Move "send ZLP write start" after last alignment packet sent

Author: cyliangtw

drivers/usb/source/USBCDC.cpp

@@ -391,6 +391,7 @@ void USBCDC::send_nb(uint8_t *buffer, uint32_t size, uint32_t *actual, bool now)
         if ((CDC_MAX_PACKET_SIZE == size) && (CDC_MAX_PACKET_SIZE == write_size)) {
             _trans_zlp = true;
         }
+
         if (now) {
             _send_isr_start();
         }
@@ -407,11 +408,6 @@ void USBCDC::_send_isr_start()
         if (USBDevice::write_start(_bulk_in, _tx_buffer, _tx_size)) {
             _tx_in_progress = true;
         }
-    } else if (!_tx_in_progress && _trans_zlp) {
-        if (USBDevice::write_start(_bulk_in, _tx_buffer, 0)) {
-            _tx_in_progress = true;
-            _trans_zlp = false;
-        }
     }
 }
 
@@ -423,6 +419,11 @@ void USBCDC::_send_isr()
 {
     assert_locked();
 
+    /* Send ZLP write start after last alignment packet sent */
+    if (_trans_zlp && USBDevice::write_start(_bulk_in, _tx_buffer, 0)) {
+        _trans_zlp = false;
+    }
+
     write_finish(_bulk_in);
     _tx_buf = _tx_buffer;
     _tx_size = 0;