Zephyr interrupt-thread synchronization - improving real time performance

[eyalasko]

Hello,

In an nRF52840, NCS2.7.0 application (based on BT NUS shell) I need to address the following case:

1. UART Interrupt handler monitors incoming bytes
2. A working thread (UART message parser & handler) is blocked on a sync object until a full message is received
3. Once the interrupt handler recognizes the end-of-UART-messgae condition, it asserts the sync object
4. The thread gets unblocked and handles the received message.

The problem I'm facing is a very looong (>20ms !) latency from the time the interrupt asserted the sync object until the thread is unblocked.

I tried:

• Using events for synchronization (k_event_post/k_event_wait)
• Using Semaphore (k_sem_give/k_sem_take)
• Changing the thread priority between priority 7 & priority 1

Nothing seems to improve the poor real-time response to a reasonable latency.

I have past experience with freertos and I've never experienced such long latencies. I assume I'm missing something here.

• Is there a recommended 'light' sync object that can mitigate that?
• Will using a workqueue yield a better realtime behavior?
• Any Kconfig parameter I should be aware of?

Thanks for any advice

[Edvin]

Hello,

You are sure that it isn't the UART RX time that is taking up the time? In what interrupts do you toggle the GPIO? If you could share some snippets, or perhaps the entire application, then I can have a look.

Best regards,

Edvin