AD9546SLASHPCBZ datasheet(161/205 Pages) AD

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Data Sheet

AD9546

Rev. 0 | Page 161 of 205

As such, the processor requires a minimum of N samples to

compute the desired average, after which the processor

continues to update the average one sample at a time. The skew

measurement processor provides the user with an indication

that it has captured the first block of N samples by setting Bit 7

of Register 0x3A33 (for an offset measurement) or

Bit 7 is Logic 0 for the first N − 1 measurements in a sequence

and Logic 1 for the Nth measurement. As such, when the user

reads the result of a skew offset or skew drift measurement, the

value is a fully averaged result if Bit 7 (associated with the

measurement type: offset or drift) is Logic 1. When Bit 7 is

Logic 0, offset or drift readings captured exhibit incomplete

averaging but are still useable. However, the variance of the

results is greater than for a fully averaged measurement.

Unlike the results from the UTSPs, the skew measurement

processor results are in absolute units of picoseconds. To access

the time skew measurement processor results, use the skew offset

and skew drift bit fields (Register 0x3A2C to Register 0x3A33

and Register 0x3A34 to Register 0x3A3B, respectively).

The result of a skew offset measurement appears in Bits[61:0]

(signed, twos complement) of Register 0x3A2C to

appears as an IRQ in Bit 4 of Register 0x300F. The decimal

value of Bits[61:0] constitute time in units of 2−16 ps (a range of

approximately ±35 sec). Skew offset measurements assume the

reference and target sources are of nearly the same frequency.

That is, skew is meaningless for two sources with differing

frequency, because the skew constantly drifts in such a case.

The skew measurement processor measures skew relative to the

assigned skew reference source (the processor computes a

coarse average of the reference period). The skew measurement

processor determines the polarity of the skew offset

measurement by considering a time window spanning ±½ unit

interval (UI), where 1 UI is the measured period of the

reference source. A negative value means time stamps from the

target source lead time stamps from the reference source by as

much as ½ UI, whereas a positive value means time stamps

from the target source lag time stamps from the reference

source by as much as ½ UI. Note that jitter can affect the

polarity of the skew measurement result.

For example, consider a 62-bit skew offset reading of 0x 3FFF

FFF8 A673 24B5 (hexadecimal), which is −31,567,174,475

decimal. To find the skew offset time, scale the decimal value by

2−16 ps, which results in −481,676.8566131591796875 ps. The

negative sign implies the target source leads the reference

source by approximately 482 ns.

The result of a skew drift measurement appears in Bits[61:0]

(signed, twos complement) of Register 0x3A34 to

2−16 picoseconds per unit interval (ps/UI). That is, the value is

relative to the measured period of the reference source. Skew

drift is a measure of the rate at which the skew offset varies

cycle by cycle.

For example, consider two signals with a nominal frequency of

100 Hz. A skew drift reading of 0x 3FFF FFF8 A673 24B5

(hexadecimal) is −31,567,174,475 decimal. To find the skew

drift, scale the decimal value by 2−16, which results in

−481,676.8566131591796875 ps/UI. Because the reference

period is 10 ms (1/100 Hz), the result indicates the target source

period is decreasing 482 ns every 10 ms, implying a frequency

offset of −48.2 ppm (−482 ns/10 ms = −48.2 × 10−6).

Two signals with the same frequency have no drift and must

yield a drift measurement of zero. However, a frequency

difference that is too great can exceed the limits of the skew

processor, which is 1/16th UI. Upon reaching this limit, the

skew measurement processor sets the status Bit 5 of Register

0x300C to Logic 1.

TAGGED SKEW MEASUREMENT TIME STAMPS

The skew measurement processor can operate on tagged time

stamps from the various time stamp sources shown in

Figure 109. When the selected skew reference and/or skew

measurement time stamp source is generating tagged time

stamps, the user can program the skew measurement processor

to only pay attention to the tagged time stamps, while ignoring

untagged time stamps. To enable tagged skew reference time

stamp processing, program Bit 5 of Register 0x2A15 to Logic 1.

To enable tagged skew target time stamp processing program

Bit 5 of Register 0x2A16 to Logic 1.

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