ICS9214 Datasheet, PDF (11/16 Pages) Integrated Circuit Systems – Rambus XDR Clock Generator

Integrated
Circuit
Systems, Inc.
ICS9214
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 R_RC at the IREFY pin sets the maximum
current. The minimum current is zero.
The voltage at the IREFY pin, V_IREFY, is by design equal to 1 V nominally, and the driver current is seven times the current
flowing through R_RC. So, the output low current can be estimated as I_OL = 7/ R_RC.
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, Z_OUT, must match the impedance of the channel, Z_CH , to minimize
secondary reflections. Z_OUT 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 R_T 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 V_TS and 1.2 V for V_T. The resistor values R1 = 38.3 Ohms, R₂ = 19.1 Ohms, R₃
= 54.9 Ohms and R_RC = 200 Ohms can be used to match a 28 Ohm channel.
Table 5. Example Resistor Values and Termination Voltages for a 50 Ohm Channel¹
Symbol
Parameter
Value Tolerance
Unit
R₁
Termination resistor
39.2
+/- 1%
Ω
R₂
Termination resistor
66.5
+/- 1%
Ω
R₃
Termination resistor
93.1
+/- 1%
Ω
R_T
Termination resistor
49.9
+/- 1%
Ω
R_RC
Swing control resistor
200
+/- 1%
Ω
V_TS
Source termination voltage
2.5
+/-5%
V
V_T
Termination voltage
1.2
+/-5%
V
Notes:
1 A different set of resistors is used in Figure 2 when testing
for maximum output current of the clock driver (I_OLABS).
These resistors are: R₁ = 34Ω, R₂ = 31.8Ω, R₃ = 48.7Ω,
R_T=28Ω, R_RC = 147Ω
Supply Voltage
CLK_INC
CLK_INT
Inp ut
CLK_INT
VTH
Inp ut
XDR
Clock Generator
a. Differential input
XDR
Clock Generator
b. Single-ended input
Figure 1. Differential and single-ended reference clock inputs
0809D—04/07/06
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