CM3 vs A04/A06 pin differences: 10 additional pins?

I’ve been studying the schematics and I noticed 10 pins on the SODIMM connector that are NC for the compute module. They are:

  • 124 - MICBIAS → (TRS Connector)
  • 126 - MICP → (TRS Connector)
  • 128 - MICN → (TRS Connector)
  • 130 - IGPIO0 → (PWRON from power IC)
  • 132 - IGPIO1 → (??)
  • 154 - HPOL → (Audio IC)
  • 156 - HPOR → (Audio IC)
  • 158 - AUD_SWITCH → (Audio IC)
  • 160 - USB0_DP → DB_P (USB IC)
  • 162 - USB0_DM → DB_N (USB IC)

All of these pins on the pi compute module are NC. So perhaps the A04 and A06 modules have more capabilities with access to these pins? At the very least, I think I can say that the pi won’t have audio input support from the TRS jack, but maybe the A04/A06 will?

Does anyone have insight into some of these pins? Seems like the main thing is that audio might be handled differently between the two types of boards.

EDIT: For reference here are the pins from the DevTerm schematics overlayed with raspberry pi’s pin documentation:


SEE!!! I didnt spell it out as well as you… all i really did was express interest in more scematics… and i got those… with the delay, i decided to upgrade from the a04 to the a06… the only things i know that are different about those 2 is the cpu, gpu and memory specs… it’d be nice to know the details yer pointing out… as i have no real idea what makes them different other than computer capability… a big facotr and major selling point, sure… but i my desire to know more has intensified. insert meme here

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Yeah I’d love to see the schematics for the A04/06 modules. These are custom SODIMM boards right? Or can they be purchased elsewhere like the pi CM?

I pre-ordered with the pi because it’s familiar to me, but I have no doubt I’ll pickup one of the other modules after launch. It’s neat they can just be swapped out!

My guess is that the only difference with these pins is how audio output is handled, and the pi not supporting mic input.

The other pins seem related to USB.

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i have searched far and wide as best I can… If there is any data on the modules themselves other than the devterm page… its hidden from me. the closest i’ve been able to get is detailed info on the cortex a* mail-t* chips… struggling to even find diagrams for those.


Closer inspection on the USB pins. It looks like the A04/06 has an additional native USB that directly connect the USB C port. Both the pi and A04/06 have usb data lines going to a usb hub IC (I think)… which I’m guessing is how the keyboard and other external usb work.

But without the A04/06 it I can’t find how the pi would connect the the USB C port. I’m probably missing something…


There’s a mark on USB-C charging port of DevTerm’s tech spec:

  • Depends on the “Core Module” features.

Could this mean that CM3 can’t charge the batteries?

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this is a great point. i mean in only ever trust using dedicated chargers for my batteries and i always recommend that as the way to go… but all that said. I just got2 pairs of 35Es and the XTAR PB2S charger… that thing will fast charge my phone while i hot swap the batteries… so being able to power/charge or hotswap batts is a really neat feature. I was actually considering how to implement a powersaving mode that would allow me to run on only a single 18650 since the battery board does mention the ability to hotswap, though it does also mention that reliability is not guaranteed.

Ah good point! But I think this statement is referring to the USB-C communication - rather than charging.

Looking at the schematics, the USB-C vbus is definitely connected to the battery charge circuit, regardless of the core module. In fact I think the batteries should charge from USB C even if the core module isn’t plugged in. I’m fairly certain that the only difference between the core modules in terms of USB-C is that the A04/06 has native USB through the USB-C port, while on the Pi CM the USB-C is only for charging and not communication.

Just for reference, here are the pins I’m taking about from the DevTerm schematics, along side the RPi pin documentation.

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It’s also possible that the mail-t* gpus may require more power than the videocore. im just speculating about a need for more power. shrugs

That exactly what the text from CPi say. Some module will allow to use the USB-C connector for more than charging, other no.

And yes, charging will always work whatever the module plugged in.


the only other possibility i can think of for usb-c is OTG-Host… i find that oddly exciting and i dont even know why… i mean… native usb-a ports…


The USB-C will always work to charge the battery, whatever the module used.

When a compatible module is used, the USB-C will only work in one direction considering how it is connected, hard to tell if it ill be device or master, but considering that there is no way to change the direction (CC1 and CC2 are fixed) this will only work in device mode, so to connect your DevTerm to something, and not the opposite.

Though the DevTerm do also have some host USB-A socket.

Oh and it is a USB-C socket yes, but will only do USB 2. No USB3 speed or whatnot related with USB3.

For the TL:DR about the USB-C socket on the DevTerm:

  • It will never work as a host (aka like a USB-A socket) only as a device (like USB-B, microB, miniB, …)
  • It will always charge the DevTerm whatever module you use
  • It will only provide maximum USB2 speed with compatible modules.
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Super speed or not, it’s still easier to plug in than USB-A.

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