AD9545 datasheet(112/157 Pages) AD

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AD9545

Data Sheet

Rev. A | Page 112 of 157

USER ACCESS TO TIME STAMPS

USER ACCESS TO TIME STAMPS OVERVIEW

The user can observe time stamps from the various timestamp

sources (see the Timestamps section) via the user timestamp

processor. The AD9545 has two user timestamp processors with

their output results available via the register map as shown in

Figure 88.

The user selects the desired timestamp source via the 5-bit

Timestamp Source x (where x is 0 or 1) bit fields in Bits[D4:D0]

of Register 0x2A12 and Register 0x2A13. Each Timestamp

Source x bit field gives the user access to the 11 timestamp

sources, but with the interconnection constraints per Table 51.

The user timestamp processors work in conjunction with the

auxiliary NCOs in that the user assigns an auxiliary NCO to a

particular user timestamp processor by means of the Timebase

Source x bit (where x is 0 or 1) in Bit D7 of Register 0x2A12 and

processors can be associated with either one of the auxiliary

NCOs. The user timestamp processor uses elapsed time

information from its assigned auxiliary NCO as the reference

time for reporting user timestamps.

READING USER TIME STAMPS

The user time stamp processors make their time stamp results

available via the Event x time bit fields (where x is 0 or 1) in

operation prior to reading an Event x time bit field. The

processors perform time stamp conversions as the time stamps

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

user time stamp processors are event driven. Because of this

event driven process, the user cannot be inherently certain that

a time stamp read from one of the Event x time bit fields is

relevant. There are two methods for mitigating this problem.

The first method is to read the Event x time bit fields

periodically. It is best to read the bit fields at a rate greater than

the expected rate of the time stamp source assigned to the input

of the user time stamp processor. 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 AD9545

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

time stamp processor completes a time stamp conversion, it sets

the corresponding Timestamp x event status bit (where x is 0 or

1) in Bits[D3:D2] of Register 0x300F (assuming the status bit is

unmasked). When the appropriate event occurs, issue an

IO_UPDATE operation and read the appropriate Event x time

bit field (see Figure 88).

The event driven nature of the user time stamp processor means

that the user may miss one or more user time stamps if multiple

time stamps occurred before the user reads one of the Event x

time bit fields. The User Time Stamp x missed count bit fields

(where x is 0 or 1) in Register 0x3A1E and Register 0x3A2A

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

stamps. User Time Stamp x missed count = 0 means the current

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

nonzero value in the User Time Stamp x missed count register

means the user missed the indicated number of time stamps

(missed time stamps are lost); therefore, the current time stamp

constitutes the first one in a new series of time stamps.

In operation, the user must issue an IO_UPDATE operation,

which refreshes both the Event x time and User Time Stamp x

missed count bit fields with the most recent time stamp. After

assertion of the IO_UPDATE bit, the user must read both bit

fields simultaneously, because they occupy adjacent address

locations in the register map. Doing so provides for assessing

the status of the User Time Stamp x missed count value, which

is an integral part of capturing the time stamp value. The User

Time Stamp x missed count bit field clears 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 the

User Time Stamp x low resolution bit (where x is 0 or 1) in

Bit D0 of Register 0x3A1F and Register 0x3A2B. In general, the

user must discard a time stamp when its associated User Time

Stamp x low resolution bit is Logic 1.

There are two categories of user time stamps. The first category

is generic time stamps, which the user reads via the Event x

time bit fields. The second category is time stamps strictly

associated with the internal time scales of the auxiliary NCOs

(see the Auxiliary NCOs section).

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