CPU Frequency

[bjorn]

Hi!

I have a question regarding the CPU Frequency on my BUBBA.
The usermanual and productflyer claims that BUBBAs CPU runs @200MHz, but when i look in "dmesg" i see the following output:

CPU: ARM920Tid(wb) [41129200] revision 0 (ARMv4T)
Machine: Atmel AT91RM9200-DK
Memory policy: ECC disabled, Data cache writeback
On node 0 totalpages: 16384
DMA zone: 16384 pages, LIFO batch:3
DMA32 zone: 0 pages, LIFO batch:0
Normal zone: 0 pages, LIFO batch:0
HighMem zone: 0 pages, LIFO batch:0
Clocks: CPU 159 MHz, master 79 MHz, main 18.432 MHz

Do i interpret the information wrong or is my bubba actually running @159MHz?

/Bjorn

[johannes]

Hi Bjorn,

Yes, true. We use a "200 MHz" ARM processor, but it's a little more complicated than that:

Optimizations are available for memory bus speed (increasing data throughput) and for CPU core speed (increasing calculation performance). Maximum memory bus speed is 80 MHz, and maximum CPU core speed is 200 MHz. Also, the ratio between those (the divider) must be a whole integer number. Thus, the choices are:

Core = 200 MHz and bus = 67 MHz (divider = 3)
Core = 160 MHz and bus = 80 MHz (divider = 2)

Note that the second option has 20% slower core speed and 20% faster bus speed.

We actually started off with the first option on our first prototypes, but changed to the second one since benchmarking showed that this was superior.

I hope that clarifies this matter. Let me know otherwise!

[bjorn]

Thanks for your answer, this explains what i see!

Is there a way for me to change this setting, for doing my own benchmarking?

/Bjorn

[johannes]

Yes, there is of course, but it's quite complicated. It requires reading the CPU:s datasheet to find out what register changes are required, changing and re-compiling the bootloader, programming the boot FLASH memory (nvram), quite a few steps that would require a long explanation. I don't have time for it right now, but will perhaps come back with more information later on. Sorry..