DM385 DDR3/DDR3L memory configuration support

Hello,

Reviewing the technical documents for DM38x

   a) errata - 2.1.2 special requirements for PLL_DDR for even M2 Post Divider

                      3.1.3 customers have to follow the DDR3 layout guidelines, and then fillout the Seed DDR3 out clock polarity and DQS levelling spreadsheet.

   b) datasheet - 32bit DDR3, DDR3L implementation (normally 32bit is 2 x16bit devices), 400 - 533Mhz DDRclk, 800-1066 operation

                      6.2 - DDR3 1.5v, DDR3L 1.35v

                      8.1.3 - suggests that memory device capacity is 128 to 4Gbyte

   c) technical reference manual

                       section 7.2 sizing, RSIZE number of rows, PAGESIZE number of columns, IBANK, EBANK number of chip selects, 15bit address bus (A14.0)

                       so supporting two devices we can have 2DDR3 with 2 chip selects and 16bit width, 

                       so supporting two devices we can have 2DDR3 with 1 chip selects and 32bit width, (more memory bandwidth 32bit access)

The customer can have a 1Gbyte x 16, 2Gbyte x 16, 1Gbyte x (2x16), 2Gbyte x (2x16), depending on the memory bandwidth needs for their application.   

Regards,

Joe Quintal

Hi Joe,

Thanks for the help.  Here are more questions from the customer:

8.13 in the data sheet

Memory device capacity: 64Mb, 128Mb, 256Mb, 512Mb, 1Gb, 2Gb, and 4Gb devices

These say bit not byte. 4Gb is only 500MB. Is he saying the Datasheet is wrong?

I guess what I really need to know is what is the largest Gbit of RAM I can use and can I just use 1 chip.

If I can just use one do I just leave the other 16 address lines open?

The DM385 DDR3 interface has a 32 bit data bus with a single chip select. Since DDR3 memory devices do not come in x32 configurations, you're choices are to use either use x16 or x8 memory devices. This is by definition in the JDEC spec.

The maximum DDR3 address space supported on the DM385 is 1GByte (8GBit). This means no matter what memory device configuration you use, we can only address up to 1GByte. That's the max total DDR3 we can have on the DM385.

The ideal configuration is to use two x16 memory devices, one on each half of the 32-bit data bus. This would give you maximum bandwidth because you would be moving 32 bits of data at a time. So for maximum bandwidth, you would use two x16 4Gbit memory devices for a total of 8Gbit of storage with 32 bits of data.

If you only want to use one memory device, I assume for either space savings or power savings, then you could use a single x16 8Gbit memory device. This would still give you the full 1GByte of memory storage. The disadvantage is that you now only have a 16-bit data bus so only half the bandwidth compared to if you were to use all 32 bits using two memory devices.

If you end up only using a single x16 memory device, which is okay to do, then you can leave the other 16 data lines (upper half of the data bus) unconnected. Then in software you would enable to internal pull-down resistors on those unused IO lines to keep them from floating wildly.

What is the reason for only wanting a single memory device? I ask because the use cases I typically see have the whole data bus populated with memory devices. Just saying it's not as common to only use half the bus.

Brian,

So I will admit I made an educated assumption. I agree with you that the datasheet conflicts with the TRM and generally I always take the datasheet as the overriding buck-stops-here source of information. In this case however, it appeared to me the misinformation is due to a carryover from a previous device in the same family. The previous generation device (DM8148) had two DDR EMIFs with a total of 2GByte address space. Since the DM385 only has one DDR EMIF, I made my assumption based on that.

What I've done is passed this thread onto our product team to get the definitive answer, they'll likely check with the chip design team to get this information.

Sorry about the confusion, we'll get this cleared up.

Regards

Hello,

We are building a new EVM that has the DM388, for DDR3.  In this application we are using (2) DDR3 devices.   MT41K256M16TW - Micron memory.

dm388_newEVM_DDRschpage.jpg - here is the device used.   These EVMs are being tested in the next month.

The Micron MT41K256M16 device is being utilized.   It says its a 4Gb, x16 device.  I have attached the datasheet.

www.micron.com/.../mt41k256m16ha-125

Brad,

Have you heard any update on the RAM options for the DM385? I am needing to pick out my RAM options and it depends on your answer.

Thank you,