AT91 Clock speeds

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kvikdl
Posts: 12
Joined: Thu Jul 16, 2009 8:58 am

AT91 Clock speeds

Fri Jan 22, 2010 10:25 pm

Hi!

I'd like to ask what clock speeds should I use on the 9260 and the 9261 boards. I'm experiencing instabilities on both cards with default settings. The default is 180MHz, 2 divider. I might be wrong here.
I'm using the Olimex boards and I've changed the SDRAM init section according to an old uboot source (I suspect sightly different SDRAM modules on this card) and the clock to 175 / div 4 for the 9260 board. Now firstboot loads much more stable.
I also experienced some ethernet instability (seems like resets the ethernet controller and the nand flash but not sure) in user programs on the 9261 board and I modified the clock settings also to 175/4 but left the SDRAM init section as it was.
I don't remember where the 175/4 came from it seems that 180/2 is default in the bootloader, also it might differ for the two card, not sure.

What settings should I use?

Thanks!

KV
kvikdl
Posts: 12
Joined: Thu Jul 16, 2009 8:58 am

Re: AT91 Clock speeds

Sat Jan 23, 2010 2:53 pm

Hi!

I see now that there was a default 200 / 4 settings compiled in the bootloader and the one I set in EBOOT is only active after the image is downloaded and the OS is started.
Still, I don't know how to choose correct settings for the clocks (core, division (affects master clock, sdram, nand, etc)). As the default is not working correctly.
I also don't know how to calculate the SDRAM timings, refresh time and what to choose for clock (again). Where do these values came from?

Thanks!

KV
guangyu.ren
Posts: 23
Joined: Tue Mar 31, 2009 7:22 am

Re: AT91 Clock speeds

Tue Jan 26, 2010 5:37 am

Please understand, the master clock (like 180/2) is the working clock of EBI interface. That means all the timings on the devices connected to EBI will be multiples of the master clock cycle, such as RAS, CAS pulse. In AT91 products, those timing settings are in form of number of master clock cycles. For example, when master clock is 90MHz and RAS setting is 2, then RAS pulse will be 22.2ns.
So if you readjusted master clock or your device model is changed, you will need to recalculate those timing settings.
You should be able to find those settings in ns/us in the datasheet of the SDRAM or other devices you choose.

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