mr_ghz
Full Member level 5
Hi all
I designed a CPU board with an AMCC 405EP controller. The whole board works well with a Linux and QT.
By EMC testing we saw a strange behaviour of the board: The 266MHz CPU clock has a strong jitter on it, but the 66MHz input clock has not.
Let me explain the design a little bit more: The CPU SYSClk is driven by a 66MHz crystaloscillator. Pegel, jitter everything OK. The PLL inside the CPU runs on a VCO frequency of 800MHz (66MHz*12). The forward divider is set to :3 this gives the corefrequency of 266MHz. On all frequencies which are created from this coreclock (MemClk, PLBClk, ...) I measure the jitter. The jitter looks like an instabile loopfilter of the PLL.
PLL has a separete VCC as mentioned in the datasheet, also supply noise is low.
The PLL has some 'tunebits' which are not clearly described in the datasheet. When I use the given values --> jitter occurs. The I played a little bit with the tuningbits. I found several combinations which runs fine (also by cooling / heating the board, over- and undervoltage).
Do anyone know this behaviour of this CPU?
Do anyone know the exact meaning of these tuningbits?
Some more ideas?
Thanks in advance
I designed a CPU board with an AMCC 405EP controller. The whole board works well with a Linux and QT.
By EMC testing we saw a strange behaviour of the board: The 266MHz CPU clock has a strong jitter on it, but the 66MHz input clock has not.
Let me explain the design a little bit more: The CPU SYSClk is driven by a 66MHz crystaloscillator. Pegel, jitter everything OK. The PLL inside the CPU runs on a VCO frequency of 800MHz (66MHz*12). The forward divider is set to :3 this gives the corefrequency of 266MHz. On all frequencies which are created from this coreclock (MemClk, PLBClk, ...) I measure the jitter. The jitter looks like an instabile loopfilter of the PLL.
PLL has a separete VCC as mentioned in the datasheet, also supply noise is low.
The PLL has some 'tunebits' which are not clearly described in the datasheet. When I use the given values --> jitter occurs. The I played a little bit with the tuningbits. I found several combinations which runs fine (also by cooling / heating the board, over- and undervoltage).
Do anyone know this behaviour of this CPU?
Do anyone know the exact meaning of these tuningbits?
Some more ideas?
Thanks in advance
