TMS320F28379D: Floating Charge on No Connect Pins

Dajon McGill

Hey Team,

I have a customer that is using the TMS320F28379DPTPQ, which has 3 pins, FLT1, FLT2, and ERRORSTS, that are defined as 'no connect' in the datasheet.

For deep space application they are trying to avoid "floating metal" since it is possible that charge can build up over time. Our thinking is that if these pins are output pins, then they are fine, but if they are inputs with weak pull-up/down or other means of weak bias, then there could be a concern about the charge being able to change the state of the pin, and that could be bad.

What are these pins, and if the above risk is there, what can we do externally to pull them high or low or bias to something benign.

Best,
Dajon McGill

over 5 years ago

0 Vamsi Gudivada over 5 years ago

TI__Guru* 87155 points

Hi Dajon,

Thank you for approaching us on this. We will analyze this and get back to you before December 18th.

Thanks and regards,
Vamsi

0 Vamsi Gudivada over 5 years ago

TI__Guru* 87155 points

Hi Dajon,

I discussed with our design team. ERRORSTS pin can be pulled down on this device (not applicable for other devices - FYI for others that may refer to this post).

I will let you know on the flash test pins by the end of this week.

Thanks and regards,

Vamsi

0 Vamsi Gudivada over 5 years ago in reply to Vamsi Gudivada

TI__Guru* 87155 points

Hi Dajon,

Thank you for approaching me offline on this. I am still working on the flash test pin connection recommendation. I will get back to you by the end of this week.

Thanks and regards,

Vamsi

0 Jason Whiles over 5 years ago in reply to Vamsi Gudivada

TI__Expert 6100 points

Dajon,

In this case I would recommend tying FLTP1 and FLPT2 to VSS through a pulldown resistor. A 10kOhm value should be acceptable.

Best regards,
Jason

0 Dajon McGill over 5 years ago in reply to Jason Whiles

TI__Expert 8131 points

Jason,

So pulling FLTP1 & FLPT2 down with the same 10k resistor is okay and should negate any floating charge concerns?

Pulling ERRORSTS down as well should result in the same affect?

Thanks for the clarification!

Best,
Dajon McGill

0 Jason Whiles over 5 years ago in reply to Dajon McGill

TI__Expert 6100 points

Dajon,

I recommend separately tying down FLTP1 to VSS and FLTP2 to VSS through their own resistors.

Yes, pulling down ERRORSTS with its own pulldown resistor should also be fine. If current consumption is a concern, just ensure that the ERRORSTS is not in output mode driving against the pull resistor polarity. I don't think it is on by default but this can be checked in the TRM and software implementation (I apologize I don't have access to the TRM at this moment).

Best regards,

Jason

0 Dajon McGill over 5 years ago in reply to Jason Whiles

TI__Expert 8131 points

Jason,

Could you expand on why you recommend tying FLTP1 and FLTP2 down through separate resistors instead of shorting them through one?

Best,
Dajon McGill

0 Jason Whiles over 5 years ago in reply to Dajon McGill

TI__Expert 6100 points

Dajon,

We don't want these pins to be shorted to each other.

Best regards,

Jason

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TMS320F28379D: Floating Charge on No Connect Pins