OMAP-L137: OMAP-L137 PRU external memory access timing

Michele Sponchiado

Part Number: OMAP-L137
Other Parts Discussed in Thread: OMAPL138

Hi, dear TI processor forum.

I am currently using an OAMP-L137 in an embedded application, and we started developing a simple PRU program to access some registers on an external FPGA in real-time.

I have found little to no mention in the documentation about the timing access of the PRU to the external memory:

the PRU wiki states for the LOAD/STORE instructions 1 (or 2) + word count CPU cycle, so it seems the PRU can't handle slow external memories, with a read access timing of 4 or more PRU cycles
- is this correct?
- or is the PRU using the same timing of the ARM core?

Best regards

Michele

over 5 years ago

0 Biser Gatchev-XID over 5 years ago

TI__Guru**** 393215 points

Hi Michele,

The PRU wiki you refer to is commenting accesses to the memory located inside the PRU itself. Please refer to this wiki to understand how access to external memories works: processors.wiki.ti.com/.../PRU_Read_Latencies

I have asked the PRU experts to add comments if they have any. They will post directly here.

0 melissaw over 5 years ago in reply to Biser Gatchev-XID

TI__Genius 15540 points

Hi Michele,

As mentioned in the wiki link Biser provided, PRU accesses to memories outside of the PRUSS are going to be indeterministic. However, below are some basic steps to estimate the “best case” read latency to a particular MMR:

1. Enable the PRU Cycle Counter by setting bit 3 in the PRU Control register.
2. Have the PRU read a particular MMR.
3. Check the PRU Cycle Count Register for the number of PRU cycles it took to read that MMR. (Note that on OMAPL138, the PRU cores run at ½ the CPU frequency.)

You may also find this training on “Designing a PRU Application” helpful-- training.ti.com/sitara-processors-building-blocks-for-pru-development-appdesign.

Regards,
Melissa

0 Michele Sponchiado over 5 years ago in reply to melissaw

Intellectual 635 points

Hi, Mr. Gatchev!
Thanks for your reply, it is very useful for my tests!
Best regards
Michele

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OMAP-L137: OMAP-L137 PRU external memory access timing