Add USB FS support

The implementation derived from https://github.com/Disasm/usb-otg-workspace
Fork of synopsys-usb-otg from github temporary used.

Cargo.toml

@@ -26,3 +26,12 @@ features = ["unproven"]
 git = "https://github.com/katyo/gd32vf103-pac"
 branch = "upcoming"
 
+[dependencies.synopsys-usb-otg]
+git = "https://github.com/katyo/synopsys-usb-otg"
+branch = "gd32v-ng-cs"
+features = ["fs"]
+optional = true
+
+[features]
+default = []
+usb_fs = ["synopsys-usb-otg"]

src/lib.rs

@@ -29,3 +29,6 @@ pub mod spi;
 pub mod time;
 pub mod timer;
 pub mod watchdog;
+
+#[cfg(feature = "usb_fs")]
+pub mod otg_fs;

src/otg_fs.rs

@@ -0,0 +1,53 @@
+//! USB OTG full-speed peripheral
+//!
+//! Requires the usb_fs feature.
+
+use crate::{
+    gpio::{
+        gpioa::{PA11, PA12},
+        Floating, Input,
+    },
+    pac,
+    time::Hertz,
+};
+pub use synopsys_usb_otg::UsbBus;
+use synopsys_usb_otg::UsbPeripheral;
+
+pub type UsbBusType = UsbBus<USB>;
+
+#[allow(dead_code)]
+pub struct USB {
+    pub usb_global: pac::USBFS_GLOBAL,
+    pub usb_device: pac::USBFS_DEVICE,
+    pub usb_pwrclk: pac::USBFS_PWRCLK,
+    pub pin_dm: PA11<Input<Floating>>,
+    pub pin_dp: PA12<Input<Floating>>,
+    pub hclk: Hertz,
+}
+
+unsafe impl Sync for USB {}
+
+unsafe impl UsbPeripheral for USB {
+    const REGISTERS: *const () = pac::USBFS_GLOBAL::ptr() as *const ();
+
+    const HIGH_SPEED: bool = false;
+    const FIFO_DEPTH_WORDS: usize = 320;
+    const ENDPOINT_COUNT: usize = 4;
+
+    fn enable() {
+        let rcu = unsafe { &*pac::RCU::ptr() };
+
+        riscv::interrupt::free(|| {
+            // Enable USB peripheral
+            rcu.ahben.modify(|_, w| w.usbfsen().set_bit());
+
+            // Reset USB peripheral
+            rcu.ahbrst.modify(|_, w| w.usbfsrst().set_bit());
+            rcu.ahbrst.modify(|_, w| w.usbfsrst().clear_bit());
+        });
+    }
+
+    fn ahb_frequency_hz(&self) -> u32 {
+        self.hclk.0
+    }
+}