No hard fault with SoftDevice and RAM off after WFI

[AD1170]

Hi Folks,

i have implementet a sleep mode by using only the RTC timer with the 32KHz OSC and the WFI instruction. The current consumtion is around 1.7µA (all peripherals including the RAM and HSXOSC is turned off). On timeout, the chip wake up and a hard fault interrupt is generated. In the Hard_Fault() handler the NVIC_SystemReset() is called and the controller will restart. The wakeup / restart reason is detected with the "NRF_POWER->GPREGRET" register. I have assigned a special value to the register before go to sleep mode. This works fine without SoftDevice. But if i use the SoftDevice (SD310 in this case) the controller hangs after wake up. The SD is disabled before power down the peripherals. It seems to be there is no hard fault condition generated in this case. Why?

[AD1170]

If the controller awake (from WFI), the next statement from flash is executed or the interrupt handler address, in this case the RTC1 interrupt vector, is fetched from the vector table. In this moment, you are right, there is no (stack-) RAM to save the PC. So i think the hard fault is the last way out and is called without saving any data on stack. I think, the problem is the changed vector table when soft device is flashed on the chip.

[Martijn]

Then you answered your own question, right? For the softdevice to forward any irq handler (including hardfault) you need stack space.

[AD1170]

But the softdevice is disabled and the interrupts should forwarded to the application.

[Martijn]

correct, but to be able to forward any interrupts it needs some ram to do so.

[AD1170]

Meanwhile i have fond a solution using the watch dog instead of RTC timer. This is not a pretty solution but it works for the first time.