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TMDX654IDKEVM: AM65x ICSSG XFR2VBUS Read slow

Cedrick
Expert 8270 points
Part Number: TMDX654IDKEVM

Hi,

Good day.

Our customer has his question about the am65x idk board. Kindly see blow.

I am currently using the XFR2VBUS modules to access DDR memory from ICSSG. Reading and writing is working fine. The problem is the speed to access data. I wrote a small test case to measure the speed to read and write from PRU to DDR.

What I found out is that writing to DDR is possible with approximitly 11 Gbit/s but reading is only possible with a tenth of that speed. 1,2 Gbit/s.

Why is the bandwidth so low when reading data from DDR in comparison to write?

The setup was done by writing 50Mbyte to DDR and than reading it back. For reading I used the auto address increment funciton of VBUS. The board was initialized with the corresponding launch.js script for CCS.

This difference in access time is not plausible for me and should be faster when the ICSSG provides gigabit ethernet. How can I speed up the read speed with VBUS?

Can you please help us with this? Thank you.


Regards,

Cedrick

  • 0
    Genius 5695 points

    Hi,

    I'm pretty sure this is due to the latency of the AM65x memory interface.

    Assuming that 1.2 Gbit/s == 150,000,000 Bytes/s and a XFR2VBUS width of 64 bytes your customer is achieving ~2.3 million reads/s or ~426ns/access. That is at least in the ballpark range of access latencies to DDR memory I've seen from the R5f on the AM65x.

    The PRU can read data via the DMA hardware. I'm pretty sure that's what the icss_dualmac firmware for which sources come with the AM65x PDK (unfortunately outdated, as far as I understand) uses.

    I guess the XFR2VBUS is just not designed for this purpose - that's what XFRDMA (same as XFR2PSI as far as I understand) is for. The DMA has larger FIFOs that help cover the latency.

    I'm sure one of your colleagues can give you a more definitive answer.

    Regards,

    Dominic