SPI slave read without debugger

[KV]

I am using nRF52840 DK as a SPI slave with an external MCU configured as SPI master. The SPI clock frequency is set to 1 MHz. I am using the following pins for SPI:

P0.29 - SPIS_CS_PIN P0.28 - SPIS_MISO_PIN P0.4 - SPIS_MOSI_PIN P0.3 - SPIS_SCK_PIN

Using the SPI slave driver in SDK 13.0.0 and S140 softdevice, I am seeing the SPI read is failing when the debugger is detached. SPI_XFER_DONE event does not happen. I am using a timer for SPI_XFER_DONE timeout purposes so that it does not get stuck waiting for SPI completion. The timeout is set to 100 millisecs (larger timeouts does not make any difference).

I start my SPI communication without debugger connection and I notice failures. Then in the middle of my test, I attach the debugger to running target and then communication is successful. Then as soon as I detach, the communication starts to fail again.

I am using IAR 8.11.1 as my IDE and segger J-Link as my debugger. The same behavior is seen if I use IAR I-Jet as my debugger also.

EDIT: 06/28/2017: If the low power mode was disabled (by commenting out sd_app_evt_wait() ), then this issue goes away. Setting constant latency mode to 1 did not make any difference. See attached code in the comments section.

[Jørgen Holmefjord]

To answer your question about placement of constant latency configuration: the reason why you get an error when placing it right before the while-loop in main is that you have enabled softdevice at this point. The softdevice restricts access to some peripherals, including the POWER peripheral. The peripherals can only be accessed through softdevice-safe APIs. To configure constant latency mode with softdevice enabled, you can call sd_power_mode_set(NRF_POWER_MODE_CONSTLAT).

[KV]

I was able to resolve this issue by following the suggestion in this post