AD9551SLASHPCBZ datasheet(36/40 Pages) AD

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AD9551

Rev. B | Page 36 of 40

REFA Frequency Control (Register 0x1E to Register 0x25)

Table 29.

Address

Bit

Bit Name

Description

0x1E

Enable SPI control of

REFA SDM

Controls REFA frequency division functionality.

0 = REFA frequency division, as defined by the A[3:0] pins (default).

1 = contents of Register 0x1F to Register 0x25 define REFA frequency division via NA,

MODA, and FRACA.

Bypass REFA SDM

Controls bypassing of the REFA SDM.

0 = allow integer-plus-fractional division (default).

1 = allow only integer division.

Enable REFA SDM

Controls REFA SDM enable and hold functionality.

0 = reset REFA SDM and stop its clocks.

1 = REFA SDM enabled (default).

Enable REFB

Controls REFB enable and power-down functionality.

0 = power down REFB input receiver (ineffective unless Register 0x1A[1] = 1).

1 = normal operation (default).

Unused

Unused.

Disable REF SDM PRBS

Controls the PRBS generator for both the REFA and REFB SDMs.

0 = PRBS generator enabled (default).

1 = PRBS generator disabled.

[1:0]

Select 19.44 MHz input

mode divider

Selects the divider value when the 19.44 MHz input mode is in effect.

00 = 1 (default).

01 = 1.

10 = 2.

11 = 4.

These bits are ineffective unless the A[3:0] pins = 1111 or the B[3:0] pins = 1111.

0x1F

[7:0]

FRACA

Bits[19:12] of the 20-bit fractional part of the REFA SDM.

0x20

[7:0]

FRACA

Bits[11:4] of the 20-bit fractional part of the REFA SDM.

0x21

[7:4]

FRACA

Bits[3:0] of the 20-bit fractional part of the REFA SDM.

Default is FRACA = 0100 0000 0000 0000 0000 (262,144).

Note that FRACA assumes twos complement format.

[3:0]

Unused

Unused.

0x22

[7:2]

6-bit integer divide value for the REFA SDM. Default divide value is 16.

[1:0]

Unused

Unused.

0x23

Unused

This bit must be programmed to 0, even though the default value is 1.

[6:0]

MODA

Bits[18:12] of the 19-bit modulus of the REFA SDM.

0x24

[7:0]

MODA

Bits[11:4] of the 19-bit modulus of the REFA SDM.

0x25

[7:4]

MODA

Bits[3:0] of the 19-bit modulus of the REFA SDM.

Default is MODA = 000 0000 0000 0000 0000.

[3:0]

Unused

Unused.

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