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MQX3.8 LPM on K60 - slow to enter WAIT mode - (why WFI going to VLPR?)

Question asked by TugboatCaptain on Jan 18, 2013
Latest reply on Mar 14, 2013 by TugboatCaptain

In the LPM demo code (MQX3.8 - Basic Examples - lowpower) the code that changes low power mode works correctly and takes around 2 mSec to change from RUN to WAIT or from WAIT to RUN. (changing the clock config and operating mode).  When I port the code to my project, it works correctly however switching to LPM_OPERATION_MODE_WAIT (line 2) takes about 150mSec (way too long).  Narrowed it down to one line of assembly code within the Cpu_SetOperationMode function in cpu.c, that is the WFI (wait for interrupt instruction).  My project and the demo use the same BSP build and same hardware. Note: modes referred to here are MQX modes, i.e. using MQX_WAIT which means Kinetis_VLPR, not using MQX_SLEEP which corresponds to Kinetis_WAIT.

Below is an oversimplified view of code that runs at two speeds.

while (TRUE)


      // full speed code here

     _lpm_set_clock_configuration(BSP_CLOCK_CONFIGURATION_2MHZ);  // line 1, takes about 0.7msec

     _lpm_set_operation_mode2(LPM_OPERATION_MODE_WAIT);                 // line 2, takes 1.7msec in demo, 150mSec ported to my project    

     // low power slower code here

     _lpm_set_operation_mode(LPM_OPERATION_MODE_RUN));                   // line 3 , takes 0.7 msec

     _lpm_set_clock_configuration(BSP_CLOCK_CONFIGURATION_DEFAULT));  // line 4, takes 0.8msec



Why might the switch to LPM_OPERATION_MODE_WAIT take so long when it hits the WFI instruction, and what interrupt releases it in the demo code?


WFI is needed to invoke Kinetis_WAIT (=MQX_SLEEP) and Kinetis_STOP (=MQX_STOP) modes for the chip, but why does the MQX LPM call WFI when transitioning to Kinetis_VLPR (=MQX_WAIT)?  No need to hang around and wait for an interrupt.