SPI inter-byte duration

[Henry Choi]

Hello, I've been working on porting a bit-banged control of a peripheral to use nRF51 SPI--which is just SPI rather than SPIM (therefore no DMA). Observe the SPICLK waveform when I choose 4 MHz:

The byte itself is emitted at 4 MHz. But the huge inter-byte distance remains constant down to about 1 MHz CLK speed. I guess that's just how it is with SPI (not SPIM). In the waveform above, one can see that the maximum effective SPI speed is roughly 14 us / 1 B => 570 KHz, which is only slightly faster than the maximum effective SPI speed when I bit-bang using GPIO on an STM32L1 (Cortex M3) ~ 400 KHz. It nearly negates the arguments for using the SPI peripheral over bit-bang (except for some battery savings by using WFE).

I doubt there is anything I can do to improve the situation, but thought I'd check with experts just in case. Thanks for reading.

[Wojtek]

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

[Hung Bui]

@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.

[Henry Choi]

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?

[Hung Bui]

@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) ?

[Henry Choi]

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?