6

When the outputs are configured for low drive operation, they

will provide a minimum 12mA of drive current regardless of

the selected output power supply. If the outputs are configured

for high drive operation, they will provide a minimum 24mA of

drive current under a 3.3V power supply and 20mA when pow-

ered from a 2.5V supply.

The UT7R995/C features split power supply buses for Banks 1

and 2, Bank 3, and Bank 4. These independent power supplies

enable the user to obtain both 3.3V and 2.5V output signals

must run from a 3.3V power supply. Table 12 summarizes the

various power supply options available with the UT7R995/C.

Notes:

1.4 Reference Clock Interfaces

When an external, LVCMOS/LVTTL, digital clock is used to

drive the UT7R995 and UT7R995C, the reference clock signal

should drive the XTAL1 input of the RadClock, while the

XTAL2 output should be left unconnected (see Figure 4). Note,

for the UT7R995 only, the XTAL2 pin is defined as a no-

connect.

In addition to a digital clock reference, the UT7R995C can in-

terface to a quartz crystal. When interfacing to a quartz crystal,

XTAL1 and XTAL2 are the input and output, respectively, of

an inverting amplifier within the RadClock. This inverting am-

by the quartz crystal and its surrounding RLC network. Figure

5 shows a typical pierce-oscillator with tank-circuit that will

support reliable startup of fundamental and odd-harmonic, AT-

cut, quartz crystals.

3.3V

3.3V or 2.5V

3.3V or 2.5V

3.3V or 2.5V

N/C

External

Digital

Oscillator

NC/XTAL2

XTAL1

Figure 4. External Digital Clock Oscillator Interface

Figure 5. Pierce Crystal Oscillator with Tank Circuit

Y1

Rdc

R1

C1

C2

L1

Cdc

XTAL1

XTAL2

UT7R995C

Fundamental Frequency Pierce Crystal Oscillator

Rdc = ~10M

Ω;

L1 = Not Used;

Cdc = Not Used

C2 is used to tune the circuit for stable oscillation.

Typical values for C2 range from 30pF to 50pF.

R1 and C1 are selected to create a time constant that facilitates the funda-

As an example, selecting a value of 100

Ω for R1 and 80pF for C1 would fa-

cilitate the reliable operation of a 20MHz, AT-cut, quartz crystal.

Higher Frequency Pierce Crystal Oscillator

Rdc = ~10M

Ω;

Cdc = ~1.5nF;

C2 = Tuning capacitor

similar to prior example

R1 and C1 are selected to create a time constant that facilitates the overtone

Additionally, L1 is selected such that its relationship with C1 facilitates a

As an example, selecting the following component values will result in a

50MHz Pierce Crystal Oscillator based upon an 3rd overtone, AT-cut,

quartz crystal having a fundamental frequency of 16.6666MHz.

Rdc = 10M

Ω;

Cdc = 1.5nF;

C2 = 30pF;

R1 = 50

Ω;

C1 = 55pF;

L1 = 300nH

*

1

*

2

1

C

R

f

F

π

=

Equation 2.

Equation 3.

*

1

*

2

1

C

R

f

OT

π

=

()

1

*

1

*

2

1

C

L

f

M

π

=

Equation 4.