FS6131

Table 9: Crystal Loop Tuning Bits

Name

Description

Crystal Loop Charge Pump current

Bit 53 = 0

Bit 52 = 0

Current = 1.5

μA

Bit 53 = 0

Bit 52 = 1

Current = 5

μA

Bit 53 = 1

Bit 52 = 0

Current = 8

μA

XLCP[1:0]

(Bits 53-52)

Bit 53 = 1

Bit 52 = 1

Current = 24

μA

XLROM[2:0]

(Bits 51-49)

Crystal Loop Divider ROM select and Crystal Oscillator Power-Down

(see Error! Reference source not found.)

Crystal Loop Voltage fine Tune ENable

Bit = 0

Disabled (fine tune is inactive)

XLVTEN

(Bit 61)

Bit = 1

Enabled (fine tune is active)

Crystal Loop SWAP polarity

Bit = 0

Use with an external VCXO that increases in

frequency in response to an increasing voltage at

the XTUNE pin.

XLSWAP

(Bit 54)

Bit = 1

Use with a VCXO that increases in frequency in

response to a decreasing voltage at the XTUNE

pin.

Use this setting for Internal VCXO

Crystal Loop Power Down Enable

Bit = 0

Disabled (crystal loop operates)

XLPDEN

(Bit 55)

Bit = 1

Enabled

(crystal loop is powered down)

XCT[3:0]

(Bits 59-56)

Crystal Coarse Tune (see Table 11)

Table 10: Crystal Loop Control ROM

XLROM

[2]

XLROM

[1]

XLROM

[0]

VCXO Divider

Crystal Frequency (MHz)

0

0

0

1

-

0

0

1

3072

24.576

0

1

0

3156

25.248

0

1

1

2430

19.44

1

0

0

2500

20.00

1

0

1

4000

32.00

1

1

0

3375

27.00

1

1

1

Crystal oscillator power-down

6.1 VCXO Coarse Tune

The VCXO may be coarse tuned by a programmable adjustment of the crystal load capacitance via XCT[3:0]. The actual amount of

frequency warping caused by the tuning capacitance will depend on the crystal used. The VCXO tuning capacitance includes an

external 6pF load capacitance (12pF from the XIN pin to ground and 12pF from the XOUT pin to ground). The fine tuning capability of

the VCXO can be enabled by setting the XLVTEN bit to a logic-one, or disabled by setting the bit to a logic-zero.

Rev. 4 | Page 20 of 44 | www.onsemi.com