228 Chapter 7
Digital Signal Interface Module
Clock Timing
Clock Timing for Serial Data
Figure 7-6 shows the clock timing for a serial port configuration. Notice that the serial transmission includes
frame pulses that mark the beginning of each sample where the clock delineates the beginning of each bit.
For serial transmission, the clock and the bit rates are the same, but the sample rate varies depending on the
number of bits per word that are entered using the
Word Size softkey. The number of bits per word is the same
as the number of bits per sample.
Figure 7-6 Clock Timing for a Serial Port Configuration
Clock Timing for Phase and Skew Adjustments
The N5102A module provides phase and skew adjustments for the clock relative to the data and can be used
to align the clock with the valid portion of the data. The phase has a 90 degree resolution (0, 90, 180, and
270 degree selections) for clock rates from 10 to 200 MHz and a 180 degree resolution (0 and 180 degree
selections) for clock rates below 10 MHz and greater than 200 MHz.
The skew is displayed in nanoseconds with a maximum range of ±5 ns using a maximum of ±127 discrete
steps. Both the skew range and the number of discrete steps are variable with a dependency on the clock rate.
The skew range decreases as the clock rate is increased and increases as the clock rate is decreased. The
maximum skew range is reached at a clock rate of approximately 99 MHz and is maintained down to a clock
rate of 25 MHz. For clock rates below 25 MHz, the skew adjustment is unavailable.
A discrete step is calculated using the following formula:
The number of discrete steps required to reach the maximum skew range decreases at lower frequencies. For
example, at a clock rate of 50 MHz, 127 steps would exceed the maximum skew range of ±5 ns, so the actual
number of discrete steps would be less than 127.
Figure 7-7 is an example of a phase and skew adjustment and shows the original clock and its phase position
relative to the data after each adjustment. Notice that the skew adjustment adds to the phase setting.
Clock
4 bits per word
1 Sample
Frame Marker
Data Bits
1
256 Clock Rate×
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