From c793adbd8676522a5fa34c89feb7b38899f34b67 Mon Sep 17 00:00:00 2001
From: mikeller <github@ike.ch>
Date: Sun, 10 May 2020 04:10:20 +1200
Subject: [PATCH] Fixed problem with USB VCP when sending packets with a length
 of a multiple of 64 bytes.

---
 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 0522cf2c3c..38c05b05e4 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 */