AD9546SLASHPCBZ datasheet(157/205 Pages) AD

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Data Sheet

AD9546

Rev. 0 | Page 157 of 205

USER TIME STAMP PROCESSOR (UTSP)

UTSP OVERVIEW

The user can view time stamps from the various time stamp

sources (see the Time Stamps section) via the UTSPs. The

AD9546 has two UTSPs with their respective output results

available via the register map, as shown in Figure 107. The

UTSPs convert time stamps originating from a given time

stamp source (see Table 92) to a time scale meaningful to the

user. The time scale conversion is necessary because the time

stamp sources use an arbitrary time scale internal to the device,

which is meaningless to the user.

The user selects the desired time scale source for a UTSP via

Bits[7:6] of Register 0x2A12 (for UTSP 0) and Register 0x2A13 (for

UTSP 1). The value of Bits[7:6] selects the desired time scale per

Table 93. Note that either one of the UTSPs can use either one of

the auxiliary NCO time scales.

Table 93. UTSP Time Scale Source

Bits[7:6] (Decimal)

Description

Auxiliary NCO 0 time scale (default)

Common time scale

Auxiliary NCO 1 time scale

Common time scale

To attach a particular time stamp source to a UTSP, use

Bits[4:0] (unsigned) of Register 0x2A12 (for UTSP 0) and

desired time stamp source per Table 94.

Table 94. UTSP Input Time Stamp Source

Bits[4:0] (Decimal)

Description

REFA TDC

REFAA TDC

REFB TDC

REFBB TDC

DPLL0 feedback TDC

DPLL1 feedback TDC

Auxiliary REF0 TDC (default for UTSP 0)

Auxiliary REF1 TDC (default for UTSP 1)

Auxiliary NCO 0 time scale

Auxiliary NCO 1 time scale

Auxiliary REF0 TDC

Auxiliary REF2 TDC

Auxiliary REF3 TDC

IUTS 0

IUTS 1

15 to 31

Not applicable

READING USER TIME STAMPS

The converted input time stamps are available at the output of

the UTSP via Bits[79:0] in Register 0x3A14 to Register 0x3A1D

(for UTSP 0) and Register 0x3A20 to Register 0x3A29 (for

UTSP 1). However, the user must assert an IO update operation

(see the IO Update section) prior to reading Bits[79:0]. The

UTSPs perform time stamp conversions as the time stamps

arrive from their prescribed time stamp sources. As such, the

UTSPs are event driven. Because of this event driven process,

the user cannot be certain that the data residing in Bits[79:0] is

indeed relevant. There are two methods for mitigating this

problem.

The first method is to read Bits[79:0] periodically. However, to

avoid missing time stamps, the user must read Bits[79:0] at a

rate greater than the expected rate of the time stamp source

assigned to the input of the UTSP. Because consecutive time

stamps arriving from a time source cannot have the same value

(by definition), it guarantees that a time stamp reading

different from the last constitutes a new time stamp.

The second method is to use the IRQ feature set of the AD9546

(see the Interrupt Request (IRQ) section). Whenever a UTSP

completes a time stamp conversion, the corresponding status Bit

2 (for UTSP 0) or Bit 3 (for UTSP 1) of Register 0x300F

(assuming the status bit is unmasked) is set. Upon the status bit

becoming Logic 1, assert an IO update operation, and read

Bits[79:0] (see Figure 107).

The event driven nature of the UTSP means that the user may

miss one or more user time stamps if multiple time stamps

occurred before the user reads Bits[79:0]. Bits[7:0] in

provide a way for the user to check for any missed input time

stamps. When Bits[7:0] indicate a value of zero, then the

current user time stamp is the latest one in a contiguous series.

When Bits[7:0] indicate a nonzero value, then the user missed

the indicated number of time stamps. Therefore, the current

time stamp constitutes the first one in a new series of time

stamps. Missed time stamps are lost and are not recoverable.

In operation, the user must assert an IO update operation,

which refreshes Bits[79:0] and Bits[7:0] with the most recent

values. After asserting an IO update operation, the

recommendation is to read Bits[79:0] immediately followed by

reading Bits[7:0]. Doing so enables immediate assessment of

the value of Bits[7:0] as an integral part of capturing the time

stamp value. Bits[7:0] clear automatically when read.

Because the user specifies the expected input period for a TDC

via the register map, the TDC knows approximately how much

time must elapse between time stamp events. If an event

happens far outside the expected event period, the TDC flags

the time stamp associated with that event as an error via Bit 0 of

In general, the user must discard a time stamp when Bit 0 is

Logic 1.

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