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-- FILE:      uart.vhd

-- BRIEF:     UART TX/RX module

-- LICENSE:   MIT

-- COPYRIGHT: Richard James Howe (2019, 2020)

--

-- The UART (Universal Asynchronous Receiver/Transmitter) is one of the simplest

-- serial communication methods available. It is often used for debugging, for

-- issuing commands to embedded devices and sometimes even for uploading firmware

-- to devices. The data format and speed are configurable but there is no method

-- for automatically configuring a UART, both sides must have agreed on the

-- settings before hand. Configurable options include; baud, use of an

-- even of odd parity bit, number of data bits and number of stop bits.

--

-- The clock is not transmitted as part of the signal (which is why baud

-- must be agreed upon before hand), a single packet starts with a 'start bit',

-- where the line goes low. The receiver must synchronize to this start bit (it

-- resets the clock that generates pulse at the sample rate and baud when it

-- encounters a start bit).

--

-- A transmission with 8 data bits, 1 parity bit and 1 stop bit looks like

-- this:

-- ____   ______________________________   ________________

--     \_/|0|1|2|3|4|5|6|7|P|S|         \_/

--     Start   Data     Parity Stop    |--- More data --->

--

-- Start bits are always low, stop bits high. The most common format is

-- 8 data bits, no parity bit, and 1 stop bit at either 9600 or 115200 baud.

--

-- For the receiver a clock that is a multiple of the baud is used so the

-- bits can be sampled with a higher frequency than the bit rate.

--

-- Some notes:

-- * We can transmit from an 8-bit UART to a less than 8-bit UART fine, so

-- long as parity is not used, as

-- * Certain UARTs have the ability to transmit a BREAK signal by holding

-- the line low for a period greater than the packet length. We can transmit

-- that by lowering the baud rate and transmitting all zeroes. Receiving a

-- break (correctly) would require changing the receiver.

--

--

-- See:

-- * <https://en.wikipedia.org/wiki/Universal_asynchronous_receiver-transmitter>

--

-- NOTE: We could replace this entire package with an entirely software driven

-- solution. The only hardware we would need two timers driven at the sample

-- rate (one for RX, one for TX) and a deglitched RX signal. An interrupt

-- would be generated on the timers expiry.

--

library ieee, work;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

package uart_pkg is

        constant uart_8N1: std_ulogic_vector(7 downto 0) := "10000100";

        type uart_generics is record

                clock_frequency:    positive; -- clock frequency of module clock

                delay:              time;     -- gate delay for simulation purposes

                asynchronous_reset: boolean;  -- use asynchronous reset if true

        end record;

        component uart_tx is

                generic (

                        g:       uart_generics;

                        N:       positive;

                        format:  std_ulogic_vector(7 downto 0) := uart_8N1;

                        use_cfg: boolean);

                port (

                        clk:    in std_ulogic;

                        rst:    in std_ulogic;

                        cr:    out std_ulogic;

                        baud:   in std_ulogic; -- Pulse at baud

                        tx:    out std_ulogic;

                        ok:    out std_ulogic;

                        ctr:    in std_ulogic_vector(format'range);

                        ctr_we: in std_ulogic;

                        we:     in std_ulogic; -- di write enable

                        di:     in std_ulogic_vector(N - 1 downto 0));

        end component;

        component uart_rx is

                generic (

                        g:       uart_generics;

                        N:       positive;

                        D:       positive;

                        format:  std_ulogic_vector(7 downto 0) := uart_8N1;

                        use_cfg: boolean);

                port (

                        clk:     in std_ulogic;

                        rst:     in std_ulogic;

                        cr:     out std_ulogic;

                        baud:    in std_ulogic;

                        sample:  in std_ulogic;

                        failed: out std_ulogic_vector(1 downto 0);

                        ctr:     in std_ulogic_vector(7 downto 0);

                        ctr_we:  in std_ulogic;

                        rx:      in std_ulogic;

                        we:     out std_ulogic;

                        do:     out std_ulogic_vector(N - 1 downto 0));

        end component;

        component uart_fifo is

                generic (

                        g:           uart_generics;

                        data_width:  positive;

                        fifo_depth:  positive;

                        read_first:  boolean := true);

                port (

                        clk:   in  std_ulogic;

                        rst:   in  std_ulogic;

                        di:    in  std_ulogic_vector(data_width - 1 downto 0);

                        we:    in  std_ulogic;

                        re:    in  std_ulogic;

                        do:    out std_ulogic_vector(data_width - 1 downto 0);

                        -- optional

                        full:  out std_ulogic := '0';

                        empty: out std_ulogic := '1');

        end component;

        component uart_baud is -- Generates a pulse at the sample rate and baud

                generic (

                        g:    uart_generics;

                        init: integer;

                        N:    positive := 16;

                        D:    positive := 3);

                port (

                        clk:      in std_ulogic;

                        rst:      in std_ulogic;

                        we:       in std_ulogic;

                        cnt:      in std_ulogic_vector(N - 1 downto 0);

                        cr:       in std_ulogic := '0';

                        sample:  out std_ulogic;

                        baud:    out std_ulogic);

        end component;

        component uart_top is

                generic (

                        clock_frequency:    positive; -- clock frequency of module clock

                        delay:              time;     -- gate delay for simulation purposes

                        asynchronous_reset: boolean;  -- use asynchronous reset if true

                        baud:               positive := 115200;

                        format:             std_ulogic_vector(7 downto 0) := uart_8N1;

                        fifo_depth:         natural := 0;

                        use_cfg:            boolean := false;

                        use_tx:             boolean := true;

                        use_rx:             boolean := true

                        ); -- Use FIFO to buffer results?

                port (

                        clk:    in std_ulogic;

                        rst:    in std_ulogic;

                        tx:            out std_ulogic;

                        tx_fifo_full:  out std_ulogic;

                        tx_fifo_empty: out std_ulogic;

                        tx_fifo_we:     in std_ulogic;

                        tx_fifo_data:   in std_ulogic_vector(7 downto 0);

                        rx:             in std_ulogic;

                        rx_fifo_full:  out std_ulogic;

                        rx_fifo_empty: out std_ulogic;

                        rx_fifo_re:     in std_ulogic;

                        rx_fifo_data:  out std_ulogic_vector(7 downto 0);

                        reg:             in std_ulogic_vector(15 downto 0);

                        clock_reg_tx_we: in std_ulogic;

                        clock_reg_rx_we: in std_ulogic;

                        control_reg_we:  in std_ulogic);

        end component;

        component uart_tb is

                generic(g: uart_generics);

        end component;

        function parity(slv:std_ulogic_vector; even: boolean) return std_ulogic;

        function parity(slv:std_ulogic_vector; even: std_ulogic) return std_ulogic;

        constant ctr_use_parity:  integer := 0;

        constant ctr_even_parity: integer := 1;

        function ctr_stop_bits(ctr: std_ulogic_vector(7 downto 0)) return integer;

        function ctr_data_bits(ctr: std_ulogic_vector(7 downto 0)) return integer;

end package;

package body uart_pkg is

        function parity(slv: std_ulogic_vector; even: boolean) return std_ulogic is

                variable z: std_ulogic := '0';

        begin

                if not even then

                        z := '1';

                end if;

                for i in slv'range loop

                        z := z xor slv(i);

                end loop;

                return z;

        end;

        function parity(slv:std_ulogic_vector; even: std_ulogic) return std_ulogic is

                variable z: boolean := false;

        begin

                if even = '1' then

                        z := true;

                end if;

                return parity(slv, z);

        end;

        function ctr_stop_bits(ctr: std_ulogic_vector(7 downto 0)) return integer is

                variable ii: std_ulogic_vector(1 downto 0);

        begin

                ii := ctr(3 downto 2);

                return to_integer(unsigned(ii));

        end function;

        function ctr_data_bits(ctr: std_ulogic_vector(7 downto 0)) return integer is

                variable ii: std_ulogic_vector(3 downto 0);

        begin

                ii := ctr(7 downto 4);

                return to_integer(unsigned(ii));

        end function;

end;

library ieee, work;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

use work.uart_pkg.all;

entity uart_top is

        generic (

                clock_frequency:    positive; -- clock frequency of module clock

                delay:              time;     -- gate delay for simulation purposes

                asynchronous_reset: boolean;  -- use asynchronous reset if true

                baud:               positive := 115200;

                format:             std_ulogic_vector(7 downto 0) := uart_8N1;

                fifo_depth:         natural := 0; -- FIFO depth, 0,1 = no FIFO

                use_cfg:            boolean := false; -- allow run time configuration of baud rate and format?

                use_tx:             boolean := true;  -- instantiate TX functionality?

                use_rx:             boolean := true   -- instantiate RX functionality?

        );

        port (

                clk:    in std_ulogic;

                rst:    in std_ulogic;

                tx:            out std_ulogic;

                tx_fifo_full:  out std_ulogic;

                tx_fifo_empty: out std_ulogic;

                tx_fifo_we:     in std_ulogic;

                tx_fifo_data:   in std_ulogic_vector(7 downto 0);

                rx:             in std_ulogic;

                rx_fifo_full:  out std_ulogic;

                rx_fifo_empty: out std_ulogic;

                rx_fifo_re:     in std_ulogic;

                rx_fifo_data:  out std_ulogic_vector(7 downto 0);

                reg:             in std_ulogic_vector(15 downto 0);

                clock_reg_tx_we: in std_ulogic;

                clock_reg_rx_we: in std_ulogic;

                control_reg_we:  in std_ulogic);

end entity;

architecture structural of uart_top is

        constant g: uart_generics := (

                clock_frequency => clock_frequency,

                delay => delay,

                asynchronous_reset => asynchronous_reset

        );

        constant tx_init: integer  := g.clock_frequency / (baud * 16); -- 54 = 115200 @ 100 MHz

        constant rx_init: positive := tx_init - 1; -- 50 = 115200 @ 100 MHz + Fudge Factor

        constant N: positive := 8;

        signal rx_ok, rx_nd, rx_push, rx_re: std_ulogic := '0';

        signal rx_pushed: std_ulogic_vector(rx_fifo_data'range) := (others => '0');

        signal tx_pop, tx_ok: std_ulogic := '0';

        signal tx_popped: std_ulogic_vector(tx_fifo_data'range) := (others => '0');

        signal tx_fifo_empty_b, rx_fifo_full_b: std_ulogic := '0';

        signal tx_sample, tx_baud, tx_cr: std_ulogic := '0';

        signal rx_sample, rx_baud, rx_cr, rx_we: std_ulogic := '0';

        signal rx_fail: std_ulogic_vector(1 downto 0) := (others => '0');

        signal rx_ok_buf: std_ulogic := '0';

        signal do, do_c, do_n: std_ulogic_vector(rx_fifo_data'range) := (others => '0');

        signal fail_c, fail_n: std_ulogic_vector(1 downto 0) := (others => '0');

        signal nd_c, nd_n: std_ulogic := '0'; -- new data

begin

        rx_ok_buf <= not (fail_c(0) or fail_c(1)) after g.delay;

        rx_ok <= rx_ok_buf;

        rx_pushed <= do after g.delay;

        rx_nd <= nd_c and rx_ok_buf after g.delay; -- no new data if there are errors

        rx_re <= rx_push;

        process (clk, rst)

        begin

                if use_rx then

                        if rst = '1' and g.asynchronous_reset then

                                do_c   <= (others => '0') after g.delay;

                                fail_c <= (others => '0') after g.delay;

                                nd_c   <= '0' after g.delay;

                        elsif rising_edge(clk) then

                                if rst = '1' and not g.asynchronous_reset then

                                        do_c   <= (others => '0') after g.delay;

                                        fail_c <= (others => '0') after g.delay;

                                        nd_c   <= '0' after g.delay;

                                else

                                        do_c <= do_n     after g.delay;

                                        nd_c <= nd_n     after g.delay;

                                        fail_c <= fail_n after g.delay;

                                end if;

                        end if;

                end if;

        end process;

        process (do_c, do, nd_c, rx_we, rx_re, fail_c, rx_fail)

        begin

                if use_tx then

                        do_n   <= do_c   after g.delay;

                        nd_n   <= nd_c   after g.delay;

                        fail_n <= fail_c after g.delay;

                        if rx_we = '1' then

                                nd_n   <= '1'     after g.delay;

                                do_n   <= do      after g.delay;

                                fail_n <= rx_fail after g.delay;

                        elsif rx_re = '1' then

                                nd_n   <= '0' after g.delay;

                        end if;

                end if;

        end process;

        ugen0: if fifo_depth <= 1 generate

                        tx_pop        <= tx_fifo_we;

                        tx_popped     <= tx_fifo_data;

                        tx_fifo_full  <= not tx_ok;

                        tx_fifo_empty <= tx_ok;

                        rx_push       <= rx_fifo_re;

                        rx_fifo_data  <= rx_pushed;

                        rx_fifo_full  <= rx_nd;

                        rx_fifo_empty <= not rx_nd;

        end generate;

        ugen1: if fifo_depth > 1 generate

                tx_fifo_empty <=               tx_fifo_empty_b;

                tx_pop        <= tx_ok and not tx_fifo_empty_b;

                ugen1_fifo_tx: if use_tx generate

                        uart_fifo_tx_0: work.uart_pkg.uart_fifo

                                generic map(g => g,

                                           data_width => tx_fifo_data'length,

                                           fifo_depth => fifo_depth)

                                port map (clk  => clk, rst => rst,

                                          di   => tx_fifo_data,

                                          we   => tx_fifo_we,

                                          re   => tx_pop,

                                          do   => tx_popped,

                                          full => tx_fifo_full,

                                          empty => tx_fifo_empty_b);

                end generate;

                rx_fifo_full <=                         rx_fifo_full_b;

                rx_push      <= rx_nd and rx_ok and not rx_fifo_full_b;

                ugen1_fifo_rx: if use_rx generate

                        uart_fifo_rx_0: work.uart_pkg.uart_fifo

                                generic map(g => g,

                                           data_width => rx_fifo_data'length,

                                           fifo_depth => fifo_depth,

                                           read_first => false)

                                port map (clk  => clk, rst => rst,

                                          di   => rx_pushed,

                                          we   => rx_push,

                                          re   => rx_fifo_re,

                                          do   => rx_fifo_data,

                                          full => rx_fifo_full_b, empty => rx_fifo_empty);

                end generate;

        end generate;

        uart_tx_gen: if use_tx generate

                baud_tx: work.uart_pkg.uart_baud

                        generic map(g => g, init => tx_init, N => reg'length, D => 3)

                        port map(

                                clk    => clk,

                                rst    => rst,

                                we     => clock_reg_tx_we,

                                cnt    => reg,  -- 0x32/50 is 1152000 @ 100MHz clk

                                cr     => tx_cr,

                                sample => tx_sample,

                                baud   => tx_baud);

                tx_0: work.uart_pkg.uart_tx

                        generic map(g => g, N => N, format => format, use_cfg => use_cfg)

                        port map(

                                clk     => clk,

                                rst     => rst,

                                cr      => tx_cr,

                                baud    => tx_baud,

                                ok      => tx_ok,

                                we      => tx_pop,

                                ctr     => reg(reg'high downto reg'high - 7),

                                ctr_we  => control_reg_we,

                                di      => tx_popped,

                                tx      => tx);

        end generate;

        uart_rx_gen: if use_rx generate

                baud_rx: work.uart_pkg.uart_baud

                        generic map(g => g, init => rx_init, N => reg'length, D => 3)

                        port map(

                                clk    => clk,

                                rst    => rst,

                                we     => clock_reg_rx_we,

                                cnt    => reg,

                                cr     => rx_cr,

                                sample => rx_sample,

                                baud   => rx_baud);

                rx_0: work.uart_pkg.uart_rx

                        generic map(g => g, N => N, D => 3, format => format, use_cfg => use_cfg)

                        port map(

                                clk     => clk,

                                rst     => rst,

                                baud    => rx_baud,

                                sample  => rx_sample,

                                cr      => rx_cr,

                                failed  => rx_fail,

                                ctr     => reg(reg'low + 7 downto reg'low),

                                ctr_we  => control_reg_we,

                                we      => rx_we,

                                do      => do,

                                rx      => rx);

        end generate;

end architecture;

library ieee, work;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

use work.uart_pkg.all;

entity uart_fifo is

        generic (g: uart_generics;

                data_width: positive;

                fifo_depth: positive;

                read_first: boolean := true);

        port (

                clk:   in  std_ulogic;

                rst:   in  std_ulogic;

                di:    in  std_ulogic_vector(data_width - 1 downto 0);

                we:    in  std_ulogic;

                re:    in  std_ulogic;

                do:    out std_ulogic_vector(data_width - 1 downto 0);

                -- optional

                full:  out std_ulogic := '0';

                empty: out std_ulogic := '1');

end uart_fifo;

architecture behavior of uart_fifo is

        type fifo_data_t is array (0 to fifo_depth - 1) of std_ulogic_vector(di'range);

        signal data: fifo_data_t := (others => (others => '0'));

        function rindex_init return integer is

        begin

                if read_first then

                        return 0;

                end if;

                return fifo_depth - 1;

        end function;

        signal count:  integer range 0 to fifo_depth := 0;

        signal windex: integer range 0 to fifo_depth - 1 := 0;

        signal rindex: integer range 0 to fifo_depth - 1 := rindex_init;

        signal is_full:  std_ulogic := '0';

        signal is_empty: std_ulogic := '1';

begin

        do       <= data(rindex) after g.delay;

        full     <= is_full after g.delay;  -- buffer these bad boys

        empty    <= is_empty after g.delay;

        is_full  <= '1' when count = fifo_depth else '0' after g.delay;

        is_empty <= '1' when count = 0          else '0' after g.delay;

        process (rst, clk) is

        begin

                if rst = '1' and g.asynchronous_reset then

                        windex <= 0 after g.delay;

                        count  <= 0 after g.delay;

                        rindex <= rindex_init after g.delay;

                elsif rising_edge(clk) then

                        if rst = '1' and not g.asynchronous_reset then

                                windex <= 0 after g.delay;

                                count  <= 0 after g.delay;

                                rindex <= rindex_init after g.delay;

                        else

                                if we = '1' and re = '0' then

                                        if is_full = '0' then

                                                count <= count + 1 after g.delay;

                                        end if;

                                elsif we = '0' and re = '1' then

                                        if is_empty = '0' then

                                                count <= count - 1 after g.delay;

                                        end if;

                                end if;

                                if re = '1' and is_empty = '0' then

                                        if rindex = (fifo_depth - 1) then

                                                rindex <= 0 after g.delay;

                                        else

                                                rindex <= rindex + 1 after g.delay;

                                        end if;

                                end if;

                                if we = '1' and is_full = '0' then

                                        if windex = (fifo_depth - 1) then

                                                windex <= 0 after g.delay;

                                        else

                                                windex <= windex + 1 after g.delay;

                                        end if;

                                        data(windex) <= di after g.delay;

                                end if;

                        end if;

                end if;

        end process;

end behavior;

-- This module generates a sample pulse and a baud pulse. The sample rate

-- can be controlled by setting 'cnt'. This sample rate is then divided by

-- 2^(D+1) to get the baud.

library ieee, work;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

use work.uart_pkg.all;

entity uart_baud is

        generic (g: uart_generics; init: integer; N: positive := 16; D: positive := 3);

        port (

                clk:     in std_ulogic;

                rst:     in std_ulogic;

                we:      in std_ulogic;

                cnt:     in std_ulogic_vector(N - 1 downto 0);

                cr:      in std_ulogic := '0';

                sample: out std_ulogic;  -- sample pulse

                baud:   out std_ulogic); -- baud (sample rate / 2^(D+1))

end entity;

architecture behaviour of uart_baud is

        constant cmp_init: std_ulogic_vector := std_ulogic_vector(to_unsigned(init, cnt'length));

        signal cmp_c, cmp_n: std_ulogic_vector(cnt'range)  :=  cmp_init;

        signal cnt_c, cnt_n: std_ulogic_vector(cnt'range)  := (others => '0');

        signal div_c, div_n: std_ulogic_vector(D downto 0) := (others => '0');

        signal pul_c, pul_n: std_ulogic := '0';

        signal pulse: std_ulogic := '0';

begin

        pulse  <= (not cr) and pul_n and (pul_c xor pul_n) after g.delay; -- rising edge detector

        baud   <= (not cr) and div_n(div_n'high) and (div_c(div_c'high) xor div_n(div_n'high)) after g.delay;

        sample <= pulse;

        process (clk, rst)

        begin

                if rst = '1' and g.asynchronous_reset then

                        cmp_c <= cmp_init after g.delay;

                        cnt_c <= (others => '0') after g.delay;

                        div_c <= (others => '0') after g.delay;

                        pul_c <= '0' after g.delay;

                elsif rising_edge(clk) then

                        if rst = '1' and not g.asynchronous_reset then

                                cmp_c <= cmp_init after g.delay;

                                cnt_c <= (others => '0') after g.delay;

                                div_c <= (others => '0') after g.delay;

                                pul_c <= '0' after g.delay;

                        else

                                cmp_c <= cmp_n after g.delay;

                                cnt_c <= cnt_n after g.delay;

                                div_c <= div_n after g.delay;

                                pul_c <= pul_n after g.delay;

                        end if;

                end if;

        end process;

        process (pulse, div_c, cr, we)

        begin

                div_n <= div_c after g.delay;

                if cr = '1' or we = '1' then

                        div_n <= (others => '0') after g.delay;

                        div_n(div_n'high) <= '1' after g.delay;

                elsif pulse = '1' then

                        div_n <= std_ulogic_vector(unsigned(div_c) + 1) after g.delay;

                end if;

        end process;

        process (cmp_c, cnt_c, we, cnt, cr)

        begin

                cmp_n <= cmp_c after g.delay;

                cnt_n <= cnt_c after g.delay;

                if we = '1' then

                        cmp_n <= cnt after g.delay;

                        cnt_n <= (others => '0') after g.delay;

                        pul_n <= '0' after g.delay;

                elsif cr = '1' then

                        cnt_n <= (others => '0') after g.delay;

                        pul_n <= '0' after g.delay;

                elsif cnt_c = cmp_c then

                        cnt_n <= (others => '0') after g.delay;

                        pul_n <= '1' after g.delay;

                else

                        cnt_n <= std_ulogic_vector(unsigned(cnt_c) + 1) after g.delay;

                        pul_n <= '0' after g.delay;

                end if;

        end process;

end architecture;

library ieee, work;

use ieee.std_logic_1164.all;

use work.uart_pkg.all;

entity uart_rx is

        generic (

                g:       uart_generics;

                N:       positive;

                D:       positive;