IMX8QM Unused Power and Signal Strapping

[ibocakir_06]

Hi,

We are designing a custom board for IMX8QM processor. There are some unused hardware blocks in this project like HDMI, LVDS, MIPI-CSI, MIPI-DSI, PCIe etc. We want to be sure about the unused power and signal strapping.

For example,  according to IMX8HWDG Table 47 if HDMI-RX is not used the power pins are given below must be tied to ground. 

• VDD_HDMI_RX0_1P8 ---> GND
• VDD_HDMI_RX0_LDO0_1P0_CAP ---> GND
• VDD_HDMI_RX0_LDO1_1P0_CAP ---> GND
• VDD_HDMI_RX0_VH_RX_3P3 ---> GND

When we look at the Table 48, if HDMI-RX is not used the signal pins must be connected like below:

• HDMI_RX0_ARC_P/N ---> Leave Unconnected
• HDMI_RX0_CEC ---> Leave Unconnected
• HDMI_RX0_HPD ---> Leave Unconnected
• HDMI_RX0_MON_5V ---> Leave Unconnected
• HDMI_RX0_CLK_P/N ---> GND
• HDMI_RX0_DATAx_P/N ---> GND
• HDMI_RX0_REXT ---> GND

There is no problem with HDMI-RX block.

However there are some confusing about LVDS and MIPI-CSI or MIPI-DSI blocks. According to table 47, LVDS power supply pins must be connected to GND with 10kΩ resistor. 

• VDD_LVDS0_1P0 ---> 10 kΩ to ground
• VDD_LVDS0_1P8 ---> 10 kΩ to ground
• VDD_LVDS1_1P0 ---> 10 kΩ to ground
• VDD_LVDS1_1P8 ---> 10 kΩ to ground

There is a note for 10 kΩ to ground recommendation in the hardware design guide Page 78:

All balls can be connected and pulled down to ground as one group through one 10 kΩ resistor, or as multiple groups (according to ball map location) through multiple 10 kΩ resistors. If the associated supply is not powered, the signal balls can be left unconnected.

Question 1: With these explanation, we have shorted VDD_LVDS0_1P0(ball AV36) and VDD_LVDS1_1P0(ball AW35) and tied to ground with only one 10k resistor. Datasheet says that these power pins are in Group 1(in the same group). Is this safe? I have attached an image below.

image.png

There is an another note for unused signal pins for LVDS block in the table 48:

All balls can be connected and pulled down to ground as one group through one 10 kΩ resistor or as multiple groups (according to the ball map location) through multiple 10 kΩ resistors. This solution is applicable only if all power domains associated with each I/O group are off or tied to ground. If some of the domains remain powered, unused I/Os falling into those domains must either be left floating or tied to the ground separately, each signal through its own 10-kΩ resistor.

Note in the unused power pins says that, unused signal pins of unpowered hardware blocks can be left unconnected for LVDS. But the note just above does not mention any information leaving signal pins unconnected if the power pins is not supplied.

Question 2: Is it safe to leave unconnected LVDS signal pins if we tie LVDS power pins 10K the ground? I have attached the an image below.

ibocakir_06_0-1753262512046.png

Question 3: I have only given an example for LVDS. Can I apply these wirings to MIPI-CSI and MIPI-DSI blocks?

Need your help to finish the hardware design of this project. 

Thanks in advance.

İbrahim

i.MX8

[ibocakir_06]

Thanks for your reply. We will wire like below:

ibocakir_06_0-1754221515993.png

It will be hard to route each trace and place a resistor for each group. Anyway, which one is the safer method, we must do it.

Thanks again.

İbrahim

[JorgeCas · Accepted solution]

Hello,

1. Yes, it is safe.

2. No, each group or signal need to be connected to ground using a 10k resistor.

3. Yes, the guidelines for LVDS and MIPI signals are similar.

Best regards.