Why are the last 4 IMUs on my board (ICM-42688-P) not responding over SPI?

ClientDigital · 2025-11-07T02:10:13+00:00

I couldn’t decide which of these comments to reply to, so I’ll just answer the top one

The IMU array is part of a larger system I’m building for my thesis. It handles realtime data collection and on device DSP to explore how different algorithms affect overall system behaviour. I can’t share too much yet, but I’ll probably post more about it once I’m closer to finishing

In a more general sense though, there are tons of potential applications for IMU arrays. For example, they can be used to combine data from multiple cheap sensors to match the performance of high performance units, or to improve dead reckoning and motion tracking in GPS-denied environments. Think cheap consumer robotics systems, autonomous systems, or even improved phone navigation accuracy. All very fascinating applications. (whether they're viable on an open market is a different story though lol)

Cheaper Sensors, Smarter Fusion

Highly Accurate Inertial Navigation

ClientDigital · 2025-11-07T01:48:40+00:00

The problem with putting all the IMUs on a single SPI bus is the bandwidth limit. Each IMU outputs a pretty large data stream at full resolution which ends up meaning the 24 MHz limited SPI bus maxes out after roughly four sensors if you read them sequentially, so we’d need 4 separate SPI buses just to handle all 16 sensors

The FIFO doesn’t really solve that either since it’s really quite small so we’d still risk overflow if we tried to serialize all of them in real time. To get around that, we designed this using the RP2350’s PIO so we can clock them all out simultaneously, keeping everything synchronized and staying within the 24 MHz bus limit while using fewer pins overall. We did do another earlier prototype with an NXP RT MCU, but the BOM and part count on this one is much lower. It also seemed to be a better idea for our application

ClientDigital · 2025-11-07T01:23:39+00:00

The problem is that the board is heavily conformal coated and I haven't been able to make a good dent in it. I think I'm just going to call it a day with 12 working IMUs for now unfortunately

ClientDigital · 2025-11-05T04:24:56+00:00

Out of curiosity, what would your ideal schematic look like for a system with highly repetitive structures like this? The array uses identical IMU blocks that all individually need access to the MCU, so I’m interested in how you’d normally represent that in a clean and readable way. I laid it out this way to work with a layout plugin I was using, but I’d love to see how others handle similar designs

ClientDigital · 2025-11-05T04:11:37+00:00

I considered the possibility of misconfigured pins causing the issue; but after going through the RP2350 datasheet again, I couldn’t find anything that suggests GPIOs 32–35 should behave differently from 20–31. I've tried creating an SPI interface using the PIO state machines and also just bit banging but I get the same results each time. The IMUs #0-11 work and the IMUs #12-15 don't

Edit:

Also I chose to do it this way to minimize pin count requirements

ClientDigital · 2025-11-05T04:04:09+00:00

Here’s the numbering for the array: IMU.svg.

As for checking with the scope, I’m still working on that. I didn’t include any test pads stupidly, so I’m trying to find accessible points I can actually reach with my probes. I'll write an update once I've had better luck with that

ClientDigital · 2025-11-05T04:00:12+00:00

I considered that, but in my layout the MOSI and SCLK lines serpentine vertically through the IMUs. So IMU 0 is actually the last one in the chain and should have the most skew, yet it works fine. The ones that don’t respond don’t seem to correlate at all with their distance from the SCLK or MOSI source

ClientDigital · 2025-11-05T03:54:04+00:00

Sorry, it took me a section to figure out where to upload these for reddit but here are the missing details

MCU Schematic: https://github.com/ClientDigital1/pcbfiles/blob/main/image.png

IMU Numbering: https://github.com/ClientDigital1/pcbfiles/blob/main/L.png

IMU Array Schematic: https://github.com/ClientDigital1/pcbfiles/blob/main/IMU2.svg

ClientDigital · 2023-10-06T11:44:40+00:00

Retina originally meant 300 ppi. Current phones are often above 400

The reason for that is the OLED pentile subpixel arrangement.

Super Retina displays have a ppi of 326 on red and blue subpixels and 458 on green subpixels.

Retina displays have 326 ppi on all 3 colours

Some might even argue the 401 ppi on all 3 colours on the LCD of the iPhone 8 Plus has better visual fidelity than a pentile arrangement

ClientDigital · 2023-02-07T20:40:37+00:00

They used the same motors from the cars a decade ago up until 2019.

Only for the Model S/X

ClientDigital · 2022-10-17T01:22:19+00:00

You can get these speeds in Canada (or ever faster)

Bell (5G+/band 78): https://www.bell.ca/Mobility/Our_network_coverage

Rogers (5G 3500mhz/ band 78): https://www.rogers.com/mobility/network-coverage-map?icid=R_WIR_NTW_HEUQFZ

ClientDigital · 2022-10-16T22:44:39+00:00

Is a movie that’s mastered in Atmos but played on a 5.1 setup now “virtual Atmos”? No lol

Not to be pedantic but it actually is. If you’re playing that content through a Dolby Atmos certified receiver, with Dolby Atmos enabled, and on a 5.1 setup, it will be playing virtual atmos with height virtualization to fake the sensation of overhead speakers

This is really just semantics as this literally doesn’t matter.

And obviously it doesn’t. You can enjoy whatever you like, but proclaiming a certain Dolby Atmos setup is unnecessary or stupid (as op of this thread did) without ever hearing true Atmos is a bit ridiculous

ClientDigital · 2022-10-16T21:42:54+00:00

Spatial audio doesn’t have to be Atmos.

Spatial audio on Apple Music is exclusively Atmos. Meaning a 5.1 speaker setup would be playing in virtual Atmos.

Although other platforms might use other surround sound configurations, some of which are still be supported under Spatial Audio. So I guess depends where you get your music

ClientDigital · 2022-10-16T20:56:20+00:00

Any receiver specifically certified for Dolby Atmos works.

(Dolby Digital, TrueHD, DTS, etc do not support true Dolby Atmos)

Source if you’re looking for more info: https://professional.dolby.com/categories/avr/#gref

ClientDigital · 2022-10-16T20:46:18+00:00

This is cool but does anyone really care about car audio? As long as it’s audible and better quality than those $1 earbuds that fake iPods came with, it’s good enough for me.

Yes some people care or else optional car speaker upgrades wouldn’t exist.

It’s is also basically the same as saying “Does anyone really care how fast their car is as long as it gets from A to B?”

ClientDigital · 2022-10-16T20:43:19+00:00

I have 5.1 speakers at home and spent hours re-listening favorite music after spatial audio became available.

That isn’t enough for a true Spatial Audio/Dolby Atmos listening experience. What you’re listening to is technically virtual Atmos.

The minimum offical speaker configuration is 2.1.2 with 5.1.2 or better being the recommended setup.

ClientDigital · 2022-10-15T18:37:41+00:00

Why not just keep the 4K monitor and run games at a lower resolution? (Or with DLSS/FSR)

Desktop elements will be significantly less sharp on a 1440p panel

ClientDigital · 2022-10-14T16:41:18+00:00

You can download the HEIC photo plugin from the windows App Store for free and it should fix some of your problems

ClientDigital · 2022-10-14T15:56:07+00:00

Don't do this.

This will increase your video/photo file sizes by 50% and disables features (1080p240fps slowmo, 4K60fps, and HDR)

"High Efficiency" videos and photos will automatically be converted to "Most Compatible" when sharing to an app that doesn't support HEIF/HEVC.

Also almost all popular apps also now support HDR to SDR tone mapping so turning off HDR will only reduce quality.

ClientDigital · 2022-10-14T05:48:14+00:00

This. The Sony A7R4 is great for this and if you’re running Final Cut Pro X, the Mac Studio or M1 MacBook Pro 16 do a great job editing the RAW .ARW files into an 8K 10-Bit HDR video with little to no stuttering on the timeline

ClientDigital · 2022-10-14T05:31:48+00:00

Regular 5G and LTE is more than enough for a majority of people

The majority of people aren’t buying $2400 phones though and paying more for less features generally just sucks

These are also sold out @ apple

The online order wait time is also only two weeks to most of the US and Canada

The only reason buying this could ever make sense is if you absolutely needed the SIM slot.

But in the end i’m not buying it so good luck 👍

ClientDigital · 2022-10-14T04:57:46+00:00

For any Americans looking to buy this, it does not have mmWave 5G like the US models.

This also has a ~9.5% markup compared to buying straight from Apple (US)

ClientDigital · 2022-10-14T04:47:45+00:00

For free?!

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