1 files changed, 110 insertions, 0 deletions
diff --git a/PIC/dpram.vhd b/PIC/dpram.vhd
new file mode 100644
index 0000000..44d7adf
--- /dev/null
+++ b/PIC/dpram.vhd
@@ -0,0 +1,110 @@
+library ieee;
+use ieee.std_logic_1164.all;
+use ieee.numeric_std.all;
+
+-- Data memory with UART memory-mapped registers.
+-- Addresses 0x70-0x72 are used (unimplemented in real PIC16F84A, no header conflict):
+--   0x70  UART_TX     write: send byte (held until tx_done)
+--   0x71  UART_RX     read:  current FIFO head (FWFT), pulses rd_en to pop
+--   0x72  UART_STATUS read:  "00000" & rx_full & rx_empty & tx_busy
+
+entity dpram is
+  generic (DEPTH : positive := 128);
+  port (
+    clk      : in  std_logic;
+    we       : in  std_logic;
+    re       : in  std_logic;
+    d        : in  std_logic_vector(7 downto 0);
+    d_status : in  std_logic_vector(7 downto 0);
+    addr     : in  std_logic_vector(6 downto 0);
+    q        : out std_logic_vector(7 downto 0);
+    q_status : out std_logic_vector(2 downto 0);
+
+    port_a    : out std_logic_vector(7 downto 0);
+    port_b    : out std_logic_vector(7 downto 0);
+    we_status : in  std_logic;
+
+    -- UART interface
+    uart_tx_byte  : out std_logic_vector(7 downto 0) := (others => '0');
+    uart_tx_send  : out std_logic                    := '0';
+    uart_tx_done  : in  std_logic;
+    uart_rx_byte  : in  std_logic_vector(7 downto 0);
+    uart_rx_rd_en : out std_logic                    := '0';
+    uart_rx_empty : in  std_logic;
+    uart_rx_full  : in  std_logic
+  );
+end entity dpram;
+
+architecture rtl of dpram is
+  type mem_t is
+    array (0 to DEPTH - 1)
+    of std_logic_vector(7 downto 0);
+
+  signal mem             : mem_t     := (others => (others => '0'));
+  signal uart_tx_pending : std_logic := '0';
+
+begin
+  -- STATUS output is combinatorial
+  q_status <= mem(16#03#)(2 downto 0);
+
+  port_a <= mem(16#05#);
+  port_b <= mem(16#06#);
+
+  -- tx_send is high while a transmission is in progress
+  uart_tx_send <= uart_tx_pending;
+
+  -- SR: set when CPU writes to 0x08, clear when UART asserts tx_done
+  tx_ctrl : process(clk)
+  begin
+    if rising_edge(clk) then
+      if we = '1' and addr = "1110000" then   -- 0x70 UART_TX
+        uart_tx_byte    <= d;
+        uart_tx_pending <= '1';
+      elsif uart_tx_done = '1' then
+        uart_tx_pending <= '0';
+      end if;
+    end if;
+  end process tx_ctrl;
+
+  read : process(clk)
+  begin
+    if rising_edge(clk) then
+      uart_rx_rd_en <= '0';   -- default: no pop
+      if re = '1' then
+        if addr = "1110001" then              -- 0x71 UART_RX
+          q <= uart_rx_byte;                  -- FWFT head, valid combinatorially
+          if uart_rx_empty = '0' then
+            uart_rx_rd_en <= '1';             -- advance FIFO pointer
+          end if;
+        elsif addr = "1110010" then           -- 0x72 UART_STATUS
+          q <= "00000" & uart_rx_full & uart_rx_empty & uart_tx_pending;
+        else
+          q <= mem(to_integer(unsigned(addr)));
+        end if;
+      end if;
+    end if;
+  end process read;
+
+  write : process(clk)
+  begin
+    if rising_edge(clk) then
+      if we_status = '1' then
+        mem(16#03#) <= d_status;
+      end if;
+      if we = '1' then
+        if addr = "0000011" then              -- 0x03 STATUS: direct write
+          mem(16#03#) <= d;
+        elsif addr = "1110000" or            -- 0x70 UART_TX
+              addr = "1110001" or            -- 0x71 UART_RX (read-only but handled gracefully)
+              addr = "1110010" then          -- 0x72 UART_STATUS (read-only)
+          -- UART-mapped addresses: don't write to RAM; still update STATUS
+          mem(16#03#) <= d_status;
+        else
+          mem(16#03#) <= d_status;
+          mem(to_integer(unsigned(addr))) <= d;
+        end if;
+      end if;
+    end if;
+  end process write;
+
+end architecture rtl;