Hi all, I’m using ST’s CubeMX implementation on a F4 discovery board. I use ST’s USB middlewares with FreeRTOS. When I get a special OutputReport from PC side I have to answer nearly immediately (in 10-15 ms). Currently I cannot achieve this timing and it seems my high priority tasks can interrupt the USB callback. What do you think, is it possible? Because it’s generated code I’m not sure but can I increase the priority of the USB interrupt (if there is any)? Thank you, David

10 to 15 ms is very slow, so I’m sure its possible. Where is the USB callback function called from? If it is an interrupt then it cannot be interrupted by high priority RTOS tasks. Any non interrupt code (whether you are using an RTOS or not) can only run if no interrupts are running. Without knowing the control flow in your application its hard to know what to suggest. How is the OutputReport communicated to you? By an interrupt, a message from another task, or some other way?

The callback which receive the data from PC is called from the OTGFSIRQHandler (it’s the part of the HALPCDIRQHandler function). I think the problem is SysTickHandler’s priority is higher than OTGFSIRQHandler and it’s cannot be modified, but the scheduler shouldn’t interrupt the OTGFSIRQHandler with any task handled by the scheduler. Am I wrong that the scheduler can interrupt the OTGFS_IRQHandler?

A couple of things – First the SysTick handler should be the lowest priority interrupt, so it can get interrupted by the USB interrupt, but it cannot interrupt the USB interrupt. Even if the SysTick was running at a higher priority it would only account for a few microseconds, not so many milliseconds. Second, as per my previous email, it is physically impossible (the hardware will not allow it) for the scheduler to start running a task (i.e. non-interrupt code) while an interrupt is executing. I think you are probably looking for the problem in the wrong area. Regards.

Thank you for the answer, I think I’m a bit confused with the Cortex ISR priorities 🙂 What I can observe is if I use a much higher osDelay in my high priority task I can respond for the received USB message much faster. This is why I think tasks can mess up with my OTG interrupt.