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Data Sheet

AD9545

Rev. A | Page 115 of 157

TIMING SKEW MEASUREMENTS USING TWO TDCS

REFERENCE

TDCS

TIME STAMP SOURCES

PING-PONG

TDC

DIGITAL

CROSS

POINT MUX

SKEW

REFERENCE

SOURCE

TIME

STAMPS

TIME

STAMPS

TIME

STAMPS

TIME

STAMPS

FEEDBACK

TDCS

AUXILIARY

NCOS

TIME

STAMPS

AUXILIARY

TDCS

SKEW

MEASUREMENT

PROCESSOR

SKEW

MEASUREMENT

SOURCE

TEXT

= BIT(S) IN THE REGISTER MAP

SELECT SKEW

REFERFENCE

SELECT SKEW

MEASURE

CLOCK SOURCE

(DERIVED FROM SYSTEM CLOCK)

SKEW WINDOW SIZE

SKEW OFFSET

SKEW OFFSET FULL WINDOW

SKEW DRIFT

SKEW DRIFT FULL WINDOW

SKEW LIMIT EXCEEDED

Figure 90. Skew Measurement Block Diagram

The user time stamp processor (see the User Access to Time

Stamps section) allows the user to read time stamps originating

from various time stamp sources. The time stamps indicate

when the rising edge of a signal source occurs. Sometimes,

however, it is desirable to know the relative phase offset

between the rising edges of two different signal sources rather

than the corresponding event times of those edges. To this end,

the AD9545 provides the ability to measure and report the time

difference (timing skew) between two TDCs, which 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 90 shows a

block diagram of the skew measurement processor.

Time skew measurements involve a reference TDC and a

measurement TDC. The user assigns the reference TDC time

stamp source via the 5-bit select skew reference bit field in

Bits[D4:D0] of Register 0x2A15 and the measurement TDC

time stamp source via the 5-bit select skew measure bit field in

Bits[D4:D0] of Register 0x2A16.

Unlike the Event x time readings (see the User Access to Time

Stamps section), which are 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 the 4-bit skew window size bit field in

Bits[D3:D0] of Register 0x2A14 according to Table 52.

The selected skew window size bit field value sets the number of

averaging samples for both skew offset and skew drift

measurements. However, a particular selection can yield a

different number of averaging samples for offset measurements

vs. drift measurements, as shown in Table 52.

Table 52. Skew Measurement Window Settings

Skew Measurement

Window Value

Skew Offset of N

Skew Drift of N

128

256

512

1024

2048

4096

8192

16384

32768

65536

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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