Data Sheet

AD9912

I/O REGISTER DESCRIPTIONS

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Rev. G | 33 of 38

Table 16. (Continued)

Bits

Bit Name

Description

0

Digital PD

Removes clock from most of digital section; leave serial port usable. In contrast to full PD, setting this bit does not debias

inputs, allowing for quick wake-up.

Register 0x0011—Reserved

Register 0x0012—Reset (Autoclearing)

To reset the entire chip, the user can use the (non-autoclearing) soft reset bit in Register 0x0000.

Table 17.

Bits

Bit Name

Description

0

DDS reset

Reset of the direct digital synthesis block. Reset of this block is very seldom needed.

Register 0x0013—Reset (Continued) (Not Autoclearing)

Table 18.

Bits

Bit Name

Description

7

PD fund DDS

Setting this bit powers down the DDS fundamental output but not the spurs. It is used during tuning of the SpurKiller circuit.

3

S-div/2 reset

Asynchronous reset for S prescaler.

1

S-divider reset

Synchronous (to S-divider prescaler output) reset for integer divider.

SYSTEM CLOCK (REGISTER 0X0020 TO REGISTER 0X0022)

Register 0x0020—N-Divider

Table 19.

Bits

Bit Name

Description

[4:0]

N-divider

These bits set the feedback divider for system clock PLL. There is a fixed divide-by-2 preceding this block, as well as an offset

of 2 added to this value. Therefore, setting this register to 00000 translates to an overall feedback divider ratio of 4. See Figure

45.

Register 0x0021—Reserved

Register 0x0022—PLL Parameters

Table 20.

Bits

Bit Name

Description

7

VCO auto range

Automatic VCO range selection. Enabling this bit allows Bit 2 of this register to be set automatically.

[6:4]

Reserved

Reserved.

3

2× reference

Enables a frequency doubler prior to the SYSCLK PLL and can be useful in reducing jitter induced by the SYSCLK PLL. See

Figure 44.

2

VCO range

Selects low range or high range VCO.

0 = low range (700 MHz to 810 MHz).

1 = high range (900 MHz to 1000 MHz). For system clock settings between 810 MHz and 900 MHz, use the VCO auto range

(Bit 7) to set the correct VCO range automatically.

[1:0]

Charge pump current

Charge pump current.

00 = 250 μA.

01 = 375 μA.

10 = off.

11= 125 μA.