asterisk users - Mar 2016 - Selecting timing source for Asterisk

I have an Asterisk 13 installation with an E1 card and I thought 
that DAHDI would be the default timing source for the system:

pbxcore*CLI> module show like timing
Module Description                              Use Count  Status 
Support Level
res_timing_dahdi.so            DAHDI Timing Interface                   
0          Running              core
res_timing_pthread.so          pthread Timing Interface                 
0          Running          extended
res_timing_timerfd.so          Timerfd Timing Interface                 
13         Running              core
3 modules loaded

     How is the timing source selected?  I thought that if a DAHDI card 
was present it was automatically selected as the timing source for the 
system.

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Carlos Ch?vez
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