machine: add USBDevice.Attach and USBDevice.Detach

The USB device is attached to the bus automatically during startup,
before user code has a chance to finish its USB configuration (device
identifiers, extra HID interfaces, ...). Composite devices such as
keyboards may therefore be enumerated by the host with an incomplete
configuration.

Attach and Detach expose the soft-connect control (DP pull-up) so that
an application or library can detach in an init function, complete its
configuration, and attach again to let the host enumerate the finished
device. They can also be used to force re-enumeration without
replugging the cable.

Implemented for atsamd21, atsamd51, nrf52840, rp2040 and rp2350.

src/machine/machine_atsamd21_usb.go

@@ -68,6 +68,20 @@ func (dev *USBDevice) Configure(config UARTConfig) {
 	dev.initcomplete = true
 }
 
+// Attach connects the device to the USB bus, allowing the host to detect and
+// enumerate it. It can be used together with Detach to delay enumeration
+// until the USB configuration (device identifiers, classes, ...) is complete.
+func (dev *USBDevice) Attach() {
+	sam.USB_DEVICE.CTRLB.ClearBits(sam.USB_DEVICE_CTRLB_DETACH)
+}
+
+// Detach disconnects the device from the USB bus. To the host this appears
+// as if the device was unplugged. A subsequent Attach makes the host
+// enumerate the device again.
+func (dev *USBDevice) Detach() {
+	sam.USB_DEVICE.CTRLB.SetBits(sam.USB_DEVICE_CTRLB_DETACH)
+}
+
 func handlePadCalibration() {
 	// Load Pad Calibration data from non-volatile memory
 	// This requires registers that are not included in the SVD file.

src/machine/machine_atsamd51_usb.go

@@ -71,6 +71,20 @@ func (dev *USBDevice) Configure(config UARTConfig) {
 	dev.initcomplete = true
 }
 
+// Attach connects the device to the USB bus, allowing the host to detect and
+// enumerate it. It can be used together with Detach to delay enumeration
+// until the USB configuration (device identifiers, classes, ...) is complete.
+func (dev *USBDevice) Attach() {
+	sam.USB_DEVICE.CTRLB.ClearBits(sam.USB_DEVICE_CTRLB_DETACH)
+}
+
+// Detach disconnects the device from the USB bus. To the host this appears
+// as if the device was unplugged. A subsequent Attach makes the host
+// enumerate the device again.
+func (dev *USBDevice) Detach() {
+	sam.USB_DEVICE.CTRLB.SetBits(sam.USB_DEVICE_CTRLB_DETACH)
+}
+
 func handlePadCalibration() {
 	// Load Pad Calibration data from non-volatile memory
 	// This requires registers that are not included in the SVD file.

src/machine/machine_nrf52840_usb.go

@@ -89,6 +89,21 @@ func (dev *USBDevice) Configure(config UARTConfig) {
 	dev.initcomplete = true
 }
 
+// Attach connects the device to the USB bus by enabling the DP pull-up,
+// allowing the host to detect and enumerate it. It can be used together with
+// Detach to delay enumeration until the USB configuration (device
+// identifiers, classes, ...) is complete.
+func (dev *USBDevice) Attach() {
+	nrf.USBD.USBPULLUP.Set(1)
+}
+
+// Detach disconnects the device from the USB bus by disabling the DP pull-up.
+// To the host this appears as if the device was unplugged. A subsequent
+// Attach makes the host enumerate the device again.
+func (dev *USBDevice) Detach() {
+	nrf.USBD.USBPULLUP.Set(0)
+}
+
 func handleUSBIRQ(interrupt.Interrupt) {
 	if nrf.USBD.EVENTS_SOF.Get() == 1 {
 		nrf.USBD.EVENTS_SOF.Set(0)

src/machine/machine_rp2040_usb.go

@@ -46,6 +46,21 @@ func (dev *USBDevice) Configure(config UARTConfig) {
 	rp.USBCTRL_REGS.SIE_CTRL.SetBits(rp.USBCTRL_REGS_SIE_CTRL_PULLUP_EN)
 }
 
+// Attach connects the device to the USB bus by enabling the DP pull-up,
+// allowing the host to detect and enumerate it. It can be used together with
+// Detach to delay enumeration until the USB configuration (device
+// identifiers, classes, ...) is complete.
+func (dev *USBDevice) Attach() {
+	rp.USBCTRL_REGS.SIE_CTRL.SetBits(rp.USBCTRL_REGS_SIE_CTRL_PULLUP_EN)
+}
+
+// Detach disconnects the device from the USB bus by disabling the DP pull-up.
+// To the host this appears as if the device was unplugged. A subsequent
+// Attach makes the host enumerate the device again.
+func (dev *USBDevice) Detach() {
+	rp.USBCTRL_REGS.SIE_CTRL.ClearBits(rp.USBCTRL_REGS_SIE_CTRL_PULLUP_EN)
+}
+
 func handleUSBIRQ(intr interrupt.Interrupt) {
 	status := rp.USBCTRL_REGS.INTS.Get()
 

src/machine/machine_rp2350_usb.go

@@ -49,6 +49,21 @@ func (dev *USBDevice) Configure(config UARTConfig) {
 	rp.USB.SetMAIN_CTRL_PHY_ISO(0x0)
 }
 
+// Attach connects the device to the USB bus by enabling the DP pull-up,
+// allowing the host to detect and enumerate it. It can be used together with
+// Detach to delay enumeration until the USB configuration (device
+// identifiers, classes, ...) is complete.
+func (dev *USBDevice) Attach() {
+	rp.USB.SIE_CTRL.SetBits(rp.USB_SIE_CTRL_PULLUP_EN)
+}
+
+// Detach disconnects the device from the USB bus by disabling the DP pull-up.
+// To the host this appears as if the device was unplugged. A subsequent
+// Attach makes the host enumerate the device again.
+func (dev *USBDevice) Detach() {
+	rp.USB.SIE_CTRL.ClearBits(rp.USB_SIE_CTRL_PULLUP_EN)
+}
+
 func handleUSBIRQ(intr interrupt.Interrupt) {
 	status := rp.USB.INTS.Get()