data_in_cpu <= compare_reg(7 downto 0) & compare_reg(15 downto 8) & compare_reg(23 downto 16) & compare_reg(31 downto 24);

                                        when "0111" =>          -- IRQ_COMPARE2         (RW)

                                                data_in_cpu <= compare_reg(15 downto 8) & compare_reg(23 downto 16) & compare_reg(31 downto 24) & x"00";

                                        when "1000" =>          -- EXTIO_IN             (RO)

                                                data_in_cpu <= extio_in & x"000000";

                                        when "1001" =>          -- EXTIO_OUT            (RW)

                                                data_in_cpu <= extio_out_reg & x"000000";

                                        when "1110" =>          -- UART                 (RW)

                                                data_in_cpu <= data_read_uart & x"000000";

                                        when "1111" =>          -- UART_DIVISOR         (RW)

                                                data_in_cpu <= uart_divisor(7 downto 0) & uart_divisor(15 downto 8) & x"0000";

                                        when others =>

                                                data_in_cpu <= data_read_mem;

                                end case;

                        when others =>                          -- ROM / RAM area, external peripherals (f800 0000 - ffff fffc)

                                data_in_cpu <= data_read_mem;

                end case;

        end process;

        inst_in_cpu <= data_read_mem;

        -- peripheral register logic, write to peripheral registers

        process(clock, reset, counter_reg, data_addr_cpu, data_out_cpu, periph_access, periph_access_we, irq)

        begin

                if reset = '1' then

                        irq_vector_reg <= x"00000000";

                        irq_mask_reg <= x"0000";

                        irq_status_reg <= x"00";

                        counter_reg <= x"00000000";

                        compare_reg <= x"00000000";

                        compare_trig <= '0';

                        compare2_reg <= x"000000";

                        compare2_trig <= '0';

                        extio_out_reg <= x"00";

                        uart_divisor <= x"0000";

                elsif clock'event and clock = '1' then

                        counter_reg <= counter_reg + 1;

                        if compare_reg = counter_reg then

                                compare_trig <= '1';

                        end if;

                        if compare2_reg = counter_reg(23 downto 0) then

                                compare2_trig <= '1';

                        end if;

                        if periph_access = '1' and periph_access_we = '1' then

                                case data_addr_cpu(7 downto 4) is

                                        when "0000" =>  -- IRQ_VECTOR

                                                irq_vector_reg <= data_out_cpu(7 downto 0) & data_out_cpu(15 downto 8) & data_out_cpu(23 downto 16) & data_out_cpu(31 downto 24);

                                        when "0010" =>  -- IRQ_MASK

                                                irq_mask_reg <= data_out_cpu(23 downto 16) & data_out_cpu(31 downto 24);

                                        when "0011" =>  -- IRQ_STATUS

                                                irq_status_reg <= data_out_cpu(31 downto 24);

                                        when "0110" =>  -- IRQ_COMPARE

                                                compare_reg <= data_out_cpu(7 downto 0) & data_out_cpu(15 downto 8) & data_out_cpu(23 downto 16) & data_out_cpu(31 downto 24);

                                                compare_trig <= '0';

                                        when "0111" =>  -- IRQ_COMPARE2

                                                compare2_reg <= data_out_cpu(15 downto 8) & data_out_cpu(23 downto 16) & data_out_cpu(31 downto 24);

                                                compare2_trig <= '0';

                                        when "1001" =>  -- EXTIO_OUT

                                                extio_out_reg <= data_out_cpu(31 downto 24);

                                        when "1111" =>  -- UART_DIVISOR

                                                uart_divisor <= data_out_cpu(23 downto 16) & data_out_cpu(31 downto 24);

                                        when others =>

                                end case;

                        end if;

                        if irq_ack_cpu = '1' or exception_cpu = '1' then

                                irq_status_reg(0) <= '0';         -- IRQ_STATUS (clear master int bit on interrupt)

                        end if;

                end if;

        end process;

        -- EPC register register load on interrupts

        process(clock, reset, inst_addr_cpu, irq, irq_ack_cpu)

        begin

                if reset = '1' then

                        irq_epc_reg <= x"00000000";

                elsif clock'event and clock = '1' then

                        if ((irq = '1' and irq_ack_cpu = '0') or exception_cpu = '1') then

                                irq_epc_reg <= inst_addr_cpu;

                        end if;

                end if;

        end process;

        -- interrupt state machine

        process(clock, reset, pulse_state, interrupt, irq_status_reg, stall)

        begin

                if reset = '1' then

                        pulse_state <= irq_idle;

                        pulse_next_state <= irq_idle;

                        irq <= '0';

                elsif clock'event and clock = '1' then

                        if stall = '0' then

                                pulse_state <= pulse_next_state;

                                case pulse_state is

                                        when irq_idle =>

                                                if interrupt = '1' and irq_status_reg(0) = '1' then

                                                        pulse_next_state <= irq_int;

                                                end if;

                                        when irq_int =>

                                                irq <= '1';

                                                pulse_next_state <= irq_req;

                                        when irq_req =>

                                                if irq_ack_cpu = '1' then

                                                        irq <= '0';

                                                        pulse_next_state <= irq_ackn;

                                                end if;

                                        when irq_ackn =>

                                                pulse_next_state <= irq_done;

                                        when irq_done =>

                                                if irq_status_reg(0) = '1' then

                                                        pulse_next_state <= irq_idle;

                                                end if;

                                        when others =>

                                                pulse_next_state <= irq_idle;

                                end case;

                        end if;

                end if;

        end process;

        -- data / peripheral access delay

        process(clock, reset, irq_ack_cpu, data_access_cpu, stall)

        begin

                if reset = '1' then

                        data_access_cpu_dly <= '0';

                        data_access_cpu_dly2 <= '0';

                elsif clock'event and clock = '1' then

                        if stall = '0' then

                                data_access_cpu_dly2 <= data_access_cpu_dly;

                                if data_access_cpu = '1' and data_access_cpu_dly = '0' and data_access_cpu_dly2 = '0' then

                                        data_access_cpu_dly <= '1';

                                else

                                        data_access_cpu_dly <= '0';

                                end if;

                        end if;

                end if;

        end process;

        periph_access <= '1' when data_addr_cpu(31 downto 27) = "11110" and data_access_cpu = '1' else '0';

        periph_access_we <= '1' when periph_access <= '1' and data_w_cpu /= "0000" else '0';

        -- memory address / write enable muxes and cpu stall logic

        addr_mem <= data_addr_cpu when data_access_cpu_dly = '0' and data_access_cpu = '1' and periph_access = '0' else inst_addr_cpu;

        data_write_mem <= data_out_cpu;

        data_we_mem_s <= data_w_cpu when data_access_cpu_dly = '0' and data_access_cpu = '1' and periph_access = '0' else "0000";

        data_we_mem <= data_we_mem_s;

        busy_cpu <= (data_access_cpu and not data_access_cpu_dly);                                      -- load/store: 1 wait cycle

        stall_cpu <= stall;

        -- interrupts and peripherals

        interrupt <= '0' when (irq_cause and irq_mask_reg) = x"0000" else '1';

        irq_cause <= extio_in & not uart_write_busy & uart_data_avail & irq_compare2 & irq_compare & irq_counter2_not & irq_counter2 & irq_counter_not & irq_counter;

        irq_cpu <= irq;

        irq_vector_cpu <= irq_vector_reg;

        irq_counter <= counter_reg(18);

        irq_counter_not <= not counter_reg(18);

        irq_counter2 <= counter_reg(16);

        irq_counter2_not <= not counter_reg(16);

        irq_compare <= '1' when compare_trig = '1' else '0';

        irq_compare2 <= '1' when compare2_trig = '1' else '0';

        extio_out <= extio_out_reg;

        write_enable <= '1' when data_we_mem_s /= "0000" else '0';

        data_write_uart <= data_out_cpu(31 downto 24);

        uart:

        if uart_support = "yes" generate

                enable_uart <= '1' when periph_access = '1' and data_addr_cpu(7 downto 4) = "1110" else '0';

                enable_uart_write <= enable_uart and periph_access_we;

                enable_uart_read <= enable_uart and not periph_access_we;

                -- a simple UART

                serial: entity work.uart

                generic map (log_file => log_file)

                port map(

                        clk             => clock,

                        reset           => reset,

                        divisor         => uart_divisor(11 downto 0),

                        enable_read     => enable_uart_read,

                        enable_write    => enable_uart_write,

                        data_in         => data_write_uart,

                        data_out        => data_read_uart,

                        uart_read       => uart_read,

                        uart_write      => uart_write,

                        busy_write      => uart_write_busy,

                        data_avail      => uart_data_avail

                );

        end generate;

        no_uart:

        if uart_support = "no" generate

                enable_uart <= '0';

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

                uart_write_busy <= '0';

                uart_data_avail <= '0';

        end generate;

end arch;