Send 0-length IN packet to flush USB after full packet

USB bulk transfers are a sequence of maximum-sized packets followed by
a short packet, which signals the end of the transfer. When the last
packet of the transfer would be a full-sized packet, an additional
packet of zero length is sent to signal the transfer end.

Signed-off-by: Keith Packard <keithp@keithp.com>

diff --git a/src/ao_usb.c b/src/ao_usb.c
index 99f0715bb9536f658b8fd60c7567fcdec5b9e28b..2266572541c820b661897b22b3a6530152b71fa9 100644 (file)
--- a/src/ao_usb.c
+++ b/src/ao_usb.c
@@ -21,6 +21,7 @@
 struct ao_task __xdata ao_usb_task;
 
 static __xdata uint16_t        ao_usb_in_bytes;
+static __xdata uint16_t ao_usb_in_bytes_last;
 static __xdata uint16_t        ao_usb_out_bytes;
 static __xdata uint8_t ao_usb_iif;
 static __xdata uint8_t ao_usb_running;
@@ -321,13 +322,40 @@ ao_usb_ep0(void)
        }
 }
 
+/* Wait for a free IN buffer */
+static void
+ao_usb_in_wait(void)
+{
+       for (;;) {
+               USBINDEX = AO_USB_IN_EP;
+               if ((USBCSIL & USBCSIL_INPKT_RDY) == 0)
+                       break;
+               ao_sleep(&ao_usb_in_bytes);
+       }
+}
+
+/* Send the current IN packet */
+static void
+ao_usb_in_send(void)
+{
+       USBINDEX = AO_USB_IN_EP;
+       USBCSIL |= USBCSIL_INPKT_RDY;
+       ao_usb_in_bytes_last = ao_usb_in_bytes;
+       ao_usb_in_bytes = 0;
+}
+
 void
 ao_usb_flush(void) __critical
 {
-       if (ao_usb_in_bytes) {
-               USBINDEX = AO_USB_IN_EP;
-               USBCSIL |= USBCSIL_INPKT_RDY;
-               ao_usb_in_bytes = 0;
+       if (!ao_usb_running)
+               return;
+
+       /* If there are pending bytes, or if the last packet was full,
+        * send another IN packet
+        */
+       if (ao_usb_in_bytes || (ao_usb_in_bytes_last == AO_USB_IN_SIZE)) {
+               ao_usb_in_wait();
+               ao_usb_in_send();
        }
 }
 
@@ -336,18 +364,13 @@ ao_usb_putchar(char c) __critical
 {
        if (!ao_usb_running)
                return;
-       for (;;) {
-               USBINDEX = AO_USB_IN_EP;
-               if ((USBCSIL & USBCSIL_INPKT_RDY) == 0)
-                       break;
-               ao_sleep(&ao_usb_in_bytes);
-       }
+
+       ao_usb_in_wait();
+
+       /* Queue a byte, sending the packet when full */
        USBFIFO[AO_USB_IN_EP << 1] = c;
-       if (++ao_usb_in_bytes == AO_USB_IN_SIZE) {
-               USBINDEX = AO_USB_IN_EP;
-               USBCSIL |= USBCSIL_INPKT_RDY;
-               ao_usb_in_bytes = 0;
-       }
+       if (++ao_usb_in_bytes == AO_USB_IN_SIZE)
+               ao_usb_in_send();
 }
 
 char