mc6845.vhd

-- BBC Micro for Altera DE1
--
-- Copyright (c) 2011 Mike Stirling
--
-- All rights reserved
--
-- Redistribution and use in source and synthezised forms, with or without
-- modification, are permitted provided that the following conditions are met:
--
-- * Redistributions of source code must retain the above copyright notice,
--   this list of conditions and the following disclaimer.
--
-- * Redistributions in synthesized form must reproduce the above copyright
--   notice, this list of conditions and the following disclaimer in the
--   documentation and/or other materials provided with the distribution.
--
-- * Neither the name of the author nor the names of other contributors may
--   be used to endorse or promote products derived from this software without
--   specific prior written agreement from the author.
--
-- * License is granted for non-commercial use only.  A fee may not be charged
--   for redistributions as source code or in synthesized/hardware form without 
--   specific prior written agreement from the author.
--
-- THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
-- AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
-- THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
-- PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE
-- LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
-- CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
-- SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
-- INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
-- CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
-- ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
-- POSSIBILITY OF SUCH DAMAGE.
--
-- MC6845 CRTC
--
-- Synchronous implementation for FPGA
--
-- (C) 2011 Mike Stirling
--
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.NUMERIC_STD.ALL;

entity mc6845 is
port (
	CLOCK		:	in	std_logic;
	CLKEN		:	in	std_logic;
	nRESET		:	in	std_logic;

	-- Bus interface
	ENABLE		:	in	std_logic;
	R_nW		:	in	std_logic;
	RS			:	in	std_logic;
	DI			:	in	std_logic_vector(7 downto 0);
	DO			:	out	std_logic_vector(7 downto 0);

	-- Display interface
	VSYNC		:	out	std_logic;
	HSYNC		:	out	std_logic;
	DE			:	out	std_logic;
	CURSOR		:	out	std_logic;
	LPSTB		:	in	std_logic;
	
	-- Memory interface
	MA			:	out	std_logic_vector(13 downto 0);
	RA			:	out	std_logic_vector(4 downto 0)
	);
end entity;

architecture rtl of mc6845 is

-- Host-accessible registers
signal addr_reg			:	std_logic_vector(4 downto 0);	-- Currently addressed register
-- These are write-only
signal r00_h_total		:	unsigned(7 downto 0);	-- Horizontal total, chars
signal r01_h_displayed	:	unsigned(7 downto 0);	-- Horizontal active, chars
signal r02_h_sync_pos	:	unsigned(7 downto 0);	-- Horizontal sync position, chars
signal r03_v_sync_width :	unsigned(3 downto 0);	-- Vertical sync width, scan lines (0=16 lines)
signal r03_h_sync_width	:	unsigned(3 downto 0);	-- Horizontal sync width, chars (0=no sync)
signal r04_v_total		:	unsigned(6 downto 0);	-- Vertical total, character rows
signal r05_v_total_adj	:	unsigned(4 downto 0);	-- Vertical offset, scan lines
signal r06_v_displayed	:	unsigned(6 downto 0);	-- Vertical active, character rows
signal r07_v_sync_pos	:	unsigned(6 downto 0);	-- Vertical sync position, character rows
signal r08_interlace	:	std_logic_vector(1 downto 0);
signal r09_max_scan_line_addr		:	unsigned(4 downto 0);
signal r10_cursor_mode	:	std_logic_vector(1 downto 0);
signal r10_cursor_start	:	unsigned(4 downto 0);	-- Cursor start, scan lines
signal r11_cursor_end	:	unsigned(4 downto 0);	-- Cursor end, scan lines
signal r12_start_addr_h	:	unsigned(5 downto 0);
signal r13_start_addr_l	:	unsigned(7 downto 0);
-- These are read/write
signal r14_cursor_h		:	unsigned(5 downto 0);
signal r15_cursor_l		:	unsigned(7 downto 0);
-- These are read-only
signal r16_light_pen_h	:	unsigned(5 downto 0);
signal r17_light_pen_l	:	unsigned(7 downto 0);


-- Timing generation
-- Horizontal counter counts position on line
signal h_counter		:	unsigned(7 downto 0);
-- HSYNC counter counts duration of sync pulse
signal h_sync_counter	:	unsigned(3 downto 0);
-- Row counter counts current character row
signal row_counter		:	unsigned(6 downto 0);
-- Line counter counts current line within each character row
signal line_counter		:	unsigned(4 downto 0);
-- VSYNC counter counts duration of sync pulse
signal v_sync_counter	:	unsigned(3 downto 0);
-- Field counter counts number of complete fields for cursor flash
signal field_counter	:	unsigned(5 downto 0);

-- Internal signals
signal h_sync_start		:	std_logic;
signal h_half_way		:	std_logic;
signal h_display		:	std_logic;
signal hs				:	std_logic;
signal v_display		:	std_logic;
signal vs				:	std_logic;
signal odd_field		:	std_logic;
signal ma_i				:	unsigned(13 downto 0);
signal ma_row_start 	: 	unsigned(13 downto 0); -- Start address of current character row
signal cursor_i			:	std_logic;
signal lpstb_i			:	std_logic;


begin
	HSYNC <= hs; -- External HSYNC driven directly from internal signal
	VSYNC <= vs; -- External VSYNC driven directly from internal signal
	DE <= h_display and v_display;
	
	-- Cursor output generated combinatorially from the internal signal in
	-- accordance with the currently selected cursor mode
	CURSOR <=	cursor_i 						when r10_cursor_mode = "00" else
				'0' 							when r10_cursor_mode = "01" else
				(cursor_i and field_counter(4))	when r10_cursor_mode = "10" else
				(cursor_i and field_counter(5));

	-- Synchronous register access.  Enabled on every clock.
	process(CLOCK,nRESET)
	begin
		if nRESET = '0' then
			-- Reset registers to defaults
			addr_reg <= (others => '0');
			r00_h_total <= (others => '0');
			r01_h_displayed <= (others => '0');
			r02_h_sync_pos <= (others => '0');
			r03_v_sync_width <= (others => '0');
			r03_h_sync_width <= (others => '0');
			r04_v_total <= (others => '0');
			r05_v_total_adj <= (others => '0');
			r06_v_displayed <= (others => '0');
			r07_v_sync_pos <= (others => '0');
			r08_interlace <= (others => '0');
			r09_max_scan_line_addr <= (others => '0');
			r10_cursor_mode <= (others => '0');
			r10_cursor_start <= (others => '0');
			r11_cursor_end <= (others => '0');
			r12_start_addr_h <= (others => '0');
			r13_start_addr_l <= (others => '0');
			r14_cursor_h <= (others => '0');
			r15_cursor_l <= (others => '0');
			
			DO <= (others => '0');
		elsif rising_edge(CLOCK) then
			if ENABLE = '1' then
				if R_nW = '1' then
					-- Read
					case addr_reg is
					when "01110" =>
						DO <= "00" & std_logic_vector(r14_cursor_h);
					when "01111" =>
						DO <= std_logic_vector(r15_cursor_l);
					when "10000" =>
						DO <= "00" & std_logic_vector(r16_light_pen_h);
					when "10001" =>
						DO <= std_logic_vector(r17_light_pen_l);
					when others =>
						DO <= (others => '0');
					end case;
				else
					-- Write
					if RS = '0' then
						addr_reg <= DI(4 downto 0);
					else
						case addr_reg is
						when "00000" =>
							r00_h_total <= unsigned(DI);
						when "00001" =>
							r01_h_displayed <= unsigned(DI);
						when "00010" =>
							r02_h_sync_pos <= unsigned(DI);
						when "00011" =>
							r03_v_sync_width <= unsigned(DI(7 downto 4));
							r03_h_sync_width <= unsigned(DI(3 downto 0));
						when "00100" =>
							r04_v_total <= unsigned(DI(6 downto 0));
						when "00101" =>
							r05_v_total_adj <= unsigned(DI(4 downto 0));
						when "00110" =>
							r06_v_displayed <= unsigned(DI(6 downto 0));
						when "00111" =>
							r07_v_sync_pos <= unsigned(DI(6 downto 0));
						when "01000" =>
							r08_interlace <= DI(1 downto 0);
						when "01001" =>
							r09_max_scan_line_addr <= unsigned(DI(4 downto 0));
						when "01010" =>
							r10_cursor_mode <= DI(6 downto 5);
							r10_cursor_start <= unsigned(DI(4 downto 0));
						when "01011" =>
							r11_cursor_end <= unsigned(DI(4 downto 0));
						when "01100" =>
							r12_start_addr_h <= unsigned(DI(5 downto 0));
						when "01101" =>
							r13_start_addr_l <= unsigned(DI(7 downto 0));
						when "01110" =>
							r14_cursor_h <= unsigned(DI(5 downto 0));
						when "01111" =>
							r15_cursor_l <= unsigned(DI(7 downto 0));
						when others =>
							null;
						end case;
					end if;
				end if;
			end if;
		end if;
	end process; -- registers
	
	-- Horizontal, vertical and address counters
	process(CLOCK,nRESET)
	variable ma_row_start : unsigned(13 downto 0);
	variable max_scan_line : unsigned(4 downto 0);
	begin
		if nRESET = '0' then
			-- H
			h_counter <= (others => '0');
			
			-- V
			line_counter <= (others => '0');
			row_counter <= (others => '0');
			odd_field <= '0';
			
			-- Fields (cursor flash)
			field_counter <= (others => '0');
			
			-- Addressing
			ma_row_start := (others => '0');
			ma_i <= (others => '0');
		elsif rising_edge(CLOCK) and CLKEN='1' then		
			-- Horizontal counter increments on each clock, wrapping at
			-- h_total
			if h_counter = r00_h_total then
				-- h_total reached
				h_counter <= (others => '0');
				
				-- In interlace sync + video mode mask off the LSb of the
				-- max scan line address
				if r08_interlace = "11" then
					max_scan_line := r09_max_scan_line_addr(4 downto 1) & "0";
				else
					max_scan_line := r09_max_scan_line_addr;
				end if;
				
				-- Scan line counter increments, wrapping at max_scan_line_addr
				if line_counter = max_scan_line then
					-- Next character row
					-- FIXME: No support for v_total_adj yet
					line_counter <= (others => '0');
					if row_counter = r04_v_total then
						-- If in interlace mode we toggle to the opposite field.
						-- Save on some logic by doing this here rather than at the
						-- end of v_total_adj - it shouldn't make any difference to the
						-- output
						if r08_interlace(0) = '1' then
							odd_field <= not odd_field;
						else
							odd_field <= '0';
						end if;
						
						-- Address is loaded from start address register at the top of
						-- each field and the row counter is reset
						ma_row_start := r12_start_addr_h & r13_start_addr_l;
						row_counter <= (others => '0');	
						
						-- Increment field counter
						field_counter <= field_counter + 1;					
					else
						-- On all other character rows within the field the row start address is
						-- increased by h_displayed and the row counter is incremented
						ma_row_start := ma_row_start + r01_h_displayed;
						row_counter <= row_counter + 1;
					end if;
				else
					-- Next scan line.  Count in twos in interlaced sync+video mode
					if r08_interlace = "11" then
						line_counter <= line_counter + 2;
						line_counter(0) <= '0'; -- Force to even
					else
						line_counter <= line_counter + 1;
					end if;
				end if;
				
				-- Memory address preset to row start at the beginning of each
				-- scan line					
				ma_i <= ma_row_start;
			else
				-- Increment horizontal counter
				h_counter <= h_counter + 1;
				-- Increment memory address
				ma_i <= ma_i + 1;
			end if;
		end if;
	end process;
	
	-- Signals to mark hsync and half way points for generating
	-- vsync in even and odd fields
	process(h_counter)
	begin
		h_sync_start <= '0';
		h_half_way <= '0';
		
		if h_counter = r02_h_sync_pos then
			h_sync_start <= '1';
		end if;
		if h_counter = "0" & r02_h_sync_pos(7 downto 1) then
			h_half_way <= '1';
		end if;
	end process;
	
	-- Video timing and sync counters
	process(CLOCK,nRESET)
	begin
		if nRESET = '0' then
			-- H
			h_display <= '0';
			hs <= '0';
			h_sync_counter <= (others => '0');
			
			-- V
			v_display <= '0';
			vs <= '0';
			v_sync_counter <= (others => '0');
		elsif rising_edge(CLOCK) and CLKEN = '1' then
			-- Horizontal active video
			if h_counter = 0 then
				-- Start of active video
				h_display <= '1';
			end if;
			if h_counter = r01_h_displayed then
				-- End of active video
				h_display <= '0';
			end if;
			
			-- Horizontal sync
			if h_sync_start = '1' or hs = '1' then
				-- In horizontal sync
				hs <= '1';
				h_sync_counter <= h_sync_counter + 1;
			else
				h_sync_counter <= (others => '0');
			end if;
			if h_sync_counter = r03_h_sync_width then
				-- Terminate hsync after h_sync_width (0 means no hsync so this
				-- can immediately override the setting above)
				hs <= '0';
			end if;

			-- Vertical active video
			if row_counter = 0 then
				-- Start of active video
				v_display <= '1';
			end if;
			if row_counter = r06_v_displayed then
				-- End of active video
				v_display <= '0';
			end if;
			
			-- Vertical sync occurs either at the same time as the horizontal sync (even fields)
			-- or half a line later (odd fields)
			if (odd_field = '0' and h_sync_start = '1') or (odd_field = '1' and h_half_way = '1') then
				if (row_counter = r07_v_sync_pos and line_counter = 0) or vs = '1' then
					-- In vertical sync
					vs <= '1';
					v_sync_counter <= v_sync_counter + 1;
				else
					v_sync_counter <= (others => '0');
				end if;
				if v_sync_counter = r03_v_sync_width and vs = '1' then
					-- Terminate vsync after v_sync_width (0 means 16 lines so this is
					-- masked by 'vs' to ensure a full turn of the counter in this case)
					vs <= '0';
				end if;
			end if;
		end if;
	end process;
	
	-- Address generation
	process(CLOCK,nRESET)
	variable slv_line : std_logic_vector(4 downto 0);
	begin
		if nRESET = '0' then
			RA <= (others => '0');
			MA <= (others => '0');
		elsif rising_edge(CLOCK) and CLKEN = '1' then
			slv_line := std_logic_vector(line_counter);
		
			-- Character row address is just the scan line counter delayed by
			-- one clock to line up with the syncs.
			if r08_interlace = "11" then
				-- In interlace sync and video mode the LSb is determined by the
				-- field number.  The line counter counts up in 2s in this case.
				RA <= slv_line(4 downto 1) & (slv_line(0) or odd_field);
			else
				RA <= slv_line;
			end if;
			-- Internal memory address delayed by one cycle as well
			MA <= std_logic_vector(ma_i);
		end if;
	end process;
	
	-- Cursor control
	process(CLOCK,nRESET)
	variable cursor_line : std_logic;
	begin	
		-- Internal cursor enable signal delayed by 1 clock to line up
		-- with address outputs
		if nRESET = '0' then
			cursor_i <= '0';
			cursor_line := '0';
		elsif rising_edge(CLOCK) and CLKEN = '1' then
			if h_display = '1' and v_display = '1' and ma_i = r14_cursor_h & r15_cursor_l then
				if line_counter = 0 then
					-- Suppress wrap around if last line is > max scan line
					cursor_line := '0';
				end if;
				if line_counter = r10_cursor_start then
					-- First cursor scanline
					cursor_line := '1';
				end if;
				
				-- Cursor output is asserted within the current cursor character
				-- on the selected lines only
				cursor_i <= cursor_line;
				
				if line_counter = r11_cursor_end then
					-- Last cursor scanline
					cursor_line := '0';
				end if;
			else
				-- Cursor is off in all character positions apart from the
				-- selected one
				cursor_i <= '0';
			end if;
		end if;
	end process;
	
	-- Light pen capture
	process(CLOCK,nRESET)
	begin
		if nRESET = '0' then
			lpstb_i <= '0';
			r16_light_pen_h <= (others => '0');
			r17_light_pen_l <= (others => '0');
		elsif rising_edge(CLOCK) and CLKEN = '1' then
			-- Register light-pen strobe input
			lpstb_i <= LPSTB;
			
			if LPSTB = '1' and lpstb_i = '0' then
				-- Capture address on rising edge
				r16_light_pen_h <= ma_i(13 downto 8);
				r17_light_pen_l <= ma_i(7 downto 0);
			end if;
		end if;
	end process;
end architecture;