You Don’t Have to Get Carried Away with Complexity to Integrate the Pi CM4 in Your Project!

Thanks to its widespread adoption in schools, makerspaces and myriad other places, there's an increasing chance that a given person will be able to recognize the nearly now ubiquitous form factor of the Raspberry Pi. With its arrangement of ports and credit card size, it has almost become the de-facto form factor in the single-board computer hardware world, and a growing number of the general public would these days likely even recognize it from various news appearances and other such promotions. Successful, in the public eye, would be an understatement. While we have seen countless projects based around the fully-featured Pi form factor, fitted with all the ports needed for out of the box functionality, we far less often see projects based around the Compute Module product offering from the Pi Foundation. The Compute Modules have been kicking around for a few generations now, with the first offerings originally based on a SO-DIMM edge connector system — the same as you'd normally see used to socket RAM modules in a laptop computer. The idea of the Compute Module is that it gives designers a cleaner path to embedding the processing power of the Pi, without needing to accommodate bulky connectors and other things that might not be needed in their application — saving space, and the associated costs. It was the most bare-bones of breakout boards for that Broadcom chipset, barely more than the BGA itself. I'm not aware of too many hobbyist projects that made use of the Compute Modules. I can think of a few suggestions why — from memory, that 200-pin SO-DIMM connector is tricky enough to solder properly without melting in a reflow oven, never mind with a hand iron. Then, the challenge of correctly breaking out the required signals from a 200 pin connector, in such a small area was quite a challenge — without resorting to a four-layer board to do so! Single-sided simplicity The most recent version of the Raspberry Pi Compute Module seems to be somewhat more hobbyist friendly in the eyes of some, and perhaps to demonstrate just how straight forward it is to integrate this quad-core 64-bit beast into any project — @timonsku has shared with us his latest design — a super simple, single-sided CM4 carrier board! This is an impressive bit of home lab work, but more so it is a striking example of how simply one can integrate the re-designed compute module — I think many will agree that integrating the older hardware wasn't as straightforward a task as we would have liked... SO-DIMM? That's SO 2017! The 200-pin SO-DIMM connector has been replaced by a stacking board connector system, a pair of the Hirose DF40C-100D , and while it has a somewhat smaller contact pitch than SO-DIMM — 0.4mm vs 0.6mm respectively — the design of the contact blade, and how it sits within the connector housing is much more forgiving when it comes to the potential stresses it might see during hand assembly. We can see from the connector drawing above that there is a good amount of plastic to support contacts, this should be a benefit to anyone looking to hand solder one of these in place — there's far less chance that an errant iron movement will drag one of the connector terminals out of line, when compared to the fragile SO-DIMM connector contacts. Pin to pin placement With the CM4 module removed, we can take a bit of a closer look at the connector pinouts — and show that there's no magic trickery or tracking hidden out of sight underneath the SoM! There is some evident consideration that has gone into the pinout of the SoM connectors. Spaced as far from each other as physically possible, these 2, 100-pin connectors are what stand between the CM4 and your next application — so we can see that some effort has gone into selecting the right physical pin positions within the connector sets, to make tracking as simple as possible to implement. The most obvious example here being that of the HDMI signals, detailed below. You couldn't ask for much more than a nearly 1:1 pin mapping. If we dive into the datasheet for the official Pi CM4 IO board, we can see that the connector pinning really does try to make things easy for anyone looking to integrate the module into their product. First and foremost, there is a clear division of function between the pins assigned to the two different stacking connectors. Let's take a look at the pinout of the high-speed signal set, pictured in the board detail above. The fact alone that we see here that HDMI connector is practically a 1:1 direct mapping to the connector pins, when orientated as expected (this pin ordering again being followed by the second HDMI pin group) shows some real efforts from the Pi Foundation to simplify the design of hardware looking to host the CM4. These connectors are specified by Hirose for 10 Gbps, while still being within the realms of hand assembly with home lab tooling. If they look a little familiar, you might have seen them previously on a diminutive development board of days gone by. These are the same series of Hirose connector series that found their way into the ill-fated Edison range of tiny embedded Linux compute modules, from Intel. Despite the short lived production life of the Edison, the modules I have are still reliable — the connectors have stood the test of time! If we peek on over to the other side of the Compute Module, and take a look at the pins and placement of the signals routed to it, we can see that this set of signals more closely resembles the sort that we'd expect to see on the 40-pin GPIO connector, found on the fully-grown Pi form factor. With all of the finely balanced high-speed signals located on one connector set, and the GPIO, SD card interface and power located on the other side of the board, this should help keep noisy switching DC/DC converters and GPIO-driven servos, etc. away from the more noise-sensitive signals. It's not often I get to wrap up an article in such a short fashion! But, for the sake of a set of connectors, there really is little more else to this super-simple carrier board — and that's the intent behind the project! Timon has shared his EAGLE project sources for this board over on his GitHub, and it is a solid starting block for anyone looking to design for this module. If you're a KiCAD convert, fret not, it'll import this project without a hitch! We can see the EAGLE board file as-imported straight into KiCADs Pcbnew without issue. If you had any lingering suspicion that there simply must be more to this SoM support board than a single sided PCB, well... The layout speaks for itself! Either if for the precise mechanical placement of the connectors in the board file, or the symbol pinning alone of those 100-pin Hirose DF40's, this repo and the files that @timonsku has shared with us are a worthy starting resource for anyone looking to get compos mentis with the Compute Module 4. A word of note, however. Anyone looking to use this project as a base for a design destined to fully demonstrate the capabilities of the CM4 module would do well to exercise due diligence in drawing all those differential pairs properly. HDMI is apparently a forgiving beast, having allowed many a signal-integrity violation across our editing desk on occasion, but despite that, this board won't be getting the certification to carry the consumer-known HDMI logo any time soon! Despite that, we can see that for the purposes of prototyping and personal use, that's not going to be any issue — it might be the worlds simplest SBC carrier, but it works! For all the latest tinkering and tech from @timonsku , be sure to follow him on Twitter.The Raspberry Pi 4 Model B (📷: Gareth Halfacree) Raspberry Pi Compute Module (📷: Raspberry Pi Foundation, CC BY-SA 4.0) Everything a CM4 needs, on one layer! As bare as barebones gets. Clean. Fast &Furious: 10.0 Gbps A previous perversion in the pursuit of pushing pixels.

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This content was originally published by Tom Fleet at Hackster News -, and is syndicated here via their RSS feed. You can read the original post over there.

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