SPI inter-byte duration

RE: SPI inter-byte duration

Henry Choi — Fri, 01 Jul 2016 12:58:07 GMT

Thanks for the example code Hung. Yes, that is essentially what I am doing. It's strange because calling sd_app_evt_wait() for the SPI0 is working fine--and it was working fine even BEFORE I put in the SCB_SCR_SEVONPEND_Msk or NVIC_ClearPendingIRQ(SPI0_TWI0_IRQn). So WFE() returned for SPI0 when it shouldn't have, and now UART0 WFE is NOT returning when it should?!

The production code is quite a bit more complex, so I really can't isolate it effectively--nor do I have the time to undo all the debugging setup. This is not the 1st time where I had this kind of predicament --unable to run small isolation code due to the complexity of the full setup. I should just buy another nRF51 DK board and set it up as an isolation bed.

I need to solve a more pressing problem, so I'll come back to this later (perhaps when I get another nRF51 DK). Thanks Hung.

RE: SPI inter-byte duration

Hung Bui — Fri, 01 Jul 2016 10:49:28 GMT

@Henry: Could you double check ? It worked fine in my test. I attached the code here, SDK v7.2 S110 v7.1.

experimental_ble_app_uart - Copy.zip

RE: SPI inter-byte duration

Henry Choi — Thu, 30 Jun 2016 17:37:47 GMT

I think POWER_CLOCK_IRQn is the IRQ I was looking for. Anyway, I can't seem to get the nRF to return from sd_app_evt_wait() using the technique you describe. Can you please point out anything obviously wrong I am doing below:

In init():

    NRF_UART0->ENABLE          = UART_ENABLE_ENABLE_Enabled;
    NRF_UART0->INTENSET = UART_INTENSET_TXDRDY_Msk;//required to wake up from WFE
    NRF_UART0->TASKS_STARTTX   = 1;
    NRF_UART0->TASKS_STARTRX   = 1;
    SCB->SCR |= SCB_SCR_SEVONPEND_Msk;

void put_byte(uint8_t b) {
    NRF_UART0->TXD = b;
    while (!NRF_UART0->EVENTS_TXDRDY) {
        sd_app_evt_wait();
    }
    NRF_UART0->EVENTS_TXDRDY = 0;
    NVIC_ClearPendingIRQ(UART0_IRQn);
}

RE: SPI inter-byte duration

Henry Choi — Thu, 30 Jun 2016 16:22:39 GMT

Thank you Hung, I found other information from this forum yesterday, but did not think about NVIC_ClearPending(). I found the interrupt number for SPI1 and UART0, but what is the corresponding NVIC number for NRF_CLOCK->EVENTS_HFCLKSTARTED on nRF51822? I don't see it in the IRQn_Type enum...

RE: SPI inter-byte duration

Hung Bui — Thu, 30 Jun 2016 13:19:55 GMT

@Henry Choi: I see, but from my understanding you should still enable the interrupt on READY event (with INTENSET register), if you want to wake CPU up when in WFE.

Still from my understanding, what ARM described as "events from peripheral" actually are the events that are enabled as interrupts from the peripheral. Without enabling interrupt, the peripheral has no way to tell the CPU about its events.

Also you need to enable SEVONPEND with SCB->SCR |= SCB_SCR_SEVONPEND_Msk; to wake the chip up from pended event.

So, NVIC_EnableIRQ() is not needed if you don't want to handle the interrupt, but INTENSET should be set, and also you should clear the interrupt pending flag NVIC_ClearPendingIRQ(XXX_IRQn); after it wake the chip up.

RE: SPI inter-byte duration

Henry Choi — Wed, 29 Jun 2016 16:56:49 GMT

I am indeed using the SD. Just to be clear Hung, we are talking about WFE rather than WFI. From ARM infocenter doc, WFE should return when an "event signaled by a peripheral ..." occurs. Don't you think the nRF peripheral EVENTS_XXXX all should lead to WFE() returning?

RE: SPI inter-byte duration

Hung Bui — Wed, 29 Jun 2016 15:19:58 GMT

@Henry: You should call sd_app_evt_wait() only if the softdevice is enabled. Otherwise you should call __WFE() instead. Have you made sure you enable interrupt for those event (NVIC_EnableIRQ) ?

RE: SPI inter-byte duration

Henry Choi — Wed, 29 Jun 2016 14:52:30 GMT

Thank you for the reply Wojtek and Hung, you were right: I was taking much too long in my checking for the nrf_spi_event_check(p_spi, NRF_SPI_EVENT_READY). There is still some ~4 us until I see the EVENT_READY flag in the register, but the effective BW is now up to 1.23 MHz (when using the 4 MHz SPI speed). BTW, why can't I call sd_app_evt_wait() (which just calls WFE) while waiting for NRF_SPI_EVENT_READY? I find this to be true even while waiting for NRF_UART0->EVENTS_TXDRDY (in another context) as well. Shouldn't these events wake up the nRF that is sleeping on WFE?

RE: SPI inter-byte duration

Hung Bui — Wed, 29 Jun 2016 12:50:04 GMT

@Henry: I agree with Wojtek, and have you made sure you read the RXD byte out right after you receive the READY event ? There is only 2 bytes in the RXD buffer, if you are failed to read the buffer the SPI won't clock the next byte, this is shown at Figure 58 in the nRF51 reference manual.

RE: SPI inter-byte duration

Wojtek — Wed, 29 Jun 2016 09:38:56 GMT

I think it is driver overhead that makes it so slow. Could you show your SPI-sending code?