From 34234c2c6a4cb77a60254e3db4c2d08d912684e6 Mon Sep 17 00:00:00 2001
From: Sandeep Mistry <s.mistry@bcmi-labs.cc>
Date: Fri, 3 Nov 2017 12:03:54 -0400
Subject: [PATCH] Handle receiving a ZLP in USB_Available

---
 hardware/arduino/avr/cores/arduino/USBCore.cpp | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/hardware/arduino/avr/cores/arduino/USBCore.cpp b/hardware/arduino/avr/cores/arduino/USBCore.cpp
index e00fb028e22..4c5c50bfd16 100644
--- a/hardware/arduino/avr/cores/arduino/USBCore.cpp
+++ b/hardware/arduino/avr/cores/arduino/USBCore.cpp
@@ -171,6 +171,11 @@ static inline u8 FifoFree()
 	return UEINTX & (1<<FIFOCON);
 }
 
+static inline u8 HasOUT()
+{
+	return UEINTX & (1<<RXOUTI);
+}
+
 static inline void ReleaseRX()
 {
 	UEINTX = 0x6B;	// FIFOCON=0 NAKINI=1 RWAL=1 NAKOUTI=0 RXSTPI=1 RXOUTI=0 STALLEDI=1 TXINI=1
@@ -214,7 +219,12 @@ class LockEP
 u8 USB_Available(u8 ep)
 {
 	LockEP lock(ep);
-	return FifoByteCount();
+	u8 n = FifoByteCount();
+
+	if (!n && HasOUT())
+		ReleaseRX(); // handle ZLP
+
+	return n;
 }
 
 //	Non Blocking receive