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STM32F4 project failed to compile with binary libPDMFilter_Keil.lib

Question asked by ER on Nov 16, 2015
Latest reply on Nov 16, 2015 by Clive One
I am trying to follow Application Note 3998 "PDM audio software decoding on STM32 microcontrollers" to convert the recorded PDM signal to PCM data. I copy the "pdm_filter.h" and "libPDMFilter_Keil.lib" files provided in the firmware package to the directory of source code. The code fails to compile and generate error like 

".\_build\STM32F401C-DISCO.axf: Error: L6218E: Undefined symbol PDM_Filter_Init (referred from selftest.o)."

The "PDM_Filter_Init" is a function declared in "pdm_filter.h". So I am guessing somehow the other file "libPDMFilter_Keil.lib" fails to be included in the compiling, or for some other reasons I don't understand. Could anyone point out what I am missing?