Table of Contents Constants ROM with Address Counter Programming Hook-up Pins Signals VHDL Instance Hook-up Strobes/Read Mux Constants Compiler Constants Subroutine Resources

• VHDL code
• A Constants ROM can be used to store strings
• This is useful for printing strings to the VDU and UART
• Constants ROM
  • 256-byte (max)
  • Null-terminated strings
  • ROM "assembler" creates .MIF file and Symbol table
• 8-bit Address Counter
  • Loadable with start address
  • Auto-increment address

• Single address from the IOP Peripheral spave
• Load transfer start address (W)
• Read Data (R)

• Add to Top Level VHDL code

• None - all internal

-- Decodes/Strobes
...
  signal w_ldConAdr				:	std_logic;
  signal w_rdConAdr				:	std_logic;
...
-- Interfaces
...
signal w_ConstsData			:	std_logic_vector(7 downto 0);

• Add this entity instance code to the IOP16 top file

-- Constants Unit
CONST_UNIT : entity work.ConstantsUnit
port map (	
  i_clock     => i_clk,       -- 50 MHz clock
  i_dataIn    => w_periphOut, -- Data from IOP CPU
  i_ldStr     => w_ldConAdr,  -- Write load address strobe
  i_rdStr     => w_rdConAdr,  -- Strobe to read data from ROM
  o_constData => w_ConstsData -- Data from ROM
);

• Strobes

-- Add Strobes/Selects
..
  w_ldConAdr <= '1' when (w_periphAdr=x"0E") and (w_periphWr = '1') else '0';
  w_rdConAdr <= '1' when (w_periphAdr=x"0E") and (w_periphRd = '1') else '0';
...

• Add to Read Mux

-- Peripheral bus read mux
  w_periphIn <=	...
    w_ConstsData when w_periphAdr = x"0E" else -- Read Constants ROM
...

• Assembler takes the Constants as an input file
  • Python 3
  • Prompts for input file
• Creates output file
  • Cross reference list included in Assembler listing
  • .MIF file with ROM contents
    • Load ROM file by double-clickjng ConstantsROM_256B below ConstantsUnit
• Input file is CSV
  • Header line - ['LABEL','STRING']

• Routine handles constants
• Sets constants address
• Reads from constants ROM
• Writes to VDU

02e	PRSTR	0x710E	IOW	Reg1,LD_CON_ADR	// LOAD CONSTANTS ADDRESS
02f	PRSTR2	0x610E	IOR	Reg1,RD_CON_DAT	// READ CHAR (NEXT)
030		0x5100	CMP	Reg1,0X00	// IS END OF STRING?
031		0xF033	BNE	SKIPPS		// NOT END OF STRING
032		0x3008	RTS			// RETURN IF END
033	SKIPPS	0xC029	JSR	WR2VDU		// PRINT TO VDU
034		0xD02F	JMP	PRSTR2		// LOOP UNTIL NULL TERM

• Logic Cells: 74
• Registers: 46
• Memory Bits: 2048
• M9Ks: 1