A Synthesizable model of TI's TMS9918A, TMS9928A, TMS9929A
==========================================================
Version: $Date: 2006/06/18 19:28:06 $

Copyright (c) 2006, Arnim Laeuger (arnim.laeuger@gmx.net)
See the file COPYING.


Integration
-----------

The vdp18 design exhibits a set of interface signals that is compatible to the
original chips. It mainly differs in the video and VRAM interfaces, which are
tailored to better fit a modern SoC.

  generic (
    is_pal_g      : integer := 0;
    -- Select PAL or NTSC video timing.
    -- 0 = NTSC
    -- 1 = PAL

    compat_rgb_g  : integer := 0
    -- Select full or compatibility RGB palettes.
    -- Refer to vdp18_col_pack-p.vhd for details.

  );
  port (
    -- Global Interface -------------------------------------------------------
    clk_i         : in  std_logic;
    -- Global clock input
    -- Equivalent to XTAL1/2 on TMS9918A.
    -- Drive with the target frequency of 10.7 MHz or any integer multiple.

    clk_en_10m7_i : in  std_logic;
    -- Clock enable
    -- A '1' on this input qualifies a valid rising edge on clk_i. A '0'
    -- disables the next rising clock edge, effectivley halting the design
    -- until the next enabled rising clock edge.
    -- Can be used to run the core at lower frequencies than applied on
    -- clk_i.

    reset_n_i     : in  std_logic;
    -- Asynchronous low active reset input
    -- Sets all sequential elements to a known state.
    -- NOTE: SYNC operation not supported.

    -- CPU Interface ----------------------------------------------------------
    csr_n_i       : in  std_logic;
    -- CPU-VDP read strobe, low active

    csw_n_i       : in  std_logic;
    -- CPU-VDP write strobe, low active

    mode_i        : in  std_logic;
    -- CPU interface mode select

    int_n_o       : out std_logic;
    -- CPU interrupt output, low active

    cd_i          : in  std_logic_vector(0 to  7);
    -- CPU data bus input
    -- MSB 0 ... 7 LSB

    cd_o          : out std_logic_vector(0 to  7);
    -- CPU data bus output
    -- MSB 0 ... 7 LSB

    -- VRAM Interface ---------------------------------------------------------
    vram_we_o     : out std_logic;
    -- VRAM write enable

    vram_a_o      : out std_logic_vector(0 to 13);
    -- VRAM address output
    -- MSB 0 ... 13 LSB

    vram_d_o      : out std_logic_vector(0 to  7);
    -- VRAM data output
    -- MSB 0 ... 7 LSB

    vram_d_i      : in  std_logic_vector(0 to  7);
    -- VRAM data input
    -- MSB 0 ... 7 LSB

    -- Video Interface --------------------------------------------------------
    col_o         : out std_logic_vector(0 to 3);
    -- Color code output
    -- Encoded pixel color information

    rgb_r_o       : out std_logic_vector(0 to 7);
    -- Red color information

    rgb_g_o       : out std_logic_vector(0 to 7);
    -- Green color information

    rgb_b_o       : out std_logic_vector(0 to 7);
    -- Blue color information

    hsync_n_o     : out std_logic;
    -- Horizontal synchronization pulse, active low

    vsync_n_o     : out std_logic;
    -- Vertical synchronization pulse, active low

    comp_sync_n_o : out std_logic
    -- Composite synchronization pulse, active low
  );


All 8 bit vector ports are defined (0 to 7) which declares bit 0 to be the MSB
and bit 7 to be the LSB. This has been implemented according to TI's data
sheet, thus all register/data format figures apply 1:1 for this design.
Many systems will flip the system data bus bit wise before it is connected to
this PSG. This is simply achieved with the following VHDL construct:

  signal data_s : std_logic_vector(7 downto 0);

  ...
  cd_i => data_s,
  ...

d_i and data_s will be assigned from left to right, resulting in the expected
bit assignment:

  cd_i    data_s
   0        7
   1        6
       ...
   6        1
   7        0


As this design is fully synchronous, care has to be taken when the design
replaces an TMS99x8 in asynchronous mode. No problems are expected when
interfacing the code to other synchronous components.


Design Hierarchy
----------------

  vdp18_core
    |
    +-- vdp18_clk_gen
    |
    +-- vdp18_hor_vert
    |
    +-- vdp18_ctrl
    |
    +-- vdp18_cpuio
    |
    +-- vdp18_addr_mux
    |
    +-- vdp18_pattern
    |
    +-- vdp18_sprite
    |
    \-- vdp18_col_mux

Resulting compolation sequence:

  vdp18_pack-p.vhd
  vdp18_comp_pack-p.vhd
  vdp18_core.vhd
  vdp18_clk_gen.vhd
  vdp18_clk_gen-c.vhd
  vdp18_hor_vert.vhd
  vdp18_hor_vert-c.vhd
  vdp18_ctrl.vhd
  vdp18_ctrl-c.vhd
  vdp18_cpuio.vhd
  vdp18_cpuio-c.vhd
  vdp18_addr_mux.vhd
  vdp18_addr_mux-c.vhd
  vdp18_pattern.vhd
  vdp18_pattern-c.vhd
  vdp18_sprite.vhd
  vdp18_sprite-c.vhd
  vdp18_col_mux.vhd
  vdp18_col_mux-c.vhd
  vdp18_core-c.vhd
  vdp18_core_com_pack-p.vhd

Skip the files containing VHDL configurations when analyzing the code for
synthesis.


References
----------

* TI Data book TMS9918.pdf
  http://www.bitsavers.org/pdf/ti/_dataBooks/TMS9918.pdf

* Sean Young's tech article:
  http://bifi.msxnet.org/msxnet/tech/tms9918a.txt

* Paul Urbanus' discussion of the timing details
  http://bifi.msxnet.org/msxnet/tech/tmsposting.txt

* Richard F. Drushel's article series
  "This Week With My Coleco ADAM"
  http://junior.apk.net/~drushel/pub/coleco/twwmca/index.html