AM335x Spread Spectrum Clocking

Daisuke Maeda

Guru 20345 points

I checked the related threads:
https://e2e.ti.com/support/arm/sitara_arm/f/791/t/359417
https://e2e.ti.com/support/arm/sitara_arm/f/791/p/198104/1637664#1637664

Spread spectrum clocking (SSC) is supported on output clocks of the on-chip DPLLs, but it is not supported for DDR, PER, DISP, and CORE PLLs.

An LVCMOS square-wave digital clock source with SSC can be connected to OSC0, but it seems to be very difficult to meet the reference clock requirements for OSC0.

In BeagleBone Green, OSC0 is connected to SSCG. I cannot clear whether this SSCG meets the reference clock requirements for OSC0.

http://www.seeedstudio.com/wiki/File:BEAGLEBONE_GREEN_V1.pdf

Our customer designs the medical equipment. It is required to reduce EMI.

Please tell me the better solution to reduce EMI in clocking.

Best regards,

Daisuke

over 9 years ago

0 Biser Gatchev-XID over 9 years ago

TI__Guru**** 393215 points

Hi,

BeagleBone Green is not a TI product or approved reference design. For reduction of EMI I can recommend standard PCB design practices:
- Design multilayer PCBs.
- Have large solid GND planes on at least 2 layers.
- Fill unused areas on all layers with solid GND. Stich together all GND areas with a lot of vias, especially around the PCB edges.
- Bury HF signals on internal layers.
- Provide external shielding if necessary.

0 Daisuke Maeda over 9 years ago in reply to Biser Gatchev-XID

Guru 20345 points

Hi Biser-san,

Thank you for your reply.

I recommended using the internal oscillator clock source rather than a digital clock source to the customer.

Best regards,

Daisuke

0 Thomas Langhammer over 9 years ago

Intellectual 580 points

This circiut connects the SSCG output (3.3V level) directly to the OSC0_IN input of the AM335x, which is only 1.8V level and not 3.3V tolerant. So I would expect this cicuit not to work stable, because it will damage the OSC0_IN input of the AM335x.

0 Daisuke Maeda over 9 years ago in reply to Thomas Langhammer

Guru 20345 points

Hi Thomas-san,

Thank you for your advice.

I am disappointed that BeagleBone Green is not really approved reference design.

Best regards,

Daisuke

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AM335x Spread Spectrum Clocking