IOP16 Constants Unit

Constants ROM with Address Counter

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

Programming

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

Hook-up

• Add to Top Level VHDL code

Pins

• None - all internal

Signals

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

VHDL Instance

• Code to add UART to the IOP16 top file

-- Constants Unit
CONST_UNIT : entity work.ConstantsUnit
port map (	
  i_clock     => i_clk,
  i_dataIn    => w_periphOut,
  i_ldStr     => w_ldConAdr,
  i_rdStr     => w_rdConAdr,
  o_constData => w_ConstsData
);

Hook-up Strobes/Read Mux

• 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
...

Constants Compiler

• Program - pyConstants_cpu_001.py
  • Python 3
  • Prompts for input file
• Creates two output files
  • Cross reference list of LABEL, address
  • .MIF file with ROM contents
    • Load ROM file by double-clickjng ConstantsROM_256B below ConstantsUnit
• Input file is CSV
  • Header line - ['LABEL','STRING']

Resources

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