From 3e4b51e09b0d915969ba04d3c2e29a6c281114b4 Mon Sep 17 00:00:00 2001
From: Michael Keller <github@ike.ch>
Date: Thu, 14 May 2020 02:12:02 +1200
Subject: [PATCH] Merge pull request #9789 from
 etracer65/hal_vcp_64byte_receive_fix

---
 src/main/vcp_hal/usbd_cdc_interface.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/main/vcp_hal/usbd_cdc_interface.c b/src/main/vcp_hal/usbd_cdc_interface.c
index 0522cf2c3c..cfc4400a96 100644
--- a/src/main/vcp_hal/usbd_cdc_interface.c
+++ b/src/main/vcp_hal/usbd_cdc_interface.c
@@ -299,6 +299,12 @@ static int8_t CDC_Itf_Receive(uint8_t* Buf, uint32_t *Len)
 {
     rxAvailable = *Len;
     rxBuffPtr = Buf;
+    if (!rxAvailable) {
+        // Received an empty packet, trigger receiving the next packet.
+        // This will happen after a packet that's exactly 64 bytes is received.
+        // The USB protocol requires that an empty (0 byte) packet immediately follow.
+        USBD_CDC_ReceivePacket(&USBD_Device);
+    }
     return (USBD_OK);
 }