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Altera_Forum
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13 years ago

Creating a bidirectional bus

I want to connect a USB chip (FT245R) to a CPLD. To do that I need a bidirectional 8-bit bus. I thought I could declare the pins to be bidirectional and with a mux, read to or write from the pins as ...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
13 years ago

You cannot use a "wire" in an "always" statement. It's a Verilog idiosyncranie.

What people usually do, when dealing with bi-directional signals is to decompose it in 3 variants (input, output and output enable) like this. 

input wire  USBBUS;
wire  USBBUS_i;
reg  USBBUS_o;
reg USBBUS_oe;
assign USBBUS_i = USBBUS;
assign USBBUS = USBBUS_oe == 1'b1 ? USBBUS_o : 8'hzz;
always @ (negedge clk) begin
if(write condition) begin
USBBUS_o <= outbus;
USBBUS_oe <= 1'b1;
end
else begin
USBBUS_o <= 8'hxx;
USBBUS_oe <= 1'b0;
end
if (read condition) begin
inbus <= USBBUS_i;
end
end