Raspberry Pi Compute Module

Stefan Wahren info at lategoodbye.de
Tue Sep 8 10:25:47 PDT 2015

Hi Martin,

Am 06.09.2015 um 16:52 schrieb Martin Sperl:
>> On 04.09.2015, at 09:17, Martin Sperl <martin at sperl.org> wrote:
>> So I have now instrumented (and almost destroyed) one CM so that I can get
>> the signals to emmc into my logic-analyzer.
>> (removing all the custom settings and switching to bus-width = 1)
>> Sampling 3 lines at 500MHz I see the following sequence:
>> CMD: 0xE9 @ 1MHz
>> CMD: 0xD1 @ 1MHz
>> CMD: 0xFE @ 1MHz (with some data getting transferred on D0)
>> CMD: 0xFE @ 1MHz (with some data getting transferred on D0)
>> then we got a clock switch to about 62.5MHz
>> (measuring 16 clocks gives me 0.26us, so 61.5MHz)
>> CMD: 0xFA @ 62.5MHz
>> So it looks as if the clock is wrong - maybe we are using the wrong clock
>> resulting in the wrong divider?
>> The mmc-clock is set as 100MHz in the device-tree, which with a divider
>> of 4 would give 25MHz. But if we assume a divider of 4 and 62.5MHz this
>> would give us a clock of 250MHz.
>> Checking the foundation device-trees, they have also 250MHz configured for
>> the mmc clock - even on a model 2...
>> Switching the clock to 250MHz in the device-tree changes the behavior,
>> and - with some errors due to my incompetent soldering - I can now detect
>> the eMMC and mount the partitions on the compute module.
>> I can now also confirm that the eMMC clock line shows now 25MHz!
>> I have ordered a replacement, but it takes some time to get it delivered,
>> so can please someone owning a Compute Module check that this fixes the
>> issue?
>> Note that I guess that because of this we are really overclocking the
>> SD cards on the normal RPIs as well, which results in more errors than
>> we see with the foundation kernels.
>> Another note: u-boot (at least the version I am using) is leaving the
>> eMMC clock running - I wonder if this is intended…
> [...]
> So to me it really looks like a wrong configuration of the clock
> and hence the patch updating the mmc clock to 250MHz seems reasonable.

thanks for all your investigation and efforts even if Eric's clock 
driver should fix the issue in a better way.

Unfortunately it looks to me that both of you work independently at the 
same problem :-(

Kind regards

> Thanks,
> 	Martin
> P.s: note that the above measurements were taken with a SanDisk 8GB
> Class 4 SD-Card.

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