Question:

Does Low-Power HCSL (LP-HCSL) require a DC coupled termination?

Answer:

No. One advantage of LP-HCSL is easy AC-coupling. LP-HCSL does not require a DC coupled termination like traditional HCSL does. You can add capacitors in series with LP-HCSL signals without affecting the signal swing or termination properties. With traditional HCSL, care has to be taken that a DC path to ground remains when adding AC coupling and this may require extra components.

////////////////////////////

Question:

What are the termination differences between Traditional HCSL and Low-Power HCSL?

Answer:

Traditional HCSL termination uses a 50Ω resistor to ground at the end of the PCB trace. Later, another method was introduced, placing the 50Ω to ground near the driver. This is called Source Termination and allows for the clock to pass through connectors that can be unplugged while the circuit is active (hot swapping). LP-HCSL has its termination at the driver side by definition, and it can work with end terminations. This setup is referred to as double termination (more on this in a moment). Source termination versus end termination makes no difference to the receiver.

The termination resistors (RS) are now in series with the clock line, near the driver. The driver itself is designed to have 17Ω

output impedance so it requires another 33Ω to match 50Ω PCB traces. The schematic on the right has the termination resistors

integrated so no external components are needed to drive 50Ω PCB traces.

Traditional HCSL doesn't use the driver itself as part of the termination. Besides the 50Ω termination resistor, traditional HCSL

needs an additional 33Ω series resistor to avoid a reflection and ringing between the driver and the 50Ω terminated trace. The

LP-HCSL concept combines the main termination and ringing avoidance in the same 33Ω series resistor, reducing the parts

count.

Certain applications use 85Ω differential traces (or 42.5Ω single ended). For these applications, we recommend RS=27Ω. IDT

also offers clocks with the 27Ω RS integrated for 85Ω systems.

Double Termination: Certain receivers may have a 100? differential termination resistor of in the chip. These receivers are

usually more generic, can handle a wide range of amplitudes and common-mode voltages, and may require AC coupling of the

clock signals. LP-HCSL drivers can readily drive double terminations. Having termination resistors at both the source and

receiver lowers the amplitude to about 400mVpp at each pin (800mVpp differential). Usually it is not a problem for this type

receiver to work with the ***aller amplitudes. The 400mVpp (800mVpp differential) level even exceeds the 150mVpp (300mVpp

differential) input swing spec for PCIe reference clocks.

//////////////////////////////

Question:

What are the basic functional differences between Traditional HCSL and Low-Power HCSL?

Answer:

Traditional HCSL outputs steer a constant 15mA current between true and complement outputs of a differential pair. This results

in a continuous power consumption of ~50mW from 3.3V for each differential HCSL output pair.

Low Power HCSL uses a push-pull voltage drive as opposed to current drive with traditional HCSL. This results in a current

consumption of approximately 4 to 5mA. Supply current flows in the driver only during the time a rising edge is traveling from the

driver to the end target and then back to the driver (round trip flight time).

Low Power HCSL not only reduces power significantly, it also better drives long traces, saves board area, reduces BOM costs,

and more easily drives AC-coupled tran***ission lines. This makes Low Power HCSL the choice for future designs.