From a72f7aa9bfeba0e307450769fe0cef181caef294 Mon Sep 17 00:00:00 2001
From: Michael Keller <github@ike.ch>
Date: Thu, 14 May 2020 02:10:58 +1200
Subject: [PATCH] Merge pull request #9790 from
 mikeller/fix_f7_usb_zero_lenght_packet

---
 src/main/vcp_hal/usbd_cdc_interface.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/main/vcp_hal/usbd_cdc_interface.c b/src/main/vcp_hal/usbd_cdc_interface.c
index cfc4400a963..3aa99cc23cf 100644
--- a/src/main/vcp_hal/usbd_cdc_interface.c
+++ b/src/main/vcp_hal/usbd_cdc_interface.c
@@ -261,12 +261,19 @@ void HAL_TIM_PeriodElapsedCallback(TIM_HandleTypeDef *htim)
     if (hcdc->TxState == 0) {
         // endpoint has finished transmitting previous block
         if (lastBuffsize) {
+            bool needZeroLengthPacket = lastBuffsize % 64 == 0;
+
             // move the ring buffer tail based on the previous succesful transmission
             UserTxBufPtrOut += lastBuffsize;
             if (UserTxBufPtrOut == APP_TX_DATA_SIZE) {
                 UserTxBufPtrOut = 0;
             }
             lastBuffsize = 0;
+
+            if (needZeroLengthPacket) {
+                USBD_CDC_SetTxBuffer(&USBD_Device, (uint8_t*)&UserTxBuffer[UserTxBufPtrOut], 0);
+                return;
+            }
         }
         if (UserTxBufPtrOut != UserTxBufPtrIn) {
             if (UserTxBufPtrOut > UserTxBufPtrIn) { /* Roll-back */