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Efficiently measure DDR3 signal integrity

Posted: 26 Jul 2011     Print Version  Bookmark and Share

Keywords:DDR3  signal integrity  time domain reflectometry 

DDD3 memory interface speeds have been consistently rising and are now reaching 2000Mbit/s data rates. DDR4 is also almost round the corner [6]. At these high data rates, it is becoming more difficult to get good probing solutions which allow performing signal integrity characterisation. High speed characterisation is considered important for silicon based product development cycle. It is needed to understand the limitations of the current generation of DDR interface designs and to gear up our designs for next generations of DDR.

What are our options?
Logic analysers (LA) and oscilloscopes are the two main signal analysis lab solutions available today. LA is primarily a digital data capturing device. Digital data, once captured, can be post-processed to extract meaningful information and to peform data visualisation. LA based measurement has its advantage that it can capture the entire DDR bus for analysis. Captured bus data can then be visualized in various forms of abstraction ranging from waveform level, byte/word level to transaction and protocol level. LAs come with few analogue outputs as well, through which analogue waveform from any of the LA pins can be observed.

In theory, if you connect entire DDR bus to LA pins, then you can observe the analogue waveform from any pin. However, in practice these outputs get limited by the fidelity of the signal capturing pods and cables. From signal integrity perspective these outputs are good for only a limited set of measurements like signal swing, etc.

Broadly, an LA has two ways to make contact with the DDR bus; one is to place LA probing pads in the middle of the DDR bus and second is to keep the probing pads at the end of the bus. At mid-bus, reflected wave superimposes almost in the middle of incident wave, creating distorted waveforms, leading to measurement difficulties when trying to set trigger.

End bus probing is, therefore, considered a better solution. LA probing footprints can either be placed on the main DUT board or be placed on the memory DIMM card. DIMM card placement is better as it allows us to probe the signals as seen by the memory devices on the DIMM. Special purpose DIMMs can be procured for this purpose from third party vendors [1,2] .

The most direct way to do signal integrity measurement using oscilloscope is to use Active probes. Active probes are high-bandwidth probes which buffer the signal near to the probing point. These probes contain active electronics on the probe tip and therefore, have low loading characteristics. The probe tip needs to be either soldered down to the probing point using accessories or held manually.

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Figure 1: Manual probing example—soldering tips with damping resistors for active probing. Probes attached to series resistor, which are the only accessible points on the DIMM.

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