UART4_RX (pin L48) connection

Robotics & Edge Computing Jetson & Embedded Systems Jetson AGX Xavier
1

Hi

I’d like to ask about signal UART4_RX (pin L48)

According to Product design guide (page 71, chapter 12.3) : “UART4 pins do not support UART functionality and UART4_RX pin is reserved and must be tied to GND through a 10kΩ resistor”

But in the carrier board reference schematic it’s connected to GND via 0R

We have it actually at the moment floating in our design and it also seems not to cause any problem.

Please what is behind this signal in SoM module? What can happen when we don’t have the recommended 10k (ie. we use 0R or left it floating)?

Many thanks
Daniel

2

Actually when double checking the 10k is used in your schematic P2822_B03_Concept_schematics.pdf , but 0R is used in P2822_B03_OrCAD_schematics.pdf

3

It is also 10k in B03 OrCAD schematic. And you can check the BOM for R955 in DLC too. Please just follow guide not to change UART4 design.

4

Could you please provide little more detailed information on the impacts if this pin is left floating in a first prototype?

5

…well in the P2822_B03_OrCAD_schematics.pdf dated 21-Aug-2018 that I’ve downloaded from NVidia webpage is R955 = 0R

Anyway, I’ve also like to ask more details about possible impact when pin is left floating. We also forgot to connect it in first sample of our HW :-(.

6

It is a chip related request, MUST be followed. If the board can’t be changed, you need to program this pin to pull-down as early as possible.

7

OK, thanks a lot for help

BR
Daniel

8

Hi,

regarding this topic, it seems that it is not possible to configure the L48 pin through the excel sheet Jetson_AGX_Devkit_Pinmux_Configuration_Template_23_05_2019.xlsm.

However, in the devkit we checked the value of the register 0x02434030, which seems to hold the pinmuxing configuration of that pin, and the default value is 0x415, where the bits 3:2 indicates that it sets a pull-down resistor. Please, could you confirm that having this value in that register is enough?

9

it is chip relative , the external PD is must. internal PD is not useful in this case.

10

Hi

Unfortunately we don’t have possibility to use external pull-down (signal is not accessible on the board) => what implication has it when this pin is floating? (ie. what features are affects, etc.? )

Many thanks
Daniel

11

Chip might be damaged if no pull-down.

12

By what mechanism can be the chip damaged in absence of pull-down?
I can imagine that it can deliver some false input into the SW by floating signal, but it’s bit hard to picture how chip can be damaged when there is absence of voltage/current on this pin?

Btw. in the Jetson_AGX_Series_DevKit_Pinmux_Configuration_Template_1_06.xlsm is this pin described that it has default 100k pull-down at reset state. Is this information not accurate?

Thx
Daniel

13

The damage mechanism is out of discussion range of forum, please just follow that. Regarding the pinmux setting, programming function like pull down takes time to execute, the damage might happen before that. So if can’t change board, programming this pin to pull-down as early as possible can only reduce not eliminate risk.