MBC1

Nintendo's MBC1 can be used to address up to 16 Mbit of ROM and 256 kbit of SRAM, depending on the mode MBC1 is set to.

Pinout

Pin No.	Name	Type	Comment
24	VCC	PWR	5V supply
14	GND	PWR	Ground supply
5Template:Ndash1	D4Template:NdashD0	I	Data Bus
11	RD	I	Low-Active Read Enable
22	WR	I	Low-Active Write Enable
23	CS	I	Low-Active Chip Select
10	RESET	I	Low-Active Asynchronous Reset
21Template:Ndash19	A15Template:NdashA13	I	Address Bus
18Template:Ndash14	RA18Template:NdashRA14	O	Upper ROM Address Lines
7Template:Ndash6	AA14Template:NdashAA13	O	Upper ROM/RAM Address Lines
8	RAM_CS	O	Low-Active RAM Chip Select
9	RAM_CS	O	High-Active RAM Chip Select
13	ROM_CS	O	Low-Active ROM Chip Select

Registers

Write-Accessible Registers:

• 0x0000-0x1FFF: RAM Enable register
• 0x2000-0x3FFF: ROM Bank register
• 0x4000-0x5FFF: RAM Bank register
• 0x6000-0x7FFF: Mode register

RAM Enable Register

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

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

ROM Bank Register

XX XX XX D4 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 RA18Template:NdashRA14.

RAM Bank Register

XX XX XX XX XX XX D1 D0    0x00 @ reset
                  \___/
                    \----- RAM Bank

RAM Bank selects which bank is mapped to 0xA000-0xBFFF when in mode 4 MBit/256 kbit. When in mode 16 Mbit/64 kbit, it can be used to select the upper two ROM address lines.

Mode Register

XX XX XX XX XX XX XX D0    0x00 @ reset
                     \/
                      \--- Mode

The mode bit switches between 16 Mbit ROM/64 kbit SRAM mode ('0') and 4 Mbit ROM/256 kbit SRAM mode ('1'). When in 16 Mbit/64 kbit mode, address lines AA14Template:NdashAA13 switch according to A14. When in 4 Mbit/256 kbit mode, the upper address lines AA14Template:NdashAA13 stay fixed regardless of A14.

Behavior

library IEEE;
use IEEE.std_logic_1164.all;

entity MBC1 is
	Port(
		RESET_N  : in  std_logic;
		RD_N     : in  std_logic;
		WR_N     : in  std_logic;
		CS_N     : in  std_logic;
		A        : in  std_logic_vector(15 downto 13);
		D        : in  std_logic_vector(4 downto 0);
		RA       : out std_logic_vector(18 downto 14);
		AA       : out std_logic_vector(14 downto 13);
		ROM_CS_N : out std_logic;
		RAM_CS_N : out std_logic;
		RAM_CS   : out std_logic
	);
end entity MBC1;

architecture Behavioral of MBC1 is
	
signal ram_enable_r : std_logic_vector(3 downto 0);
signal rom_bank_r   : std_logic_vector(4 downto 0);
signal ram_bank_r   : std_logic_vector(1 downto 0);
signal mode_r       : std_logic_vector(0 downto 0);

signal ram_enable_r_clk : std_logic;
signal rom_bank_r_clk   : std_logic;
signal ram_bank_r_clk   : std_logic;
signal mode_r_clk       : std_logic;

begin

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

ROM_CS_N <= '0' when ((A(15) = '0' and RD_N = '0') or RESET_N = '0') else
            '1';

RAM_CS_N <= '0' when (CS_N = '0' and A(14) = '0' and ram_enable_r = x"A" and RESET_N = '1') else
            '1';
RAM_CS <= not RAM_CS_N;

RA(18 downto 14) <= "00000"    when (A(14) = '0' or RESET_N = '0') else
                    rom_bank_r when (rom_bank_r /= "00000") else
                    "00001";

AA(14 downto 13) <= "00" when (A(14) = '0' and mode_r = "0") else
                    ram_bank_r;

ram_enable_r_clk <= '0' when (A = "000" and WR_N = '0') else
                    '1';

rom_bank_r_clk <= '0' when (A = "001" and WR_N = '0') else
                  '1';

ram_bank_r_clk <= '0' when (A = "010" and WR_N = '0') else
                  '1';

mode_r_clk <= '0' when (A = "011" and WR_N = '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(3 downto 0);
	
	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 <= "00000";
	
	elsif (rising_edge(rom_bank_r_clk)) then
	
		rom_bank_r <= D;
	
	end if;
	
end process rom_bank_p;

ram_bank_p : process (
	RESET_N,
	ram_bank_r_clk
	)
begin

	if (RESET_N = '0') then
	
		ram_bank_r <= "00";
	
	elsif (rising_edge(ram_bank_r_clk)) then
	
		ram_bank_r <= D(1 downto 0);
	
	end if;
	
end process ram_bank_p;

mode_p : process (
	RESET_N,
	mode_r_clk
	)
begin

	if (RESET_N = '0') then
	
		mode_r <= "0";
	
	elsif (rising_edge(mode_r_clk)) then
	
		mode_r <= D(0 downto 0);
	
	end if;

end process mode_p;

end architecture Behavioral;