From 48b689b665c8620e3477b183f5c5db98625b9018 Mon Sep 17 00:00:00 2001
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date: Wed, 28 Aug 2019 00:08:54 +1000
Subject: [PATCH] Add simulated UART design

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
---
 sim_uart.vhdl | 118 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 118 insertions(+)
 create mode 100644 sim_uart.vhdl

diff --git a/sim_uart.vhdl b/sim_uart.vhdl
new file mode 100644
index 0000000..c52a8bc
--- /dev/null
+++ b/sim_uart.vhdl
@@ -0,0 +1,118 @@
+-- Sim console UART, provides the same interface as potato UART by
+-- Kristian Klomsten Skordal.
+
+library ieee;
+use ieee.std_logic_1164.all;
+use ieee.numeric_std.all;
+
+library work;
+use work.wishbone_types.all;
+use work.sim_console.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 (dummy)      |
+--! | 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
+--!
+--! 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 sim_uart is
+    port(
+	clk : in std_logic;
+	reset : in std_logic;
+
+	-- Wishbone ports:
+	wishbone_in : in wishbone_master_out;
+	wishbone_out : out wishbone_slave_out
+	);
+end entity sim_uart;
+
+architecture behaviour of sim_uart is
+
+    signal sample_clk_divisor : std_logic_vector(7 downto 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 wb_ack : std_logic; --! Wishbone acknowledge signal
+
+begin
+
+    wishbone_out.ack <= wb_ack and wishbone_in.cyc and wishbone_in.stb;
+
+    wishbone: process(clk)
+	variable sim_tmp : std_logic_vector(63 downto 0);
+    begin
+	if rising_edge(clk) then
+	    if reset = '1' then
+		wb_ack <= '0';
+		wb_state <= IDLE;
+		sample_clk_divisor <= (others => '0');
+		irq_recv_enable <= '0';
+		irq_tx_ready_enable <= '0';
+	    else
+		case wb_state is
+		when IDLE =>
+		    if wishbone_in.cyc = '1' and wishbone_in.stb = '1' then
+			if wishbone_in.we = '1' then -- Write to register
+			    if wishbone_in.adr(11 downto 0) = x"000" then
+				report "FOO !";
+				sim_console_write(wishbone_in.dat);
+			    elsif wishbone_in.adr(11 downto 0) = x"018" then
+				sample_clk_divisor <= wishbone_in.dat(7 downto 0);
+			    elsif wishbone_in.adr(11 downto 0) = x"020" then
+				irq_recv_enable <= wishbone_in.dat(0);
+				irq_tx_ready_enable <= wishbone_in.dat(1);
+			    end if;
+			    wb_ack <= '1';
+			    wb_state <= WRITE_ACK;
+			else -- Read from register
+			    if wishbone_in.adr(11 downto 0) = x"008" then
+				sim_console_read(sim_tmp);
+				wishbone_out.dat <= sim_tmp;
+			    elsif wishbone_in.adr(11 downto 0) = x"010" then
+				sim_console_poll(sim_tmp);
+				wishbone_out.dat <= x"000000000000000" & '0' &
+						    sim_tmp(0) & '1' & not sim_tmp(0);
+			    elsif wishbone_in.adr(11 downto 0) = x"018" then
+				wishbone_out.dat <= x"00000000000000" & sample_clk_divisor;
+			    elsif wishbone_in.adr(11 downto 0) = x"020" then
+				wishbone_out.dat <= (0 => irq_recv_enable,
+						     1 => irq_tx_ready_enable,
+						     others => '0');
+			    else
+				wishbone_out.dat <= (others => '0');
+			    end if;
+			    wb_ack <= '1';
+			    wb_state <= READ_ACK;
+			end if;
+		    end if;
+		when WRITE_ACK|READ_ACK =>
+		    if wishbone_in.stb = '0' then
+			wb_ack <= '0';
+			wb_state <= IDLE;
+		    end if;
+		end case;
+	    end if;
+	end if;
+	end process wishbone;
+
+end architecture behaviour;
-- 
2.30.2