ADC and DAC xxx_timed() methods can't be used concurrently #1129
Description
DAC.write_timed(..., mode=pyb.DAC.CIRCULAR) and ADC.read_timed(...)
can't be used concurrently. The call to ADC.read_timed() stops the signal
generated by DAC.write_timed(). For details see http://forum.micropython.org/viewtopic.php?f=3&t=550&start=10#p3159