Hello,
I've tried to program the DM355 PLL to drive the ARM CPU at various speeds - 24MHz (pll bypass), 48MHz (PLL ON), 216 MHz, 240 MHz and 270 MHz (all PLL ON, over clocked). I'm not sure if this really works or not.
24MHz - I know that this mode works, everything just becomes real slow, but still works. USB subsystem goes kaput which is expected I think. I read somewhere in the datasheet/ARM guide that the USB subsystem requires clock input above a certain threshold in order to work properly.
48MHz - Everything is slow, but faster than 24MHz. USB subsystem is zonked.
216MHz, 240Mhz, 270MHz - I've tried these on the 216MHz version of the DM355 chip as well as on the 270MHz version of the chip. The moment we do something like this to enable these modes:
# echo 'pll270mhz' > /sys/power/state
(or pll240mhz or pllon for 216)
The USB sub system seems to reset for some reason, and thereafter it doesn't really work very well. I know I'll have to have a closer look at the ARM guide and the code.
But is there a way to check and verify that the CPU is indeed running at the desired frequency? I tried:
# cat /proc/cpuinfo
The 'BogoMIPS' doesn't really help, it seems to stay unchanged, even in the PLLbypass and 48MHz modes. Is there any other way?
Regards,
Kapil