Hello,
At the moment we are working on a product using a TI OMAP-L138 with a Micron DDR2 memory, type MT47H64M16HR-3.
During our qualification of the product we see that the drive-strength of a read from the DDR2 memory is too strong and exceeds the specified maximum voltage of the OMAP-L138.
In our design we don't use series termination and a write from OMAP to the DDR2 memory device is within its limits. Our design contains 1x DDR2 and is point to point connected. Also the DDR2 chip is close to the OMAP-L138.
Because we want to reduce the drive-strength of the DDR2 chip we use the SDCR.DDRDRIVE bits to set the reduced drive-strength. (Because we don't use a mDDR2 device 1/4 and 1/8 is not available.)
For that we have used the following procedure:
- We use a Micron MT47H64M16HR-3 chip
- The Micron drive strength is determined by the EMRS register, connected to address bus DDR_A[1]
- In SPRUH77A (Dec 2011), chapter 15.2.13, it is mentioned the OMAP’s DDR memory controller is able to issue EMRS commands during initialization sequence
- In SPRUH77A, table 15-13, the EMRS command is described. It is mentioned that DDR_A[1] is based on DDRDRIVE0 from the OMAP’s SDCR register.
- During the initialization (described in SPRUH77A, chapter 15.2.13.1), we set SDCR.DDRDRIVE0 to ‘1’ in step 8, which should correspond to reduced drive strength in the Micron DDR chip
The problem we encounter is that now two out of three boards does not boot/work with the reduced setting. We are still performing measurements to check what exactly is going wrong.
In the meantime we would like to ask the following two questions:
1.
we were not sure whether or not it is possible to use different drive strengths for the OMAP and DDR chip.
So, would it be possible to set the drive strength on the DDR chip to ‘reduced’, and after the DDR controller initialization, change the OMAP’s SDCR register on the fly to set the drive strength to ‘full’ again?
Or is it only possible to have either full or reduced drive strengths on both the OMAP and DDR chip?
To clarify I added the following table:
Nr | OMAP drive strength | Micron DDR chip drive strength | Possible |
1 | Full | Full | Yes |
2 | Reduced | Reduced | Yes |
3 | Full | Reduced | ? |
We currently think option 1 and 2 are possible, could you tell us if option 3 is also possible?
2.
Is there perhaps something know about this behavior? We checked the TI forum and found a topic on a similar problem. (http://e2e.ti.com/support/dsp/omap_applications_processors/f/42/t/214910.aspx)
We would like to know if some unreliable behavior could be expected. In the topic the problem occurs when the drive strength is set to 0.5, whereas full, 0.75 and 0.25 drive-strength is working reliable.