Usbcorev

This core allows you to embed a full-speed (12Mbps) USB 2.0 device core into your FPGA design.

Clocks

The core requires a reasonably precise 48MHz clock. You'd better derive it from a crystal oscillator.

Physical interface

Since USB uses a bit of a weird signaling on its half-duplex (almost-)differential line, you'll, need to do a little bit of work to connect it to the core. The following five signals connect to D+ and D- USB signals.

• input rx_j -- the differential value on D+/D- lines

• input rx_se0 -- single-ended zero detected: should be set when both D+ and D- lines are zero

• output tx_se0 -- transmit zeros on both USB lines; has priority over tx_j

• output tx_j -- transmit tx_j to D+ and ~tx_j to D-

• output tx_en -- enable the trasmitter

If your FPGA doesn't have a differential receiver, then you can simply use two pins and connect them as follows. However, without a differential receiver, you will be outside of the USB specs. Make sure the inputs are synchronized to the USB clock.

inout usb_dp;
inout usb_dn;

// ...

wire usb_tx_se0, usb_tx_j, usb_tx_en;
usb usb0(
    .rx_j(usb_dp),
    .rx_se0(!usb_dp && !usb_dn),

    .tx_se0(usb_tx_se0),
    .tx_j(usb_tx_j),
    .tx_en(usb_tx_en));

assign usb_dp = usb_tx_en? (usb_tx_se0? 1'b0: usb_tx_j): 1'bz;
assign usb_dn = usb_tx_en? (usb_tx_se0? 1'b0: !usb_tx_j): 1'bz;

However, if you have a differential receiver, you'd better use it. Configuring this is FPGA-specific. For Xilinx Spartan 6 family, I use four physical pins as follows.

// These pins are configured as differential inputs. Unfortunately,
// you can't use single-ended receivers nor transmitters on these pins.
input usb_sp;
input usb_sn;

// These pins are single-ended inouts.
inout usb_dp;
inout usb_dn'

// ...

IBUFDS usb_j_buf(.I(usb_sp), .IB(usb_sn), .O(usb_rx_j_presync));
synch usb_j_synch(clk_48, usb_rx_j_presync, usb_rx_j);
synch usb_se0_synch(clk_48, !usb_dp && !usb_dn, usb_rx_se0);

wire usb_tx_se0, usb_tx_j, usb_tx_en;
usb usb0(
    .rx_j(usb_rx_j),
    .rx_se0(usb_rx_se0),

    .tx_se0(usb_tx_se0),
    .tx_j(usb_tx_j),
    .tx_en(usb_tx_en));

assign usb_dp = usb_tx_en? (usb_tx_se0? 1'b0: usb_tx_j): 1'bz;
assign usb_dn = usb_tx_en? (usb_tx_se0? 1'b0: !usb_tx_j): 1'bz;

Note the synchronization after the receiver.

Whichever pins you transmit on need to have resistors after them. The exact values will depend on the internal resistance of the pins; usually something around 27 ohms will be ok.

You also need to pull the D+ line up to 3.3V via a 1.5k resistor. You can pull it directly, or via a pin on your FPGA, if you want to dynamically attach/detach to the bus. Make sure to never pull the line down, the only valid outputs of the pullup pin are 1'b1 and 1'bz.