Stub Series Terminated Logic

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Stub Series Terminated Logic (SSTL) devices are a family of electronic devices for driving transmission lines. They are specifically designed for driving the DDR (double-data-rate) SDRAM modules used in computer memory. However, they are also used in other applications, notably, some PCI Express PHYs and other high-speed devices.

Three voltage levels for SSTL are defined:

SSTL_3, 3.3 V, defined in EIA/JESD8-8 1996
SSTL_2, 2.5 V, defined in EIA/JESD8-9B 2002
SSTL_18, 1.8 V, defined in EIA/JESD8-15

All SSTL voltage specs reference a voltage that is exactly VDDQ/2. For example, the VREF for an SSTL_18 signal is exactly 0.9 Volts.

Terminations can be:

Class I (one series resistor at the source and one parallel resistor at the load)
Class II (one series resistor at the source and two parallel resistors, one at each end).
Class III (asymmetrically parallel terminated)
Class IIII (asymmetrically doubly parallel terminated)