Strength of Agilent Infiniium in
DDR Memory Testing
－InfiniiScan Technique－
If you need to validate a prototype involving an embeddedDDRmemory system, you will need to be able to verify proper signal quality of your design by measuring the data bus DQ lines eye opening.
The challenge of DDR measurement is separating the READ and WRITE memory data transferts.
This has been a top request from the engineer implementing DDR, because without the proper methods, the READ and WRITE eyes will overlap each other and signal analysis cannot be properly performed.
With Agilent Infiniium scope and InfiniiScan event identification software option, the signals can now be separated and analyzed easily. On contrary, other Scope vendor approach will require multiple channels for triggering and this method is poor as the correct setting is difficult to determine, long setup time is required andslight changes to the signal will affect the setting, making it not reusable.
The foundation of DDR SDRAM
Both the Data Strobe (DQS) and Data (DQ) signals existed in bidirectional. Double Data Rate (DDR) as the name implies allows data to be transferred at both the edges of the DQS signal. The setup time difference between the DQ and DQS signal differentiates the READ and WRITE signals. For READ, DQS and DQ are aligned while WRITE, the DQS and DQ are 90deg phase shifted.
In addition, command is issued before READ and the WRITE data. Command decides which signals (RAS CAS WE etc) to toggle. For example below, READis when RAS=H, CAS=L and WE=H.
The actual signals of DDR
Measuringlive signals from DQ and DQS, READ and WRITEdata access are observed on the same bus as in the figure below. Trying Infinite persistence eye diagram measurement at this point will overlay READ memory access and WRITE memory access making any measurement impossible.
This is mainly caused by the phase difference between DQ and DQS of the READ and WRITE signals. Therefore there is a need to separate the READ and WRITE signals.
Setup time of DQ and DQS are found to be the most effective method.
Method of the differentiating READ and WRITE: In this example, the WRITE eye is drawn
1. Start, by probing DQSand DQ signals on the DDR Memory chip (or as close as possible) and use AUTOSCALE function. Study the phase difference between the READ and WRITE signals.
Ch1: DQS (Yellow) Ch2:DQ (Green)
2. From observation, amplitude is different in READ and WRITE. WRITE amplitude is smaller because probing is made on the close to the DDR module (far end from the memory controller).
Next, select the Zone Qualify Mode of InfiniiScan: SETUP->INFINIISCAN->Zone Qualify
In order to isolate WRITE memory access as observed in the following figure, two “MustNotPass” zones are used. They are easy to position on the display as below in order to filter out “Reads”.
b
3. Now right click on the screen and set the display persistence setting to Infinite Persistance.
Now, the eye measurement of WRITE is COMPLETED!
Of course you need advanced triggering capabilities not just for DDR measurement:
InfiniiScan is the most simple & powerfultrigger tool ever created, try it for yourself !