LTC2000 Measuring LVDS Input Timing Skew

HI.

In datasheet on page 30 described "Measuring LVDS Input Timing Skew".

For example, to compare DB15P/N to DCKIP/N, first write 0x60 to register 0x18 to set LMX_EN = 1, LMX_ADR = 10000, and LMX_SEL = 0. The signal from DB15P/N will be driven onto TSTP/N. Write 0x61 to register 0x18 to set LMX_SEL = 1 and cause DCKIP/N to appear at TSTP/N with inverted polarity.

I don't understand mechanism of measuring it. When I switch to DCI how I will know the skew between signals because for once I can see only one signal.