AD9545 datasheet(121/157 Pages) AD

121 / 157 page

Data Sheet

AD9545

Rev. A | Page 121 of 157

auxiliary NCO x delta rate limit bit field prior to invoking a

phase offset.

For example, assume the programmed frequency of the

auxiliary NCO is 60 Hz (therefore, 1 UI = 1/60 sec) and the

desired limit for the rate of change of phase is 1 µs per second

(10−6 sec/sec). Note that sec/sec and UI/UI are fundamentally

equivalent; therefore, 10−6 sec/sec is 10−6 UI/UI. Because the

auxiliary NCO x delta rate limit units are 2−36 UI/UI, the

required value for the 32-bit auxiliary NCO x delta rate limit bit

field is 10−6/2−36 = 68,719 (rounded to the nearest integer) or

0x00010C6F hexadecimal. With the auxiliary NCO x delta rate

limit bit field programmed per this example, when the user

invokes a phase offset specified by the auxiliary NCO x delta bit

field, the AD9545 automatically ensures that the rate of change

of phase does not exceed 1 µs per second, as specified.

When auxiliary NCO x delta rate limit ≠ 0 and while the

auxiliary NCO is in the process of limiting the phase slew rate,

the device sets the auxiliary NCO x delta slewing bit to Logic 1

(where x is 0 or 1) in Bit D6 of Register 0x3002 or Register 0x3002.

MANUAL CYCLE ADJUSTMENT

As shown in Figure 93, the user can read the elapsed time

generated by the auxiliary NCOs via the 80-bit auxiliary NCO x

time clock bit fields (where x is 0 or 1) in Register 0x3A00 to

bit fields is a 40-bit integer part (most significant bits) and a 40-

bit fractional part (least significant bits). The integer part tracks

rollovers of the associated auxiliary phase accumulator. The

fractional part effectively represents the phase of the

accumulator within the current cycle. Note the contents of the

auxiliary NCO x time clock bit fields indicate the elapsed time

up to the moment of the most recent IO UPDATE.

The user has the option of manually overwriting the 40-bit

integer portion of the auxiliary NCO x time clock bit field based

on the contents of the auxiliary NCO x cycle absolute bit field

and the auxiliary NCO x cycle type bit (where x is 0 or 1). The

auxiliary NCO x cycle type bit (Bit D1 of Register 0x280F and

cycle absolute bit field (Register 0x2819 to Register 0x281D and

overwrite functionality, for example, is to force one of the

auxiliary NCO x time clock bit fields to indicate time of day

(assuming an auxiliary NCO frequency of 1 Hz).

When auxiliary NCO x cycle type = 0, the auxiliary NCO x

cycle absolute bit field value constitutes an unsigned 40-bit

integer that directly replaces the corresponding 40-bit integer

portion of the auxiliary NCO x time clock bit field.

When auxiliary NCO x cycle type = 1, the auxiliary NCO x

cycle absolute bit field value constitutes a signed (twos

complement) 40-bit integer that increments or decrements the

corresponding 40-bit integer portion of the Auxiliary NCO x

time clock bit field relative to its current value.

AUXILIARY NCO TIME STAMPS

The auxiliary NCOs generate time stamps marking the rollover

of the NCOs phase accumulator. The user has access to these

time stamps via the 80-bit Auxiliary NCO x time clock bit fields

(where x is 0 or 1) in Register 0x3A00 to Register 0x3A13. To

convert the 80-bit number to auxiliary NCO time, use the same

format as shown in Figure 89 (see the User Access to Time

Stamps section). Because these time stamps are relative to the

rollover rate of the NCOs phase accumulator, they are not

useful for time stamping external events.

Auxiliary NCO time stamps are also accessible via the user time

stamp processor (see the User Access to Time Stamps section).

Because user time stamps are relative to the selected time base

for the user time stamp processor, auxiliary NCO time stamps

accessed via the user time stamp processor are relative to the

most recent rollover of the selected time base.

For example, the user can program auxiliary NCO 0 with a

particular frequency (1 kHz, for example) and auxiliary NCO 1

with a much lower frequency (1 Hz, for example). By

programming the user time stamp processor to use auxiliary

NCO 1 as its time base, accessing time stamps from auxiliary

NCO 0 via the user time stamp processor indicates time relative

to the most recent rollover of auxiliary NCO 1. That is, reading

two consecutive time stamps from the user time stamp processor

in this example yields the following: a first time stamp with an

arbitrary value somewhere between 0 sec and 1 sec (because of

the 1 Hz frequency programmed into auxiliary NCO 1) and the

second time stamp with a value 1 ms later than the first

(because of the 1 kHz frequency programmed into auxiliary

NCO 0).

AUXILIARY NCO PULSE OUTPUT

In addition to generating time stamps, the auxiliary NCOs also

produce a physical output signal (see Figure 93). The output

signal is effectively a single pulse that occurs close to the zero

crossing of the auxiliary phase accumulator. The device

quantizes the pulse timing such that the peak-to-peak pulse

jitter is approximately 1.4 ns.

The user programs the duration of the pulse via the auxiliary

NCO x pulse width exponent in Bits[D7:D4] of Register 0x281E

and Register 0x285E and the auxiliary NCO x pulse width signifi-

cand bits in Bits[D3:D0] of Register 0x281E and Register 0x285E

(where x is 0 or 1). The pulse width duration is as follows:

Pulse Width = (96/fS) × (1 + S × 2(E + 5))

where:

fS is the system clock frequency.

S is the decimal value of the auxiliary NCO x pulse width

significand bit field.

E is the decimal value of the auxiliary NCO x pulse width

exponent bit field.

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