Warp-SE/cpld/RAM.v

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module RAM(
/* MC68HC000 interface */
input CLK, input [21:1] A, input nWE, input nAS, input nLDS, input nUDS,
/* AS cycle detection */
input BACT,
/* Select and ready signals */
input RAMCS, input ROMCS, output Ready,
/* Refresh Counter Interface */
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input RefReqIn, input RefUrgentIn,
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/* DRAM and NOR flash interface */
output [11:0] RA, output nRAS, output reg nCAS,
output nLWE, output nUWE, output nOE, output nROMCS, output nROMWE);
/* RAM control state */
reg [2:0] RS = 0;
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reg RAMEN = 0;
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reg RAMReady = 0;
reg RASEL = 0; // RASEL controls /CAS signal
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/* Refresh request synchronization */
reg RefReqR; // Refresh synchronization
always @(posedge CLK) RefReqR <= RefReqIn;
reg RefReq, RefUrgent; // Refresh commands
reg RefDone; // Refresh done "remember"
always @(posedge CLK) begin
RefReq <= RefReqR && !RefDone;
RefUrgent <= RefReqR && RefUrgentIn && !RefDone;
if (!RefReqR) RefDone <= 0;
else if (RS==2 || RS==3) RefDone <= 1; // RS2 || RS3 to save 1 input
end
/* RAM enable
*/
/* Refresh init conditions */
wire RAMRefFromRS0Next = RS==0 && (
// Non-urgent refresh can start during first clock of non-RAM cycle
( BACT && ~BACTr && ~RAMCS && RefReq) ||
// Urgent refresh can start during bus idle
(~BACT && RefUrgent) ||
// Urgent refresh can start during non-ram cycle
( BACT && ~RAMCS && RefUrgent));
wire RAMRefFromRS0Pre = RS==0 &&
// Urgent refresh can start during long RAM cycle after RAM access done.
BACT && RAMCS && !RAMEN && RefUrgent;
wire RAMRefFromRS0 = RAMRefFromRS0Next || RAMRefFromRS0Pre;
// Urgent refresh cannot start when BACT and RAMCS and RAMEN,
// since /RAS has already been asserted. For this we wait for RS7.
wire RAMRefFromRS7 = RS==7 && RefUrgent;
/* RAM access start condition */
wire RAMStart = RS==0 && BACT && RAMCS && RAMEN;
/* RAM enable (/AS -> /RAS) */
always @(posedge CLK) begin
if (RS==0) begin
if (RAMRefFromRS0) RAMEN <= 0;
else if (!BACT) RAMEN <= 1;
end else if (RS==7) begin
if (RAMRefFromRS7) RAMEN <= 0;
else if (BACT) RAMEN <= 0;
else if (!BACT) RAMEN <= 1;
end
end
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/* Refresh state */
reg RefRAS = 0;
assign nROMCS = ~ROMCS;
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assign nRAS = ~((~nAS && RAMCS && RAMEN) || RefRAS);
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assign nOE = ~(~nAS && nWE);
assign nLWE = ~(~nAS && ~nWE && ~nLDS && RAMEN);
assign nUWE = ~(~nAS && ~nWE && ~nUDS && RAMEN);
assign nROMWE = ~(~nAS && ~nWE);
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/* RAM address mux (and ROM address on RA8) */
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assign RA[11] = A[19];
assign RA[10] = A[21];
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assign RA[09] = RASEL ? A[20] : A[19];
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assign RA[08] = (RASEL && RAMCS) ? A[09] : A[18];
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assign RA[07] = RASEL ? A[08] : A[17];
assign RA[06] = RASEL ? A[07] : A[16];
assign RA[05] = RASEL ? A[06] : A[15];
assign RA[04] = RASEL ? A[05] : A[14];
assign RA[03] = RASEL ? A[04] : A[13];
assign RA[02] = RASEL ? A[03] : A[12];
assign RA[01] = RASEL ? A[02] : A[11];
assign RA[00] = RASEL ? A[01] : A[10];
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// Save BACT from last clock
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reg BACTr;
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always @(posedge CLK) BACTr <= BACT;
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always @(posedge CLK) begin
if (RS==0) begin
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// In RS0, RAM is idle and ready for new command.
if (RefFromRS0Next) begin
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RS <= 2;
RAMReady <= 0;
RASEL <= 1;
RAMDIS1 <= 1;
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end else if (RefFromRS0Pre) begin
// Urgent ref can start during long RAM cycle after access.
// Must insert one extra precharge state first by going to RS1.
RS <= 1;
RAMReady <= 0;
RASEL <= 0;
RAMDIS1 <= 1;
end else if (BACT && RAMCS && RAMEN) begin
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// RAM access cycle has priority over urgent refresh if RAM access already begun
RS <= 5;
RAMReady <= 0;
RASEL <= 1;
RAMDIS1 <= 0;
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end else if (RAMRefFromRS0Pre) begin
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RS <= 1;
RAMReady <= 0;
RASEL <= 0;
RAMDIS1 <= 1;
end else begin
// No RAM access/refresh requests pending
RS <= 0;
RAMReady <= 1;
RASEL <= 0;
RAMDIS1 <= 0;
end
RefRAS <= 0;
end else if (RS==1) begin
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// RS1 implements extra precharge time before refresh.
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RS <= 2;
RAMReady <= 0;
RASEL <= 1;
RAMDIS1 <= 1;
RefRAS <= 0;
end else if (RS==2) begin
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// Refresh RAS pulse asserted ater RS2.
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RS <= 3;
RAMReady <= 0;
RASEL <= 1;
RAMDIS1 <= 1;
RefRAS <= 1;
end else if (RS==3) begin
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// RS3 implements requisite RAS pulse width.
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RS <= 4;
RAMReady <= 0;
RASEL <= 0;
RAMDIS1 <= 1;
RefRAS <= 1;
end else if (RS==4) begin
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// RS4 implements precharge after RAM refresh.
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RS <= 7;
RAMReady <= 0;
RASEL <= 0;
RAMDIS1 <= 1;
RefRAS <= 0;
end else if (RS==5) begin
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// RS5 is first state of R/W operation
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RS <= 6;
RAMReady <= 0;
RASEL <= 1;
RAMDIS1 <= 0;
RefRAS <= 0;
end else if (RS==6) begin
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// RS6 is second state of R/W operation
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RS <= 7;
RAMReady <= 0;
RASEL <= 0;
RAMDIS1 <= 0;
RefRAS <= 0;
end else if (RS==7) begin
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// RS7 is final state of R/W or refresh operation.
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if (~BACT && RefUrgent) begin
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// If /AS cycle terminated and urgent refresh request,
// we know /RAS has been in precharge so we can go to RS2.
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RS <= 2;
RAMReady <= 0;
RAMDIS1 <= 1;
RASEL <= 1;
end else if (BACT && RefUrgent) begin
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// But if /AS cycle hasn't terminated and we need to refresh,
// we need to go to RS1 to add additional precharge time.
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RS <= 1;
RAMReady <= 0;
RASEL <= 0;
RAMDIS1 <= 1;
end else begin
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// Otherwise if no urgent refresh request, go to RS0.
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RS <= 0;
RAMReady <= 1;
RASEL <= 0;
RAMDIS1 <= 0;
end
RefRAS <= 0;
end
end
always @(negedge CLK) begin nCAS <= ~RASEL; end
assign Ready = ~RAMCS || RAMReady;
endmodule