P2Eval <--> TinyFPGA

Rayman · 2021-11-06 16:40

Trying to learn FPGA again and using the P2 Eval board to help.

Tried using some expensive FPGA boards before, but it's just too complex and became overwhelming and just too much work to figure anything out.
This time, using a very simple board, TinyFPGA.

Mentioning it here because using the P2 Eval board to help.
The P2 Eval provides power and a 16-bit interface for I/O with TinyFPGA

Made some notes on TinyFPGA here: http://www.rayslogic.com/TinyFPGA/TinyFPGA.htm
Notes on this interface are going here: http://www.rayslogic.com/TinyFPGA/TinyFPGA_To_P2Eval.htm

Rayman · 2021-11-06 16:45

I'm pretty sure we can also program the MachXO2 chip using the P2, but haven't figured out how yet.

BTW: The goal in mind is to create a 4-bit CPU. Starting with a 4-bit ALU. Was thinking about doing it with ancient IC chips, but this is more interesting (and less work).

Seems we have 16 I/O pins that we can easily use.
So, current plan is to use 8 as inputs and 8 as outputs.
Will make a program in FlexC that takes input via serial port and then shows the output.
Could use for automated testing of future designs.

pik33 · 2021-11-08 07:22

A simple CPU can be simply done in Verilog. Maybe I can find (and then place here) the code I did for my students at the university several years ago. The resulting 8-bit CPU eats up - if I can remember - about 600 (Edit: 379) LEs of Altera Cyclone 4

pik33 · 2021-11-08 07:31

and there it is.

module simple8(addr,din,dout,rw,md,irq,reset,clk,debug);

//(c) 2017.04.15 Piotr Kardasz
// pik33@o2.pl
// license: GPL 2.0


input wire [7:0] din;   // data in
output reg [7:0] dout;  // data out
output reg [7:0] addr;     // address
output reg rw;             // 0 - write; 1 - read
output reg md;          // 0 - memory, 1- device
input wire irq;         // irqerrupt, negedge
input wire clk;         // clock
input wire reset;           // reset
output [7:0] debug;

reg [7:0] acc;              // accumulator
reg [7:0] sp;               // program counter
reg [7:0] pc;               // stack poirqer
reg [1:0] flags, iflags;           // flags: IZC
reg [1:0] irqreq;
reg[7:0] arg,opcode;
reg[3:0] phase;
reg[7:0] ramaddr,ramdatain,ramdataout;

assign debug=pc;

(* ramstyle = "M9K" *) reg [7:0] ram[256];    // memory for the CPU
initial

begin
acc<=0;
sp<=8'hfe;
pc<=0;
flags<=0;
iflags<=0;
phase<=0;
opcode<=0;
rw<=1;
$readmemh("ram.hex",ram);
end


always@(negedge clk)

begin
if ((rw==0) && (md==0)) ram[ramaddr]<=ramdataout;
else ramdatain<=ram[ramaddr];
end


always@(posedge clk)

begin
if (~irq && irqreq==0) irqreq<=1;

if (~reset) // standard: reset active low
  begin
  acc<=0;
  sp<=8'hfe; // ff=irqerrupt vector
  pc<=0;
  flags<=0;
  iflags<=0;
  phase<=0;
  opcode<=0;
  rw<=1;
  end

else
  begin  


  case (phase)

  0: // get an opcode 

    begin
     if (irqreq==1) // interrupt acknowledge
       begin
        phase<=10;
        irqreq<=2;
        end
     else
       begin 
       rw<=1;
       ramaddr<=pc;
       md<=0;
       pc<=pc+1; 
      phase<=1;
        end
    end

  1: begin 
     opcode<=ramdatain;
     phase<=2;
      end  

  2: // decode address mode 

    begin
     if (opcode[7:6]==2'b10 ) // imm
       begin
        rw<=1;
        ramaddr<=pc;
        md<=0;
       pc=pc+1;
        phase<=3;
        end

     else if (opcode[7:6]==2'b11 ) // abs
       begin
      rw<=1;
        md<=0;
        ramaddr<=pc;
       pc=pc+1;
        phase<=4;
        end  
     else
       phase<=6; // acc or imp
     end

  3: begin 
     arg<=ramdatain;
     phase<=6;
      end    

  4: begin 
     ramaddr<=ramdatain;
      rw<=1;
      pc<=0;
     phase<=5;
      end    

  5: begin 
     arg<=ramdatain;
     phase<=6;
      end     


  6: // execute the opcode
    begin

     case (opcode)

       8'h29:       // ret
        begin
          sp<=sp+1;      
      ramaddr<=sp+1;
      rw<=1;
      md<=0;
          phase<=7;   
          end

        8'h2a:       // iret
          begin
          sp<=sp+1;
          ramaddr<=sp+1;
          rw<=1;
          md<=0;
          flags<=iflags;
          phase<=7;
         irqreq<=0;   
          end

        8'h41:       // shl a
          begin
          flags[0]<=acc[7];
          acc<=acc<<1;
          phase<=0;
          end

       8'h42:       // shr a
          begin
          flags[0]<=acc[0];
          acc<=acc>>1;
          phase<=0;
          end

        8'h61:       // bne a
          begin
          if (flags[1]==0) pc<=acc;
          phase<=0;
          end

       8'h62:       // bcs a
          begin
          if (flags[0]==1) pc<=acc;
          phase<=0;
          end

        8'h64:       // jmp a
          begin
          pc<=acc;
          phase<=0;
          end

        8'h68:       // call a
          begin
          ramaddr<=sp;
          ramdataout<=pc;
          rw<=0;
          md<=0;
          sp<=sp-1;
          pc<=acc;
          phase<=0;
          end

        8'h71:       // push a
          begin
          ramaddr<=sp;
          ramdataout<=acc;
          rw<=0;
          md<=0;
          sp<=sp-1;
          phase<=0;
          end

        8'h72:       // pop a
          begin
          sp<=sp+1;
      ramaddr<=sp+1;
          rw<=1;
          md<=1;
          phase<=8;
          end

        8'h81, 8'hc1:  // adc #, adc abs
          begin
          {flags[0],acc}<=acc+arg+flags[0];
          flags[1]<=~(&(acc+arg+flags[0]));
          phase<=0;
          end

        8'h82, 8'hc2:       // sbc #, sbc abs
          begin
          {flags[0],acc}<=acc-arg-flags[0];
          flags[1]<=~(&(acc+arg+flags[0]));
          phase<=0;
          end

        8'h84, 8'hc4:       // and #, and abs
          begin
          acc<=acc&arg;
          flags[1]<=~(&(acc&arg));
          phase<=0;
          end

        8'h85, 8'hc5:       // or #, or abs
          begin
          acc<=acc|arg;
          flags[1]<=~(&(acc|arg));
          phase<=0;
          end

        8'h86, 8'hc6:       // xor #, xor abs
          begin
          acc<=acc^arg;
          flags[1]<=~(&(acc^arg));
          phase<=0;
          end

        8'h91, 8'hd1:       // lda #, lda abs
          begin
          acc<=arg;
          phase<=0;
          end

       8'hd2:              // sta abs - sta # is nonsense
          begin
          ramaddr<=arg;
          ramdataout<=acc;
          rw<=0;
          md<=0;
          phase<=0;
          end

        8'h94, 8'hd4:       // in #, in abs
          begin
          addr<=arg;
          rw<=1;
          md<=1; 
          phase<=9;           // need to wait for i/o
      end

        8'h95, 8'hd5:       // out #, out abs
          begin
          addr<=arg;
          rw<=0;
          md<=1;
          dout<=acc;
          phase<=0;
          end

        8'ha1, 8'he1:       // bne #, bne abs
          begin
          if (flags[1]==0) pc<=arg;
          phase<=0;
          end

       8'ha2, 8'he2:       // bcs #, bcs abs
          begin
          if (flags[0]==1) pc<=arg;
          phase<=0;
          end

        8'ha4, 8'he4:       // jmp #, jmp abs
          begin
          pc<=arg;
          phase<=0;
          end

        8'ha8, 8'he8:      // call #, call abs
          begin
          ramaddr<=sp;
          ramdataout<=pc;
          rw<=0;
          md<=0;
          sp<=sp-1;
          pc<=arg;
          phase<=0;
          end

        default:
          phase<=0;

          endcase
        end


  7: // get the pc from RAM 
    begin
     pc<=ramdatain;
     phase<=0;
     end

  8: // get the acc from RAM    
    begin
     acc<=ramdatain;
     phase<=0;
     end

  9: // get the data from the device

     begin
    acc<=din;
     phase<=0;
     end    

  10: // interrupt phase 1 - save flags and return address

     begin
    iflags<=flags;
     ramaddr<=sp;
     ramdataout<=pc;
     rw<=0;
     md<=0;
     sp<=sp-1;
     phase<=11;
     end  

  11: // interrupt phase 2 - get vector from $FF

     begin
    ramaddr<=255;
     rw<=1;
     md<=0;
     phase<=12;
     end    

  12: // interrupt phase 3 - get vector from $FF

     begin
     pc<=ramdatain;
     phase<=0;
     end    

     endcase
  end   
end

endmodule

Rilke · 2021-11-08 18:51

Piotr, what is the maximum clock frequency ?

pik33 · 2021-11-09 07:17

I don't remember now. I did this thing for the student's lecture. (I teached basics of FPGA) - but generally this kind of thing works up to 100..200 MHz in Cyclone IV (DE2-115)

The goal was to show students how to make a CPU and peripherals in FPGA, so I also wrote some peripherals (switch input, LED output, 7-segment output) and address decoder, then I connected all of this using Quartus GUI to show the structure of such a system

This CPU has 8-bit (!) memory address space using FPGA's internal memory and separate, 8-bit I/O address space.

Rayman · 2021-11-09 23:30

Got a 74181 ALU coded up in Verilog.
Going to use P2 to do some automated testing to see if it's right.

Found some C code for 74181 in an old version of Mame: https://github.com/mamedev/historic-mame/blob/master/src/emu/machine/74181.c

Can compare the output of that with FlexProp C to what the fpga is putting out.
The 74181 is actually fairly complex for such an old chip.
And, getting the Verilog to match took a fair amount of time.

I probably made some mistakes somewhere, so this is going to be a big help.

Scroungre · 2021-11-10 04:40

I've got an old VHDL textbook wherein one chapter is entirely replicating an AM2901 (4-bit ALU) in an FPGA. S.

__deets__ · 2021-11-10 10:29

There is the MiSTer project that implements 80/90ies home computers and game consoles using FPGA. Including 68K processors. See eg https://github.com/MiSTer-devel/Minimig-AGA_MiSTer

Rayman · 2021-11-10 19:22

The P2 was a big help with testing the 74181 in verilog.
Found several bugs.
My carry out is wrong (always 0) when doing logic functions.
I think this is how it should be, but the actual chip doesn't care about M when outputting the carry...

Also, I think there was a bug in the carry out in that Mame code, that I fixed.
Don't know enough about their code to say for sure, but had to change something to match what I think is correct.
This page is very helpful with that: http://www.righto.com/2017/03/inside-vintage-74181-alu-chip-how-it.html

P2Eval <--> TinyFPGA

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