AD9546SLASHPCBZ datasheet(160/205 Pages) AD

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AD9546

Data Sheet

Rev. 0 | Page 160 of 205

TIMING SKEW MEASUREMENTS USING TWO TDCs

TIME STAMP SOURCES

SKEW

REFERENCE

SOURCE

SKEW

MEASUREMENT

PROCESSOR

SKEW

TARGET

SOURCE

CLOCK SOURCE

(DERIVED FROM SYSTEM CLOCK)

OFFSET

DRIFT

WINDOW

TAG

REFERENCE

TDCS

AUXILIARY NCOS

AUXILIARY

REFERENCE TDCS

DPLL FEEDBACK

TDCS

INVERSE

USER TIME

STAMPERS (IUTS)

DIGITAL CROSS

POINT MUX

TIME

STAMPS

NOTE

1. A RANGE OF BITS USES A COLON SEPARATOR

TIME

STAMPS

TIME

STAMPS

TIME

STAMPS

TIME

STAMPS

REG 0x3A3B TO 0x3A34, BITS[61:0]

REG 0x300C, BIT 5

REG 0x3A3B, BIT 7

REG 0x300F, BIT 4

REG 0x3A33, BIT 7

REG 0x3A33 TO 0x3A2C, BITS[61:0]

REG 0x2A14, BITS[3:0]

REG 0x2A15, BIT 5

REG 0x2A16, BIT 5

REG 0x2A15, BITS[4:0]

REG 0x2A16, BITS[4:0]

Figure 109. Skew Measurement Block Diagram

The UTSP (see the User Time Stamp Processor (UTSP)

section) allows the user to read time stamps originating from

various time stamp sources—the time stamps indicating the

occurrence of a rising edge at the signal source. Sometimes,

however, it is desirable to know the relative phase offset

between the rising edges of two different signal sources rather

than the corresponding time associated with each edge. To this

end, the AD9546 provides the ability to measure and report the

time difference (timing skew) between two time stamps

(usually, but not exclusively, from different sources). The time

difference between time stamps relates directly to phase offset.

Time skew (phase offset) only has meaning when two clock

signals operate at the same frequency. Otherwise, the phase

offset is not constant but drifts over time. Figure 109 shows a

block diagram of the skew measurement processor.

Time skew measurements involve a reference time stamp

source and a target time stamp source. The user assigns the

reference time stamp source via Bits[4:0] of Register 0x2A15 and

the target time stamp source via Bits[4:0] of Register 0x2A16.

The decimal value of Bits[4:0] selects the time stamp source per

Table 94, except that the default selection is REFA for both the

reference time stamp source and target time stamp source.

Unlike the results from the UTSP, which constitute individual

measurements, the offset and skew measurements from the

skew measurement processor constitute an averaged group of

sequential measurements. The user selects the number, N, of

samples averaged per measurement via Bits[3:0] of

Table 95. Skew Measurement Window Settings

Bits[3:0]

N for Skew Offset

N for Skew Drift

128

256

512

1024

2048

4096

8192

16384

32768

The selected skew window size bit field value sets the number

of averaging samples for both skew offset and skew drift

measurements. However, a given selection may yield a different

number of averaging samples for offset measurements vs. drift

measurements, as shown in Table 95.

The skew offset and drift measurements constitute a low-pass

filtered sequence of N samples. That is, the skew measurement

processor smooths out (that is, averages) the measurement

variation over N samples. After an N sample average is

established, subsequent measurements include the next sample

and the preceding N − 1 samples.

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