Vybrid DDRMC manual unclear

[kubiznak_petr]

Hi,

when investigating the contents of the DDRMC registers of MVF61NS151 (2N02G), I found unclear and possibly erroneous parts in the reference manual.

I found that the register DDRMC_CR115 (located at address 0x400AE1CC) is set to value 0x20200000 (using md command in u-boot). In the RM (rev.5) I found that bits 31-8 are reserved, always having value of 0, while only bits 7-0 are dedicated to a field called RDLVL_GTDL_2. As this explanation collides with my observation (non-zero value in the reserved field), I thought it was a bug in the RM and downloaded the latest revision (rev.7). There it is even more ambiguous though, saying that all bits of the register are reserved: "This field is reserved. Disabled featuredSet to 0x0". I also found that this "description" in the RM rev.7 replaced lot more registers, which were somehow (don't know whether correctly) described in RM rev.5.

Can someone please explain why the descriptions changed this way and what is the meaning of the value 0x20200000 in DDRMC_CR115 which I am observing?

[naoumgitnik]

Dear Petr,

Please, follow the latest Rev.7 one, in which the DDR Controller chapter got cardinally rewritten.

Regarding "why ... changed?" - Unfortunately, regardless of how hard one works, if the document contains 3500+ pages, incorrectness and typos still appear; this is why we decided to spend significant amount of time and effort to review the entire chapter to verify, correct, and even improve wording where required.

Sincerely, Naoum Gitnik.

[kubiznak_petr]

Hi Naoum,

thank you for your reply. I understand that there are some inaccuracies in such a long manual, that's not what is my question about. I ask why it changed this way, that is why lots of descriptions changed to "This field is reserved. Disabled featuredSet to 0x0", even though they surely have some functionality, which was already described, though maybe not correctly. How should I understand to the value 0x20200000 in DDRMC_CR115? Or are these registers really completely ignored, with no influence on the DDR functionality?

[naoumgitnik · Accepted solution]

Dear Petr,

There are several reasons why a user should not touch some registers (with different types of wording), and they all are applicable in the case of this change (after we have reviewed everything extremely thoroughly):

1. The default value work properly, and we do not want a user to alter it,
2. The register belongs to the generic block used for more than one product, but the feature it controls is not used in this specific product,
3. The changes in the tape-out version of the chip occurred and have to be reflected in the manual,
4. Etc.

So, answering your question - yes, these registers "as they are” do have functionality, but for the above reasons they are either irrelevant or not to be exposed to a user.

The main message to a user here is  "Imagine these registers do not exist” J.

Sincerely, Naoum Gitnik.

[kubiznak_petr]

Ok Naoum, thank you for the explanation. Petr