atusb/fw: implemented USB bus reset (to host) and polling of reset from host

- usb/usb.h, usb/atu2.c (usb_reset): reset the USB bus by detaching and
  re-attaching the device
- boot.c (main): force a USB reset before running the payload
- usb/atu2.c (usb_poll): test for USB reset from the host (in progress)
- usb/dfu.c (my_reset, dfu_init): register user USB reset handler

atusb/fw/boot.c

@@ -57,6 +57,7 @@ int main(void)
 
 	led(0);
 
+	usb_reset();
 	run_payload();
 
 	while (1);	/* not reached */

atusb/fw/usb/atu2.c

@@ -171,6 +171,12 @@ void usb_poll(void)
 {
 	uint8_t flags, i;
 
+	flags = UDINT;
+	if (flags & EORSTI) {
+		UDINT &= ~(1 << EORSTI);
+		if (user_reset)
+			user_reset();
+	}
 	flags = UEINT;
 	for (i = 0; i != NUM_EPS; i++)
 		if (1 || flags & (1 << i))
@@ -194,6 +200,13 @@ static void ep_init(void)
 }
 
 
+void usb_reset(void)
+{
+	UDCON |= 1 << DETACH;	/* detach the pull-up */
+	_delay_ms(1);
+}
+
+
 void usb_init(void)
 {
 	USBCON |= 1 << FRZCLK;		/* freeze the clock */

atusb/fw/usb/dfu.c

@@ -264,21 +264,21 @@ static int my_descr(uint8_t type, uint8_t index, const uint8_t **reply,
 }
 
 
-#if 0
 static void my_reset(void)
 {
+#if 0
 	/* @@@ not nice -- think about where this should go */
 	extern void run_payload(void);
 
 	if (did_download)
 		run_payload();
-}
 #endif
+}
 
 
 void dfu_init(void)
 {
 	user_setup = my_setup;
 	user_get_descriptor = my_descr;
-//	user_reset = my_reset;
+	user_reset = my_reset;
 }

atusb/fw/usb/usb.h

@@ -141,6 +141,7 @@ void usb_io(struct ep_descr *ep, enum ep_state state, uint8_t *buf,
 
 int handle_setup(const struct setup_request *setup);
 int set_addr(uint8_t addr);
+void usb_reset(void);
 void usb_init(void);
 void usb_poll(void);