It is mentioned that the External Bus Interface of an XMega can run at upto 2 times the CPU clock frequency. What is the point of having this? Won't the low CPU clock cause a bottleneck in the access anyway?
XMega EBI - why is it able to run at 2x the CPU clock freq?
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The EBI is clocked from the Fast Peripheral clock, running up to two times faster than the CPU and supporting speeds of up to 64 MHz.
MBedder wrote:
The EBI is clocked from the Fast Peripheral clock, running up to two times faster than the CPU and supporting speeds of up to 64 MHz.
I know. What I want to know is whats the point of running your external memory faster than your CPU..
E.g. for transferring data chunks using DMA.
but the DMA controller runs at Peripheral clock frequency, which is at max equal to CPU clock speed, right?
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According to XMEGA A manual, this is for lowering EBI access time(section 24.5).
electronic.designer wrote:
According to XMEGA A manual, this is for lowering EBI access time(section 24.5).
I saw that too. Doesn't answer my question. The CPU would still take one CPU Clock Cycle to fetch data from external RAM no matter how fast it runs, wouldn't it? So where's the gain??
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You can see your answer in Appendix A(EBI timing diagrams).Fetching data from external memory needs at least 4 cycles of CLKper2(and more) and there is not any access with one clock cycle of CPU.So doubling CLKper2 causes lowering access time.
Okay I get it. I had a confused understanding of EBI :).
Thanks a lot :)