On-die termination
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On-die termination (ODT) is the technology where the termination resistor for impedance matching in transmission lines is located inside semiconductor chip instead of motherboard.
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[edit] Tutorial of termination in electrical signal
[edit] Lumped circuit
In lower frequency (slow edge rate) applications, you can regard it as "lumped" circuit and there is no need to consider the concept of "termination". Under the low frequency condition, you can assume that every point in a wire has the same voltage as every other point for any instance in time and there is no need to consider the concept of termination.
[edit] Transmission line
If the propagation delay in a wire or interconnect is greater than 1/6 of the rise time of the digital signal, then the "lumped" circuit analysis assumption is no longer valid and you should analyze the interconnect as a transmission line. Transmission line means, when you interpret the signal path you cannot ignore the ingredient of inductance of the line, differential nature of the signal path in equivalent circuit model.
In order for transmission line to have as clean signal as possible the impedance of the every point of the transmission line should be uniform, ideally. If there is any point where the impedance of the transmission line is not uniform some reason such as disconnection, discontinuity, different material, then the signal gets affected from noise such as distortion, ringing and so forth.
[edit] Termination
When the signal path has impedance discontinuity, in other words, impedance mismatching, then you need to put equivalent amount of impedance device at the point of impedance discontinuity. You call it Termination. Resisters which can make the transmission line have uniform impedance equivalently are used for computer mother board, for example. There are several ways of termination depending on how the resister are connected to the transmission line. Parallel termination and serial termination are examples of those termination methodologies.
[edit] On-die termination
Instead of having the necessary resistive termination is located on the motherboard, the termination is located inside the semiconductor chips–technique called ODT, On-Die Termination.
[edit] Why is on-die termination needed?
Although the termination resistors on the motherboard reduce some reflections on the signal lines, they are unable to prevent reflections resulting from the stub lines that connect to the components on the module card (eg. DRAM Module). A signal propagating from the controller to the components encounters an impedance discontinuity at the stub leading to the components on the module. The signal that propagates along the stub to the component(eg. DRAM component) will be reflected back onto the signal line, thereby introducing unwanted noise into the signal.In addition, the ODT on die termination can reduce the number of resister elements and complex wirings on the mother board. Accordingly, the system design can be simpler and cost effective.
[edit] Example of ODT: DRAM
ODT(On Die Termination) is implemented with several combination of resisters on the DRAM silicon along with the other circuit trees. DRAM circuit designer can use combination of transistors which have different value of turn-on resister. In case of DDR2, there are three kinds of internal resisters 150ohm, 75ohm, 50ohm. The resisters can be combined each other to create proper equivalent impedance value to the outside of the chip, signal line(transmission line) of the motherboard being controlled by the ODT operation signal. The DRAM controller manage the ODT value through register, where the ODT value control circuit exist. Needless to say, as the DRAM frequency increases toward DDR3 the ODT value has to be adjusted to different value. Accordingly the internal ODT values in DDR3 are 120ohm, 60ohm, 40ohm an so forth.
