PRU and CPU driven by different oscillators?

[Justin Pearson]

How can I find out whether the PRU and CPU are driven by the same oscillator? Specifically, a colleague told me that the IEP timer (which I'm reading with the PRU) is driven by a 24 MHz oscillator that's PLL'd so the timer increments at 200 MHz, whereas the CPU is driven by a 25 MHz oscillator PLL'd so that the CPU runs at 1 GHz.

It seems to me that if they're driven by different oscillators, then they could drift apart over time. 

Page 1177 of the TRM (spruh73n.pdf) mentions a 32-kHz crystal oscillator, but I don't see how that's related.

Also, are these oscillators within the Sitara SoC, or somewhere else on the BBB? The SRM just references 24.576 MHz oscillator (pg 70 of e14 BBB_SRM_rev 0.9.pdf), and I'm not sure how that's related to the 24/25 MHz oscillators my colleague mentioned.

Thanks for your help.

[Graham]

The CPU in a BBB runs from a 24 MHz Oscillator.

There is a 25 MHz oscillator on the board, but that is for the Ethernet.

--- Graham

==

[Justin Pearson]

Thanks Graham. Follow-up questions: 

1. Where exactly did you find this information? I looked through the TRM and SRM but couldn't find anything definitive.

2. Is the 200-MHz PRU driven from the same 24 MHz oscillator that drives the CPU? If so, is it correct that the PRU cycle counter increments precisely once for every 5 CPU cycles? 

Thanks for your help.

-Justin

[Graham]

Well, best way is to read the schematic. (BBB Rev C schematic, dated March 21, 2014)

24 MHz Sitara clock crystal is on upper left of page 3, hooked to main oscillator I/O.

There is also a 32 kHz crystal shown there for the Real Time Clock.

The 25 MHz crystal is on page 9, hooked to the LAN8710, which is the Ethernet PHI.

I am not a PRU expert, but I don't think there is, or should be, a fixed relationship between

the PRU clock and the CPU clock.  The CPU in a Sitara is a variable speed CPU, and 

can run anywhere from a few hundred MHz to a GHz, depending on loading,

and it is under kernel control.

I would not think you would want the 200 MHz clock for the PRU to be variable

like that, since it would totally destroy the real time advantage of the PRU.

So, I suspect that the clock for the PRU is a fixed clock coming from a different

place in the clock tree than the variable speed CPU.

You should really check and verify the clock tree behavior.

--- Graham

==

[flairy...@gmail.com]

Graham is right. The oscillator used for the processor is the 24MHz crystal oscillator (Y2 in schematic), whereas, the one used by the ethernet controller or the IEP timer is 25MHz crystal oscillator (Y3 in schematic).

[Justin Pearson]

Thanks Graham. What do you mean by "check and verify the clock tree behavior"? I searched the TRM for "clock tree" but I'm still not sure how to figure out which timers make their way through various PLLs to the PRU and main CPU. 

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[din...@gmail.com]

You may find this tool from TI useful: http://processors.wiki.ti.com/index.php/AM335x_Clock_Tree_Tool

To unsubscribe from this group and all its topics, send an email to beagleboard...@googlegroups.com.

[Roberts Maria]

--------------------------------------------
On Sun, 4/30/17, <din...@gmail.com> wrote:

Subject: Re: [beagleboard] Re: PRU and CPU driven by different oscillators?
To: "BeagleBoard" <beagl...@googlegroups.com>
Date: Sunday, April 30, 2017, 2:31 PM

You may
find this tool from TI
useful: http://processors.wiki.ti.com/index.php/AM335x_Clock_Tree_Tool

On Saturday, April 29, 2017 at
10:14:31 PM UTC+3, Justin Pearson wrote:Thanks Graham. What
do you mean by "check and verify the clock tree
behavior"? I searched the TRM for "clock
tree" but I'm still not sure how to figure out
which timers make their way through various PLLs to the PRU
and main CPU. 


On Wed, Apr 26, 2017 at
10:00 AM, Graham <gra...@flex-radio.com> wrote:
Well, best way is to
read the schematic. (BBB Rev C schematic, dated March 21,

2014)24 MHz Sitara clock crystal is on upper left of
page 3, hooked to main oscillator I/O.There is

also a 32 kHz crystal shown there for the Real Time
Clock.
The 25 MHz
crystal is on page 9, hooked to the LAN8710, which is the
Ethernet PHI.
I am not
a PRU expert, but I don't think there is, or should be,

a fixed relationship betweenthe PRU clock and the

CPU clock.  The CPU in a Sitara is a variable speed CPU,
and can run anywhere from a few hundred MHz to a

GHz, depending on loading,and it is under kernel

control.
I would not
think you would want the 200 MHz clock for the PRU to be

variablelike that, since it would totally destroy

the real time advantage of the PRU.
So, I suspect that the clock for the
PRU is a fixed clock coming from a differentplace
in the clock tree than the variable speed CPU.
You should really check and verify
the clock tree behavior.
--- Graham
==
On Wednesday, April 26, 2017 at 8:02:38 AM
UTC-5, Justin Pearson wrote:Thanks Graham.
Follow-up questions: 
1.
Where exactly did you find this information? I looked
through the TRM and SRM but couldn't find anything
definitive.
2. Is the
200-MHz PRU driven from the same 24 MHz oscillator that
drives the CPU? If so, is it correct that the PRU cycle
counter increments precisely once for every 5 CPU
cycles? 
Thanks for
your help.-Justin

On Tuesday, April 25, 2017 at 7:35:19 PM UTC-7,
Graham wrote:The CPU in a BBB

runs from a 24 MHz Oscillator.There is a 25 MHz

oscillator on the board, but that is for the

Ethernet.--- Graham

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n perioada neutralitatii situatia economica a tarii s-a inrautatit. Mai intai inchiderea stramtorilor Bosfor si Dardanele dupa intrarea Imperiului Otoman in razboi de partea Puterilor Centrale a dus la reducerea volumului comertului romanesc. Mai apoi taberele beligerante s-au infruntat in Romania si in plan economic. Erau la mijloc importante rezerve petroliere si productia de cereale a tarii ce ar fi intarit potentialul uneia sau alteia din taberele aflate in razboi. in vederea continuarii inzestrarii armatei romane cu armament modern Antanta a

[Graham Haddock]

Justin:

From your questions, you seemed to have an over-simplified view as to how the clocks are generated in the Sitara.

It is not your grandfather's PIC.  

It is a very modern, sophisticated, clock system, with many of the clock subsystems being variable rate.

A "Clock Tree" is the generic term for the way all of the internal clocks are generated. If you draw a block diagram, it starts to look like a tree that branches out from the trunk and lots of the branches have more branches.

Just because they are derived from a single (24 MHz) clock source, does not mean that they are synchronous, or even have a constant relationship.  Lots of them are variable rate, such as the CPU, and almost all of the communications peripherals.  And then everything is complicated by low power modes and sleep modes.

The PRU main clock does appear to be a constant 200 MHz.

If you want to dig into the details, they are covered in the "AM335x Technical Reference Manual".

What I call the clock tree is page 1136, Figure 8-10.  Detail on the branches follows on the next ten pages.

--- Graham

==

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 n perioada neutralitatii situatia economica a tarii s-a inrautatit. Mai intai  inchiderea stramtorilor Bosfor si Dardanele  dupa intrarea Imperiului Otoman in razboi de partea Puterilor Centrale  a dus la reducerea volumului comertului romanesc. Mai apoi  taberele beligerante s-au infruntat in Romania si in plan economic. Erau la mijloc importante rezerve petroliere si productia de cereale a tarii  ce ar fi intarit potentialul uneia sau alteia din taberele aflate in razboi. in vederea continuarii inzestrarii armatei romane cu armament modern  Antanta a

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