AM6442: Termination resistors at clock input ?

Hello Geoffrey,

No, the AM64x PCIe reference‑clock pins (SERDES_REFCLK0P/N) do not contain internal termination. The reference‑clock circuitry is left open‑ended, so you must provide the standard 49.9 Ω series termination (or the appropriate external termination network) for the HCSL clock coming from the PCIe connector. For connecting the unused SERDES0_REFCLK0P and SERDES0_REFCLK0N pins, see the Pin Connectivity Requirements table recommendations of the AM64x Datasheet. Optionally, place 50Ω (49.9Ω) resistor at the clock output terminals (P and N) with respect to ground near to the processor and provision for a test point for internal board level testing.

Please refer below inputs related to internal termination for the SEDES0 clock:

(+) [FAQ] AM6442, AM6441, AM6422, AM6421, AM6412, AM6411, and AM2434, AM2432, AM2431 (ALV) Custom board hardware design - SERDES0 interface - Look Processors forum - Processors - TI E2E support forums

Look for: 

Inputs related to PCIe REFCLK and internal termination support, configuration:

Best Regards,

Borislav Lazarkov

Hi Borislav, 

Thank you. Before my customer moves forward and invest time and resources I need to double check this with you.

They found some threads on the TI forum that designers have difficulties to drive the PCIe clock to the AM64 and that they measure 50Ω integrated termination resistors :

• https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1447455/am6442-pcie-clock-level-not-high-enough-internal-resistors?tisearch=e2e-sitesearch&keymatch=AM6442%2520HCSL
• https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1444678/am6422-distorted-reference-clock-on-serdes-refclock-input/5555631#5555631

In addition, the customer asked about the compatibility LP-HCSL driver with AM64, but didn’t received a very clear answer:

• https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1582238/am6442-am6442-hcsl-vs-lp-hcsl-pcie-refclk

So do you confirm there is no doubt and they can safely design taking into account no internal resistors, and that LP-HCSL clock driver can be used to feed the AM64 ?

regards

Geoffrey 

Hello Geoffrey,

Give me 1-2 days to find more information about your issue.

Thank you for your patience!

Best Regards,

Borislav Lazarkov

Hello Geoffrey,

Geoffrey Ficara said:

In addition, the customer asked about the compatibility LP-HCSL driver with AM64, but didn’t received a very clear answer:

• https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1582238/am6442-am6442-hcsl-vs-lp-hcsl-pcie-refclk

I submitted an E2E to the LMK3H0102 team and below was the reply i shared with customer

(4) LMK3H0102: Does LMK3H0102 in LP-HCSL mode compatible with AM64 PCIe REFCLK input? - Clock & timing forum - Clock & timing - TI E2E support forums

Regards,

Sreenivasa

Hi Sreenivasa, 

If I understand well, the AM64 doesn't contain internal termination, however the LMK3H0102 support LP-HCSL with internally selectable 85 Ohm or 100 Ohm which means there is no need populate the external termination resistors as you would for the HCSL format. 

Do you confirm that we could connect both AM64 and LMK3H0102 without external termination resistors (further tests are required ovisouly)? 

Thanks, 

Geoffrey

Hello Geoffrey,

Thank you. 

Geoffrey Ficara said:

If I understand well, the AM64 doesn't contain internal termination,

We may have to update the initial answer. The SERDES0 contains internal terminations.

(+) [FAQ] AM6442, AM6441, AM6422, AM6421, AM6412, AM6411, and AM2434, AM2432, AM2431 (ALV) Custom board hardware design - SERDES0 interface - Processors forum - Processors - TI E2E support forums

Search for:

Inputs related to PCIe REFCLK and internal termination support, configuration:

We did perform some measurements to confirm. Refer below:

 AM6442: Resistor selection for AM6442 internal PCIe Refclock 

Regards,

Sreenivasa

Hi Sree, 

Ok so AM64xx does contain internal termination. 

Does it mean that we could connect both AM64 and LMK3H0102 without external termination resistors ? 

Regards

Geoffrey 

Hello Geoffrey,

Thank you. 

Geoffrey Ficara said:

Ok so AM64xx does contain internal termination. 

That is correct.

Geoffrey Ficara said:

Does it mean that we could connect both AM64 and LMK3H0102 without external termination resistors ? 

This is correct. You also have an option for disable the internal termination. The implementation has to be done by customer. 

The information on the termination is provided in the FAQ shared above.

Regards,

Sreenivasa

Dear Sreenivasa,
(For your information, I'm also in touch with Geoffrey on this topic).
Can you say how to disable the termination resistors ? I don't find anything related to this in the reference manual.
Thanks,
Thomas

Hello Thomas,

Thank you. 

As i understand we do not have the support.

The information on the required configuration is added to the below FAQ:

 [FAQ] AM6442, AM6441, AM6422, AM6421, AM6412, AM6411, and AM2434, AM2432, AM2431 (ALV) Custom board hardware design - SERDES0 interface 

Regards,

Sreenivasa

Dear Sreenivasa,
We have been discussing this case internally and some of our expert found that there is an item related to this in the device errata:
https://www.ti.com/lit/pdf/sprz457

i2306 mentions that the internal resistors are enabled by default, but it seems this has been reversed on Rev 2.0:

Hence it seems possible to use a LP-HCSL clock to feed the ref clock for Rev 2.0 parts.

It would be helpful to have a clear statement on the status from TI to avoid any confusion on what is actually implemented at the end and what should designers account for.

Have a nice day,
Thomas

Hello Thomas,

The Errata seems to be regarding the boot.

The internal terminations are enabled during power-up and we do not have a implementation to disable the internal terminations.

The E2E FAQ captures some steps for customer to implement.

Regards,

Sreenivasa