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mfolejewski / MirkoPC

Modular Integrated Raspberry Pi Computer
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plase think about 802.3at ;) #3

Open lsochanowski opened 3 years ago

lsochanowski commented 3 years ago

it will resolve 15W power problem and it will be best board in the world ;)

mfolejewski commented 3 years ago

Thanks!

  1. "15W issue" OK, I need to clarify "15W issue". I added the 3A polymer fuse, to limit the input power due to USB-C spec. Unfortunately, it turned out that this fuse has too high series resistance and during operation it causes voltage drops on the 5V bus, approx. 0.1V, which causes irritating messages. My fault. Apart from the voltage drop, nothing bad is happening. The CM4 has its own PMIC which generates 3.3V and 1.8V voltages for the CPU, eMMC and RAM. The M.2 slot has its own 3A buck regulator. The rest of the hardware is powered by 3.3V from the PMIC CM4. Only the USB ports are powered from the 5V bus - however, the voltage drop to 4.8V has no negative consequences. The USB electronics have their own LDO or converters anyway, so apart from the annoying message, nothing bad is happening. I tested NVMe + CPU @ 2.2GHz, it worked stably. I bridged the fuse in all other boards. In REV1.1, the fuse will be removed because it causes more problems than benefits. Unfortunately, powering electronics from 5V causes some problems, but it also has many advantages -> you can power the board from the USB-C port, mobile chargers or original RPi power supplies. As simple DIY project 5V rail is quite OK.

  2. NVMe power consumption Another thing - the power consumption of the NVMe disk. Since CM4 has only x1 lane and only GEN2, therefore drives specified even as 3V3 @ 3A will not consume such current (probably somewhere 50% less), due to the low PCIe bandwidth of approx. Max. 360MB / s (approx. 8-10x less than the max. throughput of NVMe disks - I'm guessing).

  3. PoE support I thought about adding the PoE option, but it would involve replacing the RJ45 connector with a more expensive one (I don't know if the footprint will be compatible) and there may also be a problem with adding the connectors (possible collision with the M.2 card).

Unfortunately, since the RJ45 connector is in a different location than on the RPi 4B, the PoE option would not be compatible with the original HATs.

The only chance is to add a 4-pin connector pointing down the board, but this would also require the design of a dedicated PoE + power supply, then it makes sense to design an industrial 12 / 24V power supply.

Initially, I had a plan to expand the project (hence the name: Modular Integrated Computer) and add at least 3 boards:

Anyway, I will consider the option of adding PoE and outputting the 5V / 3.3V system bus -> just in case and for future use -> but only for the REV2 (now I'm working on the REV1.1 to fix all the minor issues).