Characterizing DDR2 memory devices requires the ability to isolate read and write access cycles. In DDR2 memory the read and write operations can be distinguished by the timing relationships between the strobe and data lines as shown in figure 1. In read operations the data changes are synchronous with the strobe transition. During write operations the strobe transition precedes the data transition. We can use this timing difference to separate the read and write operation. In figure 1, the lower amplitude read operations data transitions occur synchronously with the strobe transitions. In the higher amplitude write operations the strobe occurs about 750 ps ahead of the data transition.
LeCroy’s WaveScan waveform search and scan feature can find data signals with selected timing relationships to the strobe and show only waveforms which meet those criteria in the scan overlay display trace. An example of this capability is shown in figure 2. The WaveScan dialog box is setup to scan for waveforms which have a hold time between 500 ps and 1 ns (750 ±250 ps). The tabs at the right of the dialog box are used to identify the strobe and data signals. Note that the measurement is setup to use both edges on the ‘hold clock’ and ‘hold data’ dialog box tabs.
WaveScan lists all the features found in each acquisition that meet the measurement criteria in the table at left of the display. Touching any listing will automatically set the zoom to display the selected feature. In this example the zoom scale is set to show a group of write operations. Each operation meeting the measurement criteria is outline with a red box as shown.
WaveScan accumulates all waveforms meeting the criteria in the measurement filter in the ScanOverlay trace (center trace in figure 2. This persistence trace contains a history of the entire series of acquisitions. We can now apply measurements to the entire group of selected waveforms by using measurement parameters on the ScanOverlay trace. You can also make measurements on selected waveforms by applying the measurement parameters to the zoom traces.
Figure 3 shows an example of the risetime parameter applied to the ScanOverlay trace. The ScanOverlay trace contains only write cycle operations that have been isolated by WaveScan.
Figure 4 shows an alternative measurement technique, Here, a selected group of write cycles has been isolated using ScanZoom. Parameter gates further isolate only the 12 write cycles and measure the delta time at level measurement parameter (P4) for the positive edges only.
WaveScan provides the tools to allow you to characterize DDR2 devices.