Difference between revisions of "MBC2"

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(recat DMG)
(→‎Behavior: add behavioral model)
Line 113: Line 113:
  
 
<source lang="vhdl">
 
<source lang="vhdl">
−
TODO
+
library IEEE;
 +
use IEEE.std_logic_1164.all;
 +
 
 +
entity MBC2 is
 +
Port(
 +
RESET_N  : in    std_logic;
 +
RD_N    : in    std_logic;
 +
WR_N    : in    std_logic;
 +
CS_N    : in    std_logic;
 +
A_HI    : in    std_logic_vector(15 downto 14);
 +
A        : in    std_logic_vector(8 downto 0);
 +
D        : inout std_logic_vector(3 downto 0);
 +
RA      : out  std_logic_vector(17 downto 14);
 +
ROM_CS_N : out  std_logic;
 +
);
 +
end entity MBC2;
 +
 
 +
architecture Behavioral of MBC2 is
 +
 
 +
type ram_type is array(natural range <>) of std_logic_vector(3 downto 0);
 +
 
 +
signal ram : ram_type(0 to 511);
 +
 
 +
signal ram_enable_r : std_logic_vector(3 downto 0);
 +
signal rom_bank_r  : std_logic_vector(3 downto 0);
 +
 
 +
signal ram_enable_r_clk : std_logic;
 +
signal rom_bank_r_clk  : std_logic;
 +
 
 +
signal ram_cs_n : std_logic;
 +
 
 +
signal A_HI_int : std_logic_vector(A_HI'range);
 +
signal A_int    : std_logic_vector(A'range);
 +
 
 +
signal RD_N_int : std_logic;
 +
signal WR_N_int : std_logic;
 +
signal CS_N_int : std_logic;
 +
 
 +
begin
 +
 
 +
-----------------------------------------------------------------------
 +
-- Signal Assignments
 +
-----------------------------------------------------------------------
 +
 
 +
A_HI_int <= A_HI and RESET_N;
 +
A_int    <= A    and RESET_N;
 +
RD_N_int <= RD_N and RESET_N;
 +
WR_N_int <= WR_N and RESET_N;
 +
CS_N_int <= CS_N and RESET_N;
 +
 
 +
ROM_CS_N <= '0' when (A_HI_int(15) = '0' and RD_N_int = '0') else
 +
            '1';
 +
 
 +
ram_cs_n <= '0' when ((CS_N_int = '0' and A_HI_int(14) = '0' and ram_enable_r = x"A") or RESET_N = '0') else
 +
            '1';
 +
 
 +
RA <= "0000"    when (A_HI_int(14) = '0') else
 +
      rom_bank_r when (rom_bank_r /= "0000") else
 +
      "0001";
 +
 
 +
ram_enable_r_clk <= '0' when (A_HI_int = "00" and A_int(8) = '0' and WR_N_int = '0') else
 +
                    '1';
 +
 
 +
rom_bank_r_clk <= '0' when (A_HI_int = "00" and A_int(8) = '1' and WR_N_int = '0') else
 +
                  '1';
 +
 
 +
-----------------------------------------------------------------------
 +
-- Registers
 +
-----------------------------------------------------------------------
 +
 
 +
ram_enable_p : process (
 +
RESET_N,
 +
ram_enable_r_clk
 +
)
 +
begin
 +
 
 +
if (RESET_N = '0') then
 +
 +
ram_enable_r <= x"0";
 +
 +
elsif (rising_edge(ram_enable_r_clk)) then
 +
 +
ram_enable_r <= D;
 +
 +
end if;
 +
 
 +
end process ram_enable_p;
 +
 
 +
rom_bank_p : process (
 +
RESET_N,
 +
rom_bank_r_clk
 +
)
 +
begin
 +
 
 +
if (RESET_N = '0') then
 +
 +
rom_bank_r <= "0000";
 +
 +
elsif (rising_edge(rom_bank_r_clk)) then
 +
 +
rom_bank_r <= D;
 +
 +
end if;
 +
 +
end process rom_bank_p;
 +
 
 +
ram_p : process (
 +
A_int,
 +
D,
 +
WR_N_int,
 +
RD_N_int,
 +
ram_cs_n
 +
)
 +
begin
 +
 
 +
-- default tri-state
 +
D <= "ZZZZ";
 +
 +
-- read
 +
if (ram_cs_n = '0' and RD_N_int = '0' and WR_N_int /= '0') then
 +
-- write deasserted
 +
D <= ram(to_integer(unsigned(A_int)));
 +
elsif (ram_cs_n = '0' and RD_N_int = '0') then
 +
-- read and write asserted
 +
D <= "0000";
 +
end if;
 +
 
 +
-- write
 +
if (rising_edge(not RD_N_int and WR_N_int and ram_cs_n)) then
 +
 +
ram(to_integer(unsigned(A_int))) <= D;
 +
 +
end if;
 +
 
 +
end process ram_p;
 +
 
 +
end architecture Behavioral;
 
</source>
 
</source>
  
 
[[Category:DMG]]
 
[[Category:DMG]]

Revision as of 12:00, 2 August 2015

Nintendo's MBC2 can be used to address up to 2 Mbit of ROM and 512×4 bit of internal RAM.

Pinout

MBC2 Pinout.png
Pin No. Name Type Comment
28 VCC PWR I/O 5V supply
14 GND PWR I/O Ground supply
21 VCC_RAM PWR RAM supply
7 GND_RAM PWR RAM Ground supply
12Template:Ndash9 D3Template:NdashD0 I/O Data Bus
1 RD I Low-Active Read Enable
27 WR I Low-Active Write Enable
26 CS I Low-Active Chip Select
15 RESET I Low-Active Asynchronous Reset
20, 22Template:Ndash25, 8, 6Template:Ndash2 A15Template:NdashA14, A8Template:NdashA0 I Address Bus
16Template:Ndash19 RA17Template:NdashRA14 O Upper ROM Address Lines
13 ROM_CS O Low-Active ROM Chip Select

Footprint: SOP127P1200X225-24

Remarks: All I/O pins are protected via diodes to VCC/GND. VCC and VCC_RAM both supply RAM -- probably depending on VCC voltage.

Registers

Write-Accessible Registers:

  • 0b00-- ---0 ---- ----: RAM Enable register
  • 0b00-- ---1 ---- ----: ROM Bank register
  • 0b10-- ---a aaaa aaaa: Internal RAM

RAM Enable Register

XX XX XX XX D3 D2 D1 D0    0x00 @ reset
            \_________/
                 \-------- RAM Enable

A value of 0x0A enables RAM access, all other values disable RAM access.

ROM Bank Register

XX XX XX XX D3 D2 D1 D0    0x00 @ reset
            \_________/
                 \-------- ROM Bank

ROM Bank selects which bank is mapped to 0x4000-0x7FFF. The written value is zero-adjusted before output on RA17Template:NdashRA14.

Behavior

library IEEE;
use IEEE.std_logic_1164.all;

entity MBC2 is
	Port(
		RESET_N  : in    std_logic;
		RD_N     : in    std_logic;
		WR_N     : in    std_logic;
		CS_N     : in    std_logic;
		A_HI     : in    std_logic_vector(15 downto 14);
		A        : in    std_logic_vector(8 downto 0);
		D        : inout std_logic_vector(3 downto 0);
		RA       : out   std_logic_vector(17 downto 14);
		ROM_CS_N : out   std_logic;
	);
end entity MBC2;

architecture Behavioral of MBC2 is

type ram_type is array(natural range <>) of std_logic_vector(3 downto 0);

signal ram : ram_type(0 to 511);

signal ram_enable_r : std_logic_vector(3 downto 0);
signal rom_bank_r   : std_logic_vector(3 downto 0);

signal ram_enable_r_clk : std_logic;
signal rom_bank_r_clk   : std_logic;

signal ram_cs_n : std_logic;

signal A_HI_int : std_logic_vector(A_HI'range);
signal A_int    : std_logic_vector(A'range);

signal RD_N_int : std_logic;
signal WR_N_int : std_logic;
signal CS_N_int : std_logic;

begin

-----------------------------------------------------------------------
-- Signal Assignments
-----------------------------------------------------------------------

A_HI_int <= A_HI and RESET_N;
A_int    <= A    and RESET_N;
RD_N_int <= RD_N and RESET_N;
WR_N_int <= WR_N and RESET_N;
CS_N_int <= CS_N and RESET_N;

ROM_CS_N <= '0' when (A_HI_int(15) = '0' and RD_N_int = '0') else
            '1';

ram_cs_n <= '0' when ((CS_N_int = '0' and A_HI_int(14) = '0' and ram_enable_r = x"A") or RESET_N = '0') else
            '1';

RA <= "0000"     when (A_HI_int(14) = '0') else
      rom_bank_r when (rom_bank_r /= "0000") else
      "0001";

ram_enable_r_clk <= '0' when (A_HI_int = "00" and A_int(8) = '0' and WR_N_int = '0') else
                    '1';

rom_bank_r_clk <= '0' when (A_HI_int = "00" and A_int(8) = '1' and WR_N_int = '0') else
                  '1';

-----------------------------------------------------------------------
-- Registers
-----------------------------------------------------------------------

ram_enable_p : process (
	RESET_N,
	ram_enable_r_clk
	)
begin

	if (RESET_N = '0') then
	
		ram_enable_r <= x"0";
	
	elsif (rising_edge(ram_enable_r_clk)) then
	
		ram_enable_r <= D;
	
	end if;

end process ram_enable_p;

rom_bank_p : process (
	RESET_N,
	rom_bank_r_clk
	)
begin

	if (RESET_N = '0') then
	
		rom_bank_r <= "0000";
	
	elsif (rising_edge(rom_bank_r_clk)) then
	
		rom_bank_r <= D;
	
	end if;
	
end process rom_bank_p;

ram_p : process (
	A_int,
	D,
	WR_N_int,
	RD_N_int,
	ram_cs_n
	)
begin

	-- default tri-state
	D <= "ZZZZ";
	
	-- read
	if (ram_cs_n = '0' and RD_N_int = '0' and WR_N_int /= '0') then
		-- write deasserted
		D <= ram(to_integer(unsigned(A_int)));
	elsif (ram_cs_n = '0' and RD_N_int = '0') then
		-- read and write asserted
		D <= "0000";
	end if;

	-- write
	if (rising_edge(not RD_N_int and WR_N_int and ram_cs_n)) then
	
		ram(to_integer(unsigned(A_int))) <= D;
	
	end if;

end process ram_p;

end architecture Behavioral;
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