Warp-SE/cpld/FSB.v
2021-10-29 06:04:59 -04:00

56 lines
1.2 KiB
Verilog

module FSB(
/* MC68HC000 interface */
input FCLK, input nAS, output reg nDTACK, output nVPA, output nBERR,
/* AS cycle detection */
output BACT,
/* Ready inputs */
input Ready0, input Ready1, input Ready2,
/* BERR inputs */
input BERR0, input BERR1,
/* Interrupt acknowledge select */
input IACS);
/* AS cycle detection */
reg ASrf = 0;
always @(negedge FCLK) begin ASrf <= ~nAS; end
assign BACT = ~nAS || ASrf;
/* Ready and BERR "remember" */
reg Ready0r, Ready1r, Ready2r;
reg BERR0r, BERR1r;
wire Ready = (Ready0 || Ready0r) &&
(Ready1 || Ready1r) &&
(Ready2 || Ready2r);
wire BERR = (BERR0 || BERR0r || BERR1 || BERR1r);
assign nBERR = ~(~nAS && BERR);
always @(posedge FCLK) begin
if (~BACT) begin
Ready0r <= 0;
Ready1r <= 0;
Ready2r <= 0;
BERR0r <= 0;
BERR1r <= 0;
end else begin
if (Ready0) Ready0r <= 1;
if (Ready1) Ready1r <= 1;
if (Ready2) Ready2r <= 1;
if (BERR0) BERR0r <= 1;
if (BERR1) BERR1r <= 1;
end
end
/* DTACK/VPA control */
reg VPA;
assign nVPA = ~(~nAS && VPA);
always @(posedge FCLK) begin
if (~BACT) begin
nDTACK <= 1;
VPA <= 0;
end else if (Ready && ~BERR) begin
nDTACK <= IACS;
VPA <= IACS;
end
end
endmodule