[Oberon] What is the status of Lola-2 and its use in the FPGA version of Project Oberon?

Skulski, Wojciech skulski at pas.rochester.edu
Sat Mar 16 06:12:33 CET 2019

I tried to send a short tutorial text to this list (26 PDF pages) but it did not get through the list. There was an example of behavioral versus structural VHDL on page 19 and 20. So let me copy-paste in order to satisfy Paul's request. The formatting was lost while copy-paste. 

 I will illustrate a shift register. A shift register is an example of a "hardened logic" provided in Xilinx FPGAs. It is not as simple as a flip flop, and not as complex as  a clock manager or wizardry inside the Xilinx I/O pin. 

Let me start with the structural implementation using a "hardened" library component SRL32E. Note how many details you need to connect, initialize, etc. This implementation gives you a complete control over all the details. The compiler does not need to guess. So this is the "structural RTL" style of coding. I kind of believe that LOLA is meant this way, more or less. 

ENTITY Delay_Line_L32 IS
GENERIC (PipeWdt: INTEGER := 14); -- default bit width of the stream
  CE : in STD_LOGIC; -- clock enable
  prgm : in STD_LOGIC_VECTOR (4 downto 0); -- delay select between 0 to 31
  din : in STD_LOGIC_VECTOR (PipeWdt-1 downto 0);
  dout : out STD_LOGIC_VECTOR (PipeWdt-1 downto 0)
end Delay_Line_L32; -- end of the interface definition

pipe: FOR i IN 0 TO PipeWdt-1 GENERATE
slice : SRLC32E
GENERIC MAP (INIT => X"00000000")
  CLK => CLK, -- Clock input
  D => din(i), -- SRL data input
  Q31 => open, -- SRL cascade output pin (connect only to next SRL)
  A => prgm, -- 5-bit shift depth select input
  CE => CE, -- Clock enable can freeze the pipe
  Q => dout(i)); -- SRL data output

Now the behavioral version (without the CE).  Note that the code is really simple and self explanatory.  The details will be inferred by the compiler. You describe what you want, and the compiler does the rest. You can check in the synthesis report what the compiler really guessed.

TYPE Tpipe IS ARRAY(PipeLen-1 downto 0) OF
STD_LOGIC_VECTOR (PipeWdt-1 downto 0);
SIGNAL pipe: Tpipe;

 -- The "dynamic shift register" should be inferred by synthesis.
process (CLK) begin
if rising_edge (CLK) then
pipe <= pipe(PipeLen-2 DOWNTO 0) & din;
end if;
end process;
dout <= pipe(conv_integer(prgm)); -- select the dynamic length

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