Proper Termination of AM335x VSS_RTC pin

Ayus said:

Hello,


In AM3359, If the  OSC1（RTC_XTALIN, OUT) is not used, how to terminate the unused VSS_RTC pin?
is it ok to connect VSS_RTC pin to GND?

Regards.

The operation of the RTC is independent of the use of the OSC1 clock source. It can be run off an internal RC oscillator or from other internal clocks. The VSS_RTC pin appears to be intended as a ground for the RTC sub-system, not specifically related to the OSC1 oscillator. The datasheet and TRM cast serious doubt in my mind over the way VSS_RTC  has been wired on the EVM schematic. Figure 4-8 in the datasheet for OSC1 wiring does not show VSS_RTC used as a floating ground as is the case with Figure 4-6 for the OSC0 wiring, where VSS_OSC is used as a virtual Kelvin ground. The datasheet specifies VSS_RTC as type "Ground" in Table 2-7 while VSS_OSC is given type "Analog".

Everything I have read suggests that VSS_RTC should be tied to board GND and not be left unconnected (or used as a virtual earth as it is on the EVM)

Some clarification from TI would be helpful.

Regards Roger.

Hello Roger,

Thank you for the information.

As per your suggestion VSS_RTC pin to GND would be answer.

Just in case, I would like to request TI people to clarify this.

Regards.

You are correct.  The RTC oscillator (OSC1) does not have a Kelvin ground pin like the high frequency system clock oscillator (OSC0).  A Kelvin ground is provided for the external OSC0 components to help reduce clock jitter since this clock may be used to source jitter sensitive peripherals.  The RTC is not sensitive to clock jitter so a Kelvin ground pin was not provided.

The VSS_RTC terminal is the RTC logic ground so it should always be connected to the PCB ground.

Regards,

Paul  

Hi Paul, thank you for your answer.

The BeagleBone and EVM schematics show VSS_RTC, not connected to PCB ground, but only used as a ground for the crystal. Can you confirm that these schematics are therefore not correct and should not be copied in customer designs.

Regards,

Roger.

Opps!  My last reply was not correct.

I remember removing the RTC oscillator Kelvin ground, but forgot it was only removed in the ZCE package not the ZCZ package.  VSS_RTC is the OSC1 Kelvin ground terminal.

 I forgot about this when drawing the three OSC1 figures in the AM335x data manual.  I will need to correct these drawings to show the VSS_RTC terminal for the ZCZ package.

 The RTC logic ground is connected to VSS on the package substrate.

 I also need to change the Ball Characteristics Table type from “GND” to “A”.

So the BeagleBone and EVM schematics are correct.

Regards,

Paul

Hi Paul, I think that's clear now.

As I now understand it on the ZCZ package, both the main oscillator, OSC0, and the external RTC oscillator, OSC1, have floating Kelvin ground terminals, VSS_OSC and VSS_RTC respectively. Can you please recommend how to wire VSS_RTC when OSC1 is not being used.

Regards

Roger

 Each Kelvin ground terminal should be only connected to the respective external crystal components as shown in the OSC0 Crystal Circuit Schematic data manual figure when a crystal is connected to the respective oscillator.

Each Kelvin ground terminal should be connected to the PCB ground when an LVCMOS digital clock source is connected to the respective oscillator.

When not using the RTC oscillator (OSC1) the VSS_RTC Kelvin ground terminal should be connected to the PCB ground.

Regards,

Paul