1/27/15
SigC667x / SigC641x User Guide, Rev B7
Copyright
Signalogic 2012-2014
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6.3 GPIO Test Mode
In GPIO only mode, the McBSPs are initialized to transmit and receive at a frequency of 1 MHz.
In all GPIO modes, GPIO sampling is done via a timer interrupt which occurs at a frequency of
900 KHz. This rate can be increased depending on the test mode used but ultimately has a limit
based on peripheral register access speeds at which errors start to occur due to code delays
caused by CPU stalls. 900 KHz is the highest rate at which all test modes reliably and
consistently operate. An issue where BIOS crashes arises when the BIOS enables multiple
interrupts concurrently causing multiple ISRs to be called at around the same time. As a
workaround to this issue, initially the timer operates at a frequency of 16 Hz then reinitializes to
the 900 KHz sampling rate after 1 sec of operation.
Due to the relatively low sampling rates of the strobe signals compared to their duration,
verification of the strobe signal timing was measured with an oscilloscope and is shown in
Figure 6-3 below.
Figure 6-3, Oscilloscope capture of strobe signal