Datasheet

AD9858
Rev. C | Page 23 of 32
Register Address (MSB) (LSB) Default
Name Ser Par Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Value Profile
Phase
Offset
Word 2
(POW2)
0x08 0x1A Phase Offset Word 2[7:0] 2
0x1B Not used Phase Offset Word 2[13:8] 2
Frequency
Tuning
Word 3
(FTW3)
0x09 0x1C Frequency Tuning Word 3[7:0] 3
0x1D Frequency Tuning Word 3[15:8] 3
0x1E Frequency Tuning Word 3[23:16] 3
0x1F Frequency Tuning Word 3[31:24] 3
Phase
Offset
Word 3
(POW3)
0x0A 0x20 Phase Offset Word 3[7:0] 3
0x21 Not used Phase Offset Word 3[13:8] 3
Reserved 0x0B 0x22 Reserved, do not write, leave at 0xFF 0xFE N/A
0x23 Reserved, do not write, leave at 0xFF 0xFF N/A
REGISTER BIT DESCRIPTIONS
Control Function Register (CFR)
The CFR comprises four bytes. CFR is used to control the
various functions, features, and modes of the AD9858. The
functionality of each bit follows. Note that the register bits are
identified according to their serial register bit locations beginning
with the most significant bit.
CFR[31:30]: Frequency Detect Mode Charge Pump Current
These bits are used to set the scale factor for the frequency
detect mode charge pump output current (see Table 7). The
charge pump delivers the scaled output current when the
control logic forces the charge pump into its frequency detect
operating mode. The charge pump’s baseline output current
(I
CP0
) is determined by the external CPISET resistor and is
given by
I
CP0
= 1.24/CPISET
The recommended nominal value of the CPISET resistor is
2.4 kΩ, which yields a baseline current of 500 μA.
Table 7.
CFR[31:30]
Frequency Detect Mode
Charge Pump Scale Value Notes
00 0 I
OUT
= 0 (default)
01 2 I
OUT
= 20 × I
CP0
10 3 I
OUT
= 40 × I
CP0
11 4 I
OUT
= 60 × I
CP0
CFR[29:27]: Final Closed-Loop Mode Charge Pump
Current
These bits are used to set the scale factor for the final closed-
loop mode charge pump output current (see Table 8). The
charge pump delivers the scaled output current when the control
logic forces the charge pump into its final closed-loop mode.
Table 8.
CFR[29:27]
Final Closed-Loop Mode
Charge Pump Scale Value Notes
0xx 0 I
OUT
= 0 (default)
100 1 I
OUT
= I
CP0
101 2 I
OUT
= 2 × I
CP0
110 3 I
OUT
= 3 × I
CP0
111 4 I
OUT
= 4 × I
CP0
CFR[26:24]: Wide Closed-Loop Mode Charge Pump
Current
These bits are used to set the scale factor for the wide closed-
loop charge pump output current (see Table 9). The charge
pump delivers the scaled output current when the control logic
forces the charge pump into its wide closed-loop operating mode.
Table 9.
CFR[26:24]
Wide Closed-Loop Mode
Charge Pump Scale Value
Notes
000 0 I
OUT
= 0 (default)
001 2 I
OUT
= 2 × I
CP0
010 4 I
OUT
= 4 × I
CP0
011 6 I
OUT
= 6 × I
CP0
100 8 I
OUT
= 8 × I
CP0
101 10 I
OUT
= 10 × I
CP0
110 12 I
OUT
= 12 × I
CP0
111 14 I
OUT
= 14 × I
CP0
CFR[23]: Auto Clear Frequency Accumulator Bit
When CFR[23] = 0 (default), a new delta frequency word is
applied to the input of the accumulator and added to the
currently stored value.
When CFR[23] = 1, this bit automatically synchronously clears
(loads zeros into) the frequency accumulator for one cycle upon
reception of the FUD sequence indicator.