ICS9214 datasheet(11/16 Pages) ICST

11 / 16 page

Integrated

Circuit

Systems, Inc.

ICS9214

0809D—04/07/06

Figure 1. Differential and single-ended reference clock inputs

Clock Output Drivers

Figure 2 shows the clock driver equivalent circuit. The differential driver produces a specified voltage swing on the channel

by switching the currents going into ODCLK_T and ODCLK_C. The external resistor RRC at the IREFY pin sets the maximum

current. The minimum current is zero.

The voltage at the IREFY pin, VIREFY, is by design equal to 1 V nominally, and the driver current is seven times the current

flowing through RRC. So, the output low current can be estimated as IOL = 7/ RRC.

The driver output characteristics are defined together with the external resistors, R1, R2, and R3. The output clock signals are

specified at the measurement points indicated in Figure 2. Table 5 shows example values for the resistors.

R1, R2, and R3 and the clock driver output impedance, ZOUT, must match the impedance of the channel, ZCH , to minimize

secondary reflections. ZOUT is specified as 1000 Ohms, minimum to accomplish this. The effective impedance can be

estimated by:

(1000R1/(1000+R1)+R2) R3/(1000R1/(1000+R1)+R2+R3)

Pull-up resistor RT terminates the transmission line at the load to minimize clock signal reflection signal reflections. Table 5

shows the resistor values for establishing and effective source termination impedance of 49.2 Ohms to match a 50 Ohm

channel. The termination voltages are 2.5 V for VTS and 1.2 V for VT. The resistor values R1 = 38.3 Ohms, R2 = 19.1 Ohms, R3

= 54.9 Ohms and RRC = 200 Ohms can be used to match a 28 Ohm channel.

Table 5. Example Resistor Values and Termination Voltages for a 50 Ohm Channel

Symbol

Parameter

Value

Tolerance

Unit

Termination resistor

39.2

+/- 1%

Ω

Termination resistor

66.5

+/- 1%

Ω

Termination resistor

93.1

+/- 1%

Ω

Termination resistor

49.9

+/- 1%

Ω

RRC

Swing control resistor

200

+/- 1%

Ω

VTS

Source termination voltage

2.5

+/-5%

Termination voltage

1.2

+/-5%

Notes:

1 A different set of resistors is used in Figure 2 when testing

for maximum output current of the clock driver (IOLABS).

These resistors are: R1 = 34Ω, R2 = 31.8Ω, R3 = 48.7Ω,

RT=28Ω, RRC = 147Ω

VTH

Inp ut

XDR

CLK_INC

CLK_INT

a. Differential input

b. Single-ended input

Clock Generator

Supply Voltage

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