Hello,
I have a question on axi read time. If we have 20 read commands, the read outstanding is 4 and read latency is 500ns, how much time is needed to read all of these data back?
I feel puzzled on this for a long time and I need a relatively accurate calculate method. Expect your help.
Best Regards,
redbud
Hello redbud,
to calculate the total latencies some information will be lack.It will depend on a master functionality, and READY states.What frequency is the ACLK?What is the burst length?The following figure is on condition READY signals are always ready and the burst length is 1.
Also It would show the fastest case.
From the figure, the total latencies would be 5 x 500ns + 7 x ACLK.
Best Regards,Yasuhiko Koumoto.
Hi Yashhiko Koumoto,
Thanks for your answer and detailed explanation.
The BL=1 is the most simple. If the BL=8, maybe the time diagram is like this?
if the read latency is large enough and the frequency of ACLK is high, then we can ignore the time of ACLK, how to estimate the total latency of these read command accurately (only use read latency, the number of command and the number of outstanding)?
Yes, if the master does not have any read buffers.
If the master has enough read buffers, the timing would be as the following.
Threfore, the latency would be 20x8xACLK-ACLK+500ns.
I agree with you.
In this case, the latency would be 500ns x 20.
The picture which I attached above would be the case the frequency of ACLK was low.
Best regards,
Yasuhiko Koumoto.
Hi Yasuhiko Koumoto,
Thank you for your detailed explanation and patience!
I learned many things from your reply. Thank you again!
On the last conclusion of "In this case, the latency would be 500ns x 20." It may be not correct. Would you like to re-analyse it?
Only use read latency, the number of read commands and the numbe of read outstanding, how to compensate for the time of ACLK which is excluded from 500ns latency? (the result may be more accurate).
wasn't your assumption that ACLK was too high to be ignored?
What accuracy do you want?
What is your opinion?
When the frequency of ACLK was too high and can be ignored, wether the latency would be 500nsx5?
if the latency is 500nsx20, it's equal to the result of one outstanding. So I think it may be not accurate.
In fact, even if the frequency of the ACLK is high, I guess that may have a method to give some compensation on it and which is more accurate than ignore the ACLK when we estimate the total read latency. for example 5.25x500? or something else? Of cause, I just guess it.
Sorry for not explaining it clearly at the last comment.
Thank you for your kindly help!
I am sorry for confusing you.
You are right.
If the ACLK frequency is very high and outstanding is 4, the read latency would be around 500nsx5.
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