Verdin Carrier Board Series Resistors Advice

Technical Support
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Hello - I’m designing a carrier board for a Verdin IMX8MP module. I’m looking at the series resistors sprinkled across SODIMM pins on the reference carrier board and trying to figure out if I need these, for EMI, backfeeding, impedance matching reasons, etc.

The IMX8MP being a typical IMX SOC - has slew rate and drive strength settings, albeit they make it very hard to determine the impedance of the output driver for things beyond DDR - and merely specify the maximum frequency each supports.

I was wondering what would motivate the many 22R series resistors on the carrier board - is it because the thought is - some modules may not offer drive strength setting? Are some of the resistors placed closer to the output of a module rather than at the SOM? Is it a waste for me to do the same as the carrier if I intend to choose a SOC module which allows for software control for EMI -or would it be wise to keep them on to limit backfeeding current?

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Hello,

In the design of carrier boards for Verdin modules, including the Verdin iMX8MP, the inclusion of series resistors on certain signal lines serves multiple purposes, such as EMI reduction, impedance matching, and protection against backfeeding. These resistors can help in managing signal integrity, especially in high-speed interfaces, by mitigating reflections and ensuring the signal edges are cleaner, which is crucial for reliable communication.

The decision to use 22R series resistors, as seen on reference designs, is influenced by a combination of factors:

  1. EMI Reduction: Series resistors can help in damping signal ringing and reducing the emission of electromagnetic interference, which is particularly important in designs that need to meet strict EMI/EMC requirements.

  2. Impedance Matching: While the iMX8MP SoC allows for some degree of control over drive strength and slew rate, these settings might not always perfectly match the impedance of the PCB traces or the connected peripherals. Series resistors can be used to fine-tune the impedance path, reducing reflections.

  3. Backfeeding Protection: In some cases, series resistors can also help in limiting the current that might backfeed into the SoC pins, offering a layer of protection against potential damage.

The placement of these resistors, whether closer to the SoC module’s output or elsewhere on the carrier board, is typically determined by the specific requirements of the signal in question, such as its speed, function, and susceptibility to noise. For high-speed signals, placing the resistor closer to the signal source can be more effective in managing reflections and EMI.

Regarding whether it is wise to include these resistors in your design, even if the SoC module allows for software control over EMI, it’s generally advisable to follow the reference design practices, especially if your carrier board will interface with a variety of peripherals or operate in environments with stringent EMI/EMC requirements. The software control over drive strength and slew rate is valuable, but it might not fully substitute the physical layer design considerations, such as series resistors, for signal integrity and protection purposes.

In summary, while the iMX8MP SoC provides some flexibility in managing signal characteristics through software, incorporating series resistors in your carrier board design can offer additional benefits in terms of signal integrity, EMI reduction, and protection. It’s a balanced approach, considering both the electrical characteristics of the signals and the practical aspects of PCB design.

Best regards,
Toradex AI Assistant

Please, also check if these links can help you solve your problem:

Verdin Family Specification | Toradex Developer Center.

3

Unfortunately, it’s hard to describe the purpose of the resistors’ placement without knowing the specific carrier board and resistors you are referring to. In most cases, 22Ω serial resistors are placed to protect module pins from short circuits and overvoltage.

The Dahlia Carrier Board and Verdin Development Board are designed for prototyping, where many user errors can occur, so they include many such resistors. In contrast, the Yavia Carrier Board, which is designed to be used primarily as a single-board computer, has almost none of these resistors.

In any case, we recommend following our reference schematics as depicted in our Carrier Board Design Guide.

Since SOM stands for System on Module, I am having trouble understanding your question. Could you please clarify?

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The carrier board here would be the Verdin Carrier Board 1.1 and the Verdin Carrier Board Design Guide schematics. For an example, on Page 65 of the Verdin carrier board design guide there are 22 ohm series resistors for the CAN bus - even though there is a CAN transceiver after it (and not some unknown user peripheral). My thought here is that the assumption is that there may be an SOC module that doesn’t have drive strength controls, so a carrier board should design around this. But if you do have drive strength - it’d be better to use the on-die ones than an external one. I2S interface also recommends 22 ohm resistors in the design guide - but if the SOC has drive strength - once again it would seem to be better to save on the components and use the built in impedance controls.

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Sure, if you wish to save one tenth of a cent, you can omit the mentioned resistors

I assume you are talking about the Verdin Development Board v1.1.
To avoid any ambiguity, please note that Mallow, Yavia, and Dahlia are all Verdin Carrier boards.

6

Yes, I mean the Verdin Development Board 1.1 - apologies.

I’m not worried about saving 1/10 of a cent, I’m worried about board space, routing time, board repair (we may have like 60 of these resistors, they need to be small, and we sometimes need to do component level repair), protection in the event of a peripheral component’s IO line going short circuit (but 22 ohms seems pretty low for that?), and how EMI suppression is better done on-chip than externally.

The verdin carrier board development guide has a number of series termination resistors:

  • Page 56, the CSI MCLK line has a 22R resistor, but the I2C SDA SCL do not.
  • Page 65, CAN TX and CAN RX have 22R resistors going directly to a CAN isolator.
  • Page 68, I2S directly recommends putting 22R series resistors on the signal lines close to the signal outputs (but if we’re trying to suppress EMI, on-chip is always better)
  • There’s also the suggestion where the resistor is there to help measure backfeeding current on page 99.
  • The RGMI reference schematic on page 29 doesn’t have the 22 ohm resistors going to ETH2 MDC, MDIO, INT, but the Verdin Development Board does (despite it not being externally accessible).

I guess my questions are these:

  • Is the on-die drive strength/ slew rate control sufficient for EMI suppression for SOM-driven signals? Is it only included on example boards because some modules may not have this control?
    • Is the SD interface on the IMX8MP done through a level translator to enable 3.3V logic- meaning the drive strength control of the IMX8MP doesn’t have an effect and external ones are necessary?
  • Is 22 ohms sufficient and/or necessary for protection of the IMX8MP module in the event of an IO line driving a short to GND?
  • Is there another reason these 22 ohm resistors are done in these cases I’m missing?
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We didn’t care too much about EMI suppression while designing boards like the Verdin Development Board, since such boards will never be used in end-user products. The main purposes of inline resistors on high-speed lines like I2S and USB are to ensure signal integrity - reduce signal reflections, dampen high-frequency noise and ringing, limit current, and, to a lesser extent, reduce EMI. For backreading, significantly higher value inline resistors are required (like 1K), so such a method is not suitable for high-speed signals.

In most cases, 22-ohm resistors can be omitted with proper PCB layout design, but they can help mitigate issues arising from mistakes in layout design and/or PCB manufacturing issues.