AM625: PRU Clock Speed

John Tobias

Part Number: AM625
Other Parts Discussed in Thread: SK-AM62

Tool/software:

Hi,

I am working on SPI bitbanging using AM625 Evm board using GPI Direct Input mode. The SPI master is streaming data continuously at the frequency speed of 12.5Mhz. I had an issue - I couldn't get all the bits.

Then, I started some testing. According to the TRM the clock speed of AM62x is 333Mhz.

So, I tried to toggle a PRU pin (P22 - GPMC0_AD5.PR0_PRU0_GPO5) using this code:

    .sect ".text"
    .global main

main:

MainLoop:

    SET r30, r30, 5         
    CLR r30, r30, 5         
    JMP MainLoop

When I scope the pin, I got 12.5Mhz for Frequency and 25Mhz for width. If the PRU is running at 333Mhz clock frequency, I should expect at least around 160Mhz for one cycle or toggle (high/low) of the said pin.

Then, I saw in SK-AM62 Starter Kit User's Guide that the PRU pins are low speed - "AM62x SKEVM has a 20 pin PRU Header which offers Low speed connection to the PRG0 Interface" and I was thinking maybe that's the real reason.

So, I tried the Beaglebone black (200Mhz clock speed) and to see if I could toggle the pin in much faster speed.

I configured the P8_12 as pruout using the command "config-pin P8_12 pruout" and I used the same code above but instead of using bit 5, I used the bit 14 and run it.

The clock frequency was 16.667 Mhz and 25Mhz for width.

My question is, how I can verify if the PRU is running the correct clock and how to set it correctly (333Mhz for AM62x or Beaglebone 200Mhz)?.

Regards,

John

3 days ago

0 Nick Saulnier 2 days ago

TI__Guru* 88125 points

Hello John,

"low speed" in this case just means basically "don't expect to use this for a 125MHz Ethernet protocol". I have been told that SPI should be fine.

We actually realized after SDK 10.0 that we were not setting the PRU clock properly by default - the clock setting in the Linux devicetree file was just a copy-paste of the default AM64x setting, and would throw a clock error. I filed a bug that we were planning to fix for the upcoming SDK 10.1. Let me check to see if the team actually resolved the bug, or if it is still outstanding...

The thread where this first came to my attention is here: https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1410928/am625-sdk-10-ti-sci-clk-recalc-rate-failed-for-dev-81-clk-20-ret--19

I'll also link you to another thread with the same question. António has put together a test program that is exactly how I have validated PRU clock frequencies in the past (wait N PRU clocks, then toggle the signal. Is the width of the signal pulse = 3/4/5 ns * N?): https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1450236/am625-help-setting-pruss-clock-to-333mhz-on-am6254

I'll also update the thread when I hear back from the developers.

Regards,

Nick

0 Nick Saulnier 2 days ago in reply to Nick Saulnier

TI__Guru* 88125 points

Ok, I asked the dev team to re-run the tests for this commit, and it DOES look like this gets rid of the clock errors on boot:
https://git.ti.com/cgit/ti-linux-kernel/ti-linux-kernel/commit/arch/arm64/boot/dts/ti/k3-am62-main.dtsi?h=ti-linux-6.6.y-cicd&id=34e6b1d215aae3e5d9c9d455edc943140ec99775

So you should be able to use this as a framework for changing the PRU clock frequency to 333MHz.

Please note that I expect the PRU clocking information presented in the Technical Reference Manual (TRM) is wrong. See the above thread for more details about how I expect the clocking to actually look like: https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1410928/am625-sdk-10-ti-sci-clk-recalc-rate-failed-for-dev-81-clk-20-ret--19

Apologies in advance that I will not be able to devote much more time to this question for the next couple of weeks. Please do keep me updated though - I am the author of the AM62x academy's multicore module (I'll link that here in case you are doing any M4F or DM R5F development as well: https://dev.ti.com/tirex/explore/node?node=A__AJa64F6ctzZNbb.TYx4mCA__AM62-ACADEMY__uiYMDcq__LATEST). In 2025 the team and I will be creating a new module for the AM62x academy called the PRU academy. One of the pages I want to add is modifying the clock frequency, so I am bookmarking this thread as a reference when we get to writing that page.

Regards,

Nick

0 John Tobias 2 days ago in reply to Nick Saulnier

Intellectual 325 points

Hi Nick,

After compiling the device tree and boot the device. I ran k3conf

$ k3conf dump parent_clock 81 14

|------------------------------------------------------------------------------|
| VERSION INFO |
|------------------------------------------------------------------------------|
| K3CONF | (version 0.3-nogit built Thu Jul 25 14:13:02 UTC 2024) |
| SoC | AM62X SR1.0 |
| SYSFW | ABI: 4.0 (firmware version 0x000a '10.0.8--v10.00.08 (Fiery Fox))') |
|------------------------------------------------------------------------------|

|--------------------------------------------------------------------------------|
| Clock information |
|--------------------------------------------------------------------------------|
| Device ID | Clock ID | Clock Name | Status | Clock Frequency |
|--------------------------------------------------------------------------------|
| 81 | 14 | DEV_ICSSM0_VCLK_CLK | CLK_STATE_READY | 250000000 |
|--------------------------------------------------------------------------------|

|-------------------------------------------------------------|
| Clock Parent information |
|-------------------------------------------------------------|
| Selected | Clock ID | Clock Name | Status | Clock Frequency |
|-------------------------------------------------------------|
|-------------------------------------------------------------|

So, the fix is to set the PRU clock to 250Mhz?.

Then, I ran the gpio toggle that I provided above and I scope it. I was still getting the same output 16.667Mhz for frequency.

Regards,

John

0 John Tobias 1 day ago in reply to John Tobias

Intellectual 325 points

Hi Nick,

I tried the patch and also tried the link you provided
https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1450236/am625-help-setting-pruss-clock-to-333mhz-on-am6254

I used the same code mentioned in the link.

 while (1 ) {
        __delay_cycles(100);
        __R30 |= 0x80;
        __delay_cycles(100);
        __R30 &= ~0x80;
    }

and I was getting a similar result (~1.63Mhz).

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AM625: PRU Clock Speed