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antonblanchard.microwatt/fpga/pp_soc_uart.vhd
Benjamin Herrenschmidt e5aa0e9dc9 uart: Remove combinational loops on ack and stall signal 
They hurt timing forcing signals to come from the master and back
again in one cycle. Stall isn't sampled by the master unless there
is an active cycle so masking it with cyc is pointless. Masking acks
is somewhat pointless too as we don't handle early dropping of cyc
in any of our slaves properly anyways.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
2020-06-13 11:38:34 +10:00

396 lines
11 KiB
VHDL
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-- The Potato Processor - A simple processor for FPGAs
-- (c) Kristian Klomsten Skordal 2014 - 2016 <kristian.skordal@wafflemail.net>
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
--! @brief Simple UART module.
--! The following registers are defined:
--! |--------------------|--------------------------------------------|
--! | Address | Description |
--! |--------------------|--------------------------------------------|
--! | 0x00 | Transmit register (write-only) |
--! | 0x08 | Receive register (read-only) |
--! | 0x10 | Status register (read-only) |
--! | 0x18 | Sample clock divisor register (read/write) |
--! | 0x20 | Interrupt enable register (read/write) |
--! |--------------------|--------------------------------------------|
--!
--! The status register contains the following bits:
--! - Bit 0: receive buffer empty
--! - Bit 1: transmit buffer empty
--! - Bit 2: receive buffer full
--! - Bit 3: transmit buffer full
--!
--! The sample clock divisor should be set according to the formula:
--! sample_clk = (f_clk / (baudrate * 16)) - 1
--!
--! If the sample clock divisor register is set to 0, the sample clock
--! is stopped.
--!
--! Interrupts are enabled by setting the corresponding bit in the interrupt
--! enable register. The following bits are available:
--! - Bit 0: data received (receive buffer not empty)
--! - Bit 1: ready to send data (transmit buffer empty)
entity pp_soc_uart is
generic(
FIFO_DEPTH : natural := 64 --! Depth of the input and output FIFOs.
);
port(
clk : in std_logic;
reset : in std_logic;
-- UART ports:
txd : out std_logic;
rxd : in std_logic;
-- Interrupt signal:
irq : out std_logic;
-- Wishbone ports:
wb_adr_in : in std_logic_vector(11 downto 0);
wb_dat_in : in std_logic_vector( 7 downto 0);
wb_dat_out : out std_logic_vector( 7 downto 0);
wb_we_in : in std_logic;
wb_cyc_in : in std_logic;
wb_stb_in : in std_logic;
wb_ack_out : out std_logic
);
end entity pp_soc_uart;
architecture behaviour of pp_soc_uart is
subtype bitnumber is natural range 0 to 7; --! Type representing the index of a bit.
-- UART sample clock signals:
signal sample_clk : std_logic;
signal sample_clk_divisor : std_logic_vector(7 downto 0);
signal sample_clk_counter : std_logic_vector(sample_clk_divisor'range);
-- UART receive process signals:
type rx_state_type is (IDLE, RECEIVE, STARTBIT, STOPBIT);
signal rx_state : rx_state_type;
signal rx_byte : std_logic_vector(7 downto 0);
signal rx_current_bit : bitnumber;
subtype rx_sample_counter_type is natural range 0 to 15;
signal rx_sample_counter : rx_sample_counter_type;
signal rx_sample_value : rx_sample_counter_type;
subtype rx_sample_delay_type is natural range 0 to 7;
signal rx_sample_delay : rx_sample_delay_type;
-- UART transmit process signals:
type tx_state_type is (IDLE, TRANSMIT, STOPBIT);
signal tx_state : tx_state_type;
signal tx_byte : std_logic_vector(7 downto 0);
signal tx_current_bit : bitnumber;
-- UART transmit clock:
subtype uart_tx_counter_type is natural range 0 to 15;
signal uart_tx_counter : uart_tx_counter_type := 0;
signal uart_tx_clk : std_logic;
-- Buffer signals:
signal send_buffer_full, send_buffer_empty : std_logic;
signal recv_buffer_full, recv_buffer_empty : std_logic;
signal send_buffer_input, send_buffer_output : std_logic_vector(7 downto 0);
signal recv_buffer_input, recv_buffer_output : std_logic_vector(7 downto 0);
signal send_buffer_push, send_buffer_pop : std_logic := '0';
signal recv_buffer_push, recv_buffer_pop : std_logic := '0';
-- IRQ enable signals:
signal irq_recv_enable, irq_tx_ready_enable : std_logic := '0';
-- Wishbone signals:
type wb_state_type is (IDLE, WRITE_ACK, READ_ACK);
signal wb_state : wb_state_type;
signal rxd2 : std_logic := '1';
signal rxd3 : std_logic := '1';
signal txd2 : std_ulogic := '1';
begin
irq <= (irq_recv_enable and (not recv_buffer_empty))
or (irq_tx_ready_enable and send_buffer_empty);
---------- UART receive ----------
recv_buffer_input <= rx_byte;
-- Add a few FFs on the RX input to avoid metastability issues
process (clk) is
begin
if rising_edge(clk) then
rxd3 <= rxd2;
rxd2 <= rxd;
end if;
end process;
txd <= txd2;
uart_receive: process(clk)
begin
if rising_edge(clk) then
if reset = '1' then
rx_state <= IDLE;
recv_buffer_push <= '0';
else
case rx_state is
when IDLE =>
if recv_buffer_push = '1' then
recv_buffer_push <= '0';
end if;
if sample_clk = '1' and rxd3 = '0' then
rx_sample_value <= rx_sample_counter;
rx_sample_delay <= 0;
rx_current_bit <= 0;
rx_state <= STARTBIT;
end if;
when STARTBIT =>
if sample_clk = '1' then
if rx_sample_delay = 7 then
rx_state <= RECEIVE;
rx_sample_value <= rx_sample_counter;
rx_sample_delay <= 0;
else
rx_sample_delay <= rx_sample_delay + 1;
end if;
end if;
when RECEIVE =>
if sample_clk = '1' and rx_sample_counter = rx_sample_value then
if rx_current_bit /= 7 then
rx_byte(rx_current_bit) <= rxd3;
rx_current_bit <= rx_current_bit + 1;
else
rx_byte(rx_current_bit) <= rxd3;
rx_state <= STOPBIT;
end if;
end if;
when STOPBIT =>
if sample_clk = '1' and rx_sample_counter = rx_sample_value then
rx_state <= IDLE;
if recv_buffer_full = '0' then
recv_buffer_push <= '1';
end if;
end if;
end case;
end if;
end if;
end process uart_receive;
sample_counter: process(clk)
begin
if rising_edge(clk) then
if reset = '1' then
rx_sample_counter <= 0;
elsif sample_clk = '1' then
if rx_sample_counter = 15 then
rx_sample_counter <= 0;
else
rx_sample_counter <= rx_sample_counter + 1;
end if;
end if;
end if;
end process sample_counter;
---------- UART transmit ----------
tx_byte <= send_buffer_output;
uart_transmit: process(clk)
begin
if rising_edge(clk) then
if reset = '1' then
txd2 <= '1';
tx_state <= IDLE;
send_buffer_pop <= '0';
tx_current_bit <= 0;
else
case tx_state is
when IDLE =>
if send_buffer_empty = '0' and uart_tx_clk = '1' then
txd2 <= '0';
send_buffer_pop <= '1';
tx_current_bit <= 0;
tx_state <= TRANSMIT;
elsif uart_tx_clk = '1' then
txd2 <= '1';
end if;
when TRANSMIT =>
if send_buffer_pop = '1' then
send_buffer_pop <= '0';
elsif uart_tx_clk = '1' and tx_current_bit = 7 then
txd2 <= tx_byte(tx_current_bit);
tx_state <= STOPBIT;
elsif uart_tx_clk = '1' then
txd2 <= tx_byte(tx_current_bit);
tx_current_bit <= tx_current_bit + 1;
end if;
when STOPBIT =>
if uart_tx_clk = '1' then
txd2 <= '1';
tx_state <= IDLE;
end if;
end case;
end if;
end if;
end process uart_transmit;
uart_tx_clock_generator: process(clk)
begin
if rising_edge(clk) then
if reset = '1' then
uart_tx_counter <= 0;
uart_tx_clk <= '0';
else
if sample_clk = '1' then
if uart_tx_counter = 15 then
uart_tx_counter <= 0;
uart_tx_clk <= '1';
else
uart_tx_counter <= uart_tx_counter + 1;
uart_tx_clk <= '0';
end if;
else
uart_tx_clk <= '0';
end if;
end if;
end if;
end process uart_tx_clock_generator;
---------- Sample clock generator ----------
sample_clock_generator: process(clk)
begin
if rising_edge(clk) then
if reset = '1' then
sample_clk_counter <= (others => '0');
sample_clk <= '0';
else
if sample_clk_divisor /= x"00" then
if sample_clk_counter = sample_clk_divisor then
sample_clk_counter <= (others => '0');
sample_clk <= '1';
else
sample_clk_counter <= std_logic_vector(unsigned(sample_clk_counter) + 1);
sample_clk <= '0';
end if;
end if;
end if;
end if;
end process sample_clock_generator;
---------- Data Buffers ----------
send_buffer: entity work.pp_fifo
generic map(
DEPTH => FIFO_DEPTH,
WIDTH => 8
) port map(
clk => clk,
reset => reset,
full => send_buffer_full,
empty => send_buffer_empty,
data_in => send_buffer_input,
data_out => send_buffer_output,
push => send_buffer_push,
pop => send_buffer_pop
);
recv_buffer: entity work.pp_fifo
generic map(
DEPTH => FIFO_DEPTH,
WIDTH => 8
) port map(
clk => clk,
reset => reset,
full => recv_buffer_full,
empty => recv_buffer_empty,
data_in => recv_buffer_input,
data_out => recv_buffer_output,
push => recv_buffer_push,
pop => recv_buffer_pop
);
---------- Wishbone Interface ----------
wishbone: process(clk)
begin
if rising_edge(clk) then
if reset = '1' then
wb_ack_out <= '0';
wb_state <= IDLE;
send_buffer_push <= '0';
recv_buffer_pop <= '0';
sample_clk_divisor <= (others => '0');
irq_recv_enable <= '0';
irq_tx_ready_enable <= '0';
else
case wb_state is
when IDLE =>
if wb_cyc_in = '1' and wb_stb_in = '1' then
if wb_we_in = '1' then -- Write to register
if wb_adr_in = x"000" then
send_buffer_input <= wb_dat_in;
send_buffer_push <= '1';
elsif wb_adr_in = x"018" then
sample_clk_divisor <= wb_dat_in;
elsif wb_adr_in = x"020" then
irq_recv_enable <= wb_dat_in(0);
irq_tx_ready_enable <= wb_dat_in(1);
end if;
-- Invalid writes are acked and ignored.
wb_ack_out <= '1';
wb_state <= WRITE_ACK;
else -- Read from register
if wb_adr_in = x"008" then
recv_buffer_pop <= '1';
elsif wb_adr_in = x"010" then
wb_dat_out <= x"0" & send_buffer_full & recv_buffer_full &
send_buffer_empty & recv_buffer_empty;
wb_ack_out <= '1';
elsif wb_adr_in = x"018" then
wb_dat_out <= sample_clk_divisor;
wb_ack_out <= '1';
elsif wb_adr_in = x"020" then
wb_dat_out <= (0 => irq_recv_enable,
1 => irq_tx_ready_enable,
others => '0');
wb_ack_out <= '1';
else
wb_dat_out <= (others => '0');
wb_ack_out <= '1';
end if;
wb_state <= READ_ACK;
end if;
end if;
when WRITE_ACK =>
send_buffer_push <= '0';
if wb_stb_in = '0' then
wb_ack_out <= '0';
wb_state <= IDLE;
end if;
when READ_ACK =>
if recv_buffer_pop = '1' then
recv_buffer_pop <= '0';
else
wb_dat_out <= recv_buffer_output;
wb_ack_out <= '1';
end if;
if wb_stb_in = '0' then
wb_ack_out <= '0';
wb_state <= IDLE;
end if;
end case;
end if;
end if;
end process wishbone;
end architecture behaviour;
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