AD9545 datasheet(43/157 Pages) AD

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Data Sheet

AD9545

Rev. A | Page 43 of 157

REFERENCE MONITOR STATE MACHINE

The REFA monitor, shown in Figure 43, is essentially a state

machine. Therefore, each REFx input (where x is A, AA, B, or

BB) has a dedicated state machine. The state machine uses

certain input parameters (see the Reference Monitor Controls

section) for performing calculations and comparisons.

The state machine operates directly on TDC time stamps,

which occur with each rising edge from the output of the R-

divider. As such, the state machine effectively operates on the

input reference period scaled up by the division factor

associated with the R-divider.

Because the ΔTOL input parameter (see Figure 43) is a

proportional value rather than an absolute offset, the state

machine can determine in or out of tolerance without direct

knowledge of the value of the R-divider. Instead, the state

machine continually observes the difference between successive

time stamps and compares the statistics of those observations

against ΔTOL to determine if the reference is fast, slow, absent,

or exhibits excessive jitter.

REFERENCE MONITOR CONTROLS

The reference monitor requires certain advance information to

function properly. The user provides this information by

programming the appropriate parameters as follows.

REFx Nominal Period (tREF)

Each reference input has a dedicated bit field for specifying

tREF: the unsigned 60-bit REFx nominal period bit field (where

x is A, AA, B, or BB) in Register 0x0404 to Register 0x040B,

tREF constitutes the nominal period of the associated input

reference and represents time in units of atto-seconds (10−18 sec)

with an upper limit of about 1 sec.

For example, the REFAA input has a nominal 2.048 MHz input

signal. The corresponding period is 1/(2.048 × 106) sec, which,

when multiplied by 1018, yields 488,281,250,000 attoseconds.

Program Register 0x0424 to Register 0x042B with the 60-bit

value corresponding to 488,281,250,000 attoseconds (0x 71).

REFx Offset Limit (∆TOL)

Each reference input has a dedicated unsigned 24-bit REFx

offset limit bit field (where x is A, AA, B, or BB) in Register 0x040C

to Register 0x040E, Register 0x042C to Register 0x042E,

portion of the corresponding tREF value and is the maximum

relative deviation the reference monitor uses for declaring a

nonfaulted reference as out of tolerance. The 24-bit number

represents fractional units of parts per billion (ppb) up to a

maximum of approximately 17 million ppb (1.7%).

Because ΔTOL is a fractional period (δp), it relates to fractional

frequency (δf) as follows:

δp = −δf/(1 + δf)

(1)

For example, consider the case where the expected maximum

relative deviation of the input reference frequency for REFAA is

2 × 10−6 (2 ppm). That is, δf = 2 × 10−6.

From Equation 1, δt = −1.999996 × 10−6, or −1.999996 ppm, or

−1999.996 ppb. Because ΔTOL is an unsigned number with

integer units of ppb, ΔTOL = 2000. Therefore, program Register

0x042C to Register 0x042E with the 24-bit value corresponding

to 2000 (0x0007D0 hexadecimal).

REFx Monitor Hysteresis (∆HYS)

Each reference input has a dedicated 3-bit REFx monitor

hysteresis bit field (where x is A, AA, B, or BB) in Bits[D2:D0]

of Register 0x040F, Register 0x042F, Register 0x044F, and

Table 29. Hysteresis Selection Table

REFx Monitor Hysteresis Bit Field Value

Scale Factor (SF)

0.03125

0.0625

0.125

0.25

0.5

0.75

0.875

The selected SF defines ΔHYS as a fractional portion of the

value of ΔTOL. That is, ΔHYS is always less than or equal to

ΔTOL.

ΔHYS = ΔTOL × (1 − SF)

Given ΔTOL = 2,000 ppb and the selected SF value is 0.0625,

determine the value of ΔHYS.

ΔHYS = 2,000 × (1 − 0.0625) ppb = 1875 ppb

ΔHYS applies to a faulted reference, whereas ΔTOL applies to a

nonfaulted reference. The reference monitor uses tREF, ΔTOL,

and ΔHYS to make fault and unfault decisions with regard to a

reference input, as shown in Figure 44.

tREF

TIME

MINIMUM DEVIATION FROM

tREF FOR AN

UNFAULTED REFERENCE TO BECOME FAULTED

MAXIMUM DEVIATION FROM

tREF FOR A

FAULTED REFERENCE TO BECOME UNFAULTED

Figure 44. Reference Monitor Fault and Unfault Parameters

REFx Jitter Tolerance (JTOL)

Each reference input has a dedicated bit field to specify JTOL: the

unsigned 16-bit REFx jitter tolerance bit field (where x is A, AA,

B, or BB) in Register 0x0413 to Register 0x0414, Register 0x0433 to

of nanoseconds (10−9 sec) up to a maximum of approximately

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