You’re very welcome James. I can’t say that I have seen too much of his but I will keep my eye out for more.
Had my sign up to get notified for months now.
Yes, I could go Barebones, but not going to pay for an old power supply and Noobs.
Still want a Pi 5, I want to try and answer some questions, and the changes to the Pi 5 compared with the Pi 4 are much better suited to what I want.
So headed to another supplier and its on its way. (Pi5 and official cooler)
Different supplier, Pcie M.2 Key-M NVMe board and case to match. Also on its way.
What I really want to understand is why the Raspberry Pi Corporation did not consider powering the Pi 5 via the GPIO +5V GND pins.
Is it something they do not recommend for the Pi 5 ??
Why did they leave all the voltage regulation to an external supply ??
Why did they not have an option for using something like a MeanWell high current supply ??
Why is the only way I can now power it, is via USB-C PD with 5V5A supply ??
Why not use the higher voltages available and regulate to 5V ??
Is the USB-C Power IN circuit similar to the Pi 4 ??
(wish they would release a schematic like the other version of the Pi, at least a reduced version)
I have successfully powered a Pi 4 via the GPIO
The Pi 4 reduced schematic shows the USB-C VBUS pin is the same as the +5V GPIO pin.
The PD_SENSE goes to an analog pin on U2 (whatever that chip does, its unlabelled)
In the USB-C standard, CC1 & CC2 are used to communicate between the supply and device to negotiate power. With this Pi 4 wiring there would be no communication, or one way at the very least.
I am guessing the Pi 5 will be different.
Briefly scanning the Raspberry Pi Forum shows I am not the only one who would like answers to the Pi 5 powering situation. The Corporation needs to release more information on how all of this is done. I can see a few Pi 5 being destroyed by lack of knowledge on the part of the someone. Saying the official supply only is not good enough for a hobbyist like me.
Anyway, will see what I can discover when my parts arrive.
Cheers
Jim
EDIT: After some further investigation.
U2 is a MxL7704 - Five Output Universal PMIC; it generates the low level voltages needed by processor and other device’s. It has internal registers and an I2C interface to the OS Kernel.
The analog inputs are to measure a voltage related to GPU temperature or external voltages.
Don’t think these Analog inputs are used on the Pi 4, except for temperature.
The chip also generates the power good signal which is determined from the VBUS input from the USB port or GPIO pin. It has an internal reference of 4.65V and flags power good if higher than that.
EDIT: Note the connection of CC1 & CC2 in the above schematic is not the way USB-C is supposed to work. It is a recognised mistake the Raspberry Pi Corporation made in design of the Pi 4 with USB-C. The pulldown resistors are required, but there should have been a PD_SENSE_1 & PD_SENSE_2 going to the ADC pins on the MxL7704.
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Hi Jim,
Very fair about not wanting to go with the Barebones kit - we’ll be getting the Pi 5 supplies in soon so we can rejig the bundle to be more appealing to a wider range of Makers!
Without being in the Raspberry Pi team its hard to say, though for cross compatability it ought to work!
(This topic covers getting the limits lifted: Powering the Raspberry Pi 5. - Raspberry Pi Forums)
Why did they leave all the voltage regulation to an external supply ??
The Blogs might cover the reasoning though I imagine there are some smarts that handle the PMIC to choose the best voltage for the load, a smarter regulator allows higher voltages to be used so that the main SoC doesnt suffer any voltage drops from a lesser power supply.
Why did they not have an option for using something like a MeanWell high current supply ??
The option is definitely still there but not officially supported (through the 5V pins). We can find the boot.txt file mods that make this work, and i believe it’s mentioned in my previous link.
Why is the only way I can now power it, is via USB-C PD with 5V5A supply ??
There are plenty of ways around it but unfortunately it limits the CPU but more on that below
Why not use the higher voltages available and regulate to 5V ??
I imagine that is what the PMIC is doing behind the scenes, but without the correct PSU identifier.
Is the USB-C Power IN circuit similar to the Pi 4 ??
This is definitely something we can test! I’ll ask @Jaryd to probe some points if you have some concerns 
In any case, the maker market being what it is - we can probably expect to see third-party solutions emerging at some point in the future… hopefully soon!
Liam
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Hi Liam,
Yeah, my questions are pretty much rhetorical.
Having listened to a couple of interviews between Eben Upton and Jeff Geerling my curiosity has been activated.
It is interesting the Spec pages for the Pi 4 says power via USB or GPIO.
The Pi 5 lists only power via the USB.
For the Pi 4 the PMIC generates the low voltages for the processor, it has a I2C interface to the Kernel and internal registers that can be read. It detects the low voltage, (4.65V or lower) and sets a register which the Kernel reads and displays the HDMI symbol if too low.
The datasheet for the Pi 5 PMIC I have not been able to find, I suspect it does a lot more than the Pi 4 PMIC. The Raspberry Pi official documentation is a treasure trove of good information. But the way it is worded is sometimes not exactly clear as to what they mean. I look forward to testing it for real.
Thanks for your replies.
Cheers
Jim
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