STM32_USB-FS-Device_Lib_V4.1.0

file                  :usb_core.c

funciton name:  RESULT Standard_ClearFeature(void)

code               :

  if (wIndex0 & 0x80)
{
/* IN endpoint */
if (_GetTxStallStatus(Related_Endpoint ))
{
ClearDTOG_TX(Related_Endpoint);
SetEPTxStatus(Related_Endpoint, EP_TX_VALID);
}
}
else
{
/* OUT endpoint */
if (_GetRxStallStatus(Related_Endpoint))
{
if (Related_Endpoint == ENDP0)
{
/* After clear the STALL, enable the default endpoint receiver */
SetEPRxCount(Related_Endpoint, Device_Property.MaxPacketSize);
_SetEPRxStatus(Related_Endpoint, EP_RX_VALID);
}
else
{
ClearDTOG_RX(Related_Endpoint);
_SetEPRxStatus(Related_Endpoint, EP_RX_VALID);
}
}
}

question: i saw that status of IN endpint was set to VALID(EP_TX_VALID) after recover from HALT;

                i wonder why we don't set the status to NAK;

                The USB spec doesn't specify how to leave the endpoints after a halt is cleared.

                but for an interrupt IN endpoint such as a HID device,at that point when the endpoint recover from halt status,the device will send an  packet with uncertain data to the host,since the host automatically query the device  by IN token in a determined interval constantly,and it can be quite possible for the packet buffer contains uncertain data at that point;

                possible advice: just set the status to NAK,firmware could set it to VALID if necessary data get ready to transfer to host later.

主要意思是在一个端点从HALT状态恢复后，发送是有效的，那样的话，比如对于一个中断型的IN端点，会自动发给主机一个数据包，这个数据包的内容是不定的，很可能包含未知的数据；

疑问就是为何不在端点恢复后将发送端点设置为NAK，在固件准备好数据后在由固件设为VALID。