It should be possible to move to the Pocketbeagle.  I moved a moderately 
complex application from the black to the pocketbeagle.  I use GPIO, both 
SPIs, I2C and USB.  Here's what I found (4.14-based beaglebone debian 
distro):

1. SPI and I2C devices should be ok providing you are using the standard 
device files for access.  I am using the SPI busses for a LCD and 
touchscreen.  In that case since I am using a specific driver for each 
device I had to make new device tree files for the pocketbeagle to tell the 
drivers how to talk to the hardware.  And I had to change /boot/uEnv.txt to 
include the new device tree files.  I am using the PRUs so also had to make 
pocketbeagle-specific device tree files for the pins they used as well.

2. Not sure about UART4 but if a device file shows up for it then I imagine 
it will work.  The pocketbeagle seems to have a good default hardware 
configuration.

3. The pocketbeagle has a different power distribution.  Like the Black 
it's still using the same PMIC but the external wiring is different.  I 
encourage you to download and look at the schematic and read-up on the PMIC 
operation.  On the PB, the USB power from the micro-USB connector is 
connected directly (through a ferrite inductor) to P1.7 with a fairly fat 
trace.  If you power your hub from P1.7 then the number of devices will be 
based on the capability of the power supply you connect to the micro-USB 
connector (and the USB cable wiring, connector limits, etc).  You could 
have an external 5V power supply that powers both the Pocketbeagle and USB 
Hub.  It would connect to P1.7 (or, alternatively, VIN, on P1.1) and also 
the Hub input.  However, if you power the pocketbeagle via VIN (P1.1) then 
you will see nothing coming out of P1.7.  In that case you have to also 
connect the USB Hub power input directly to the supply.

4. See above I was a little surprised that you powered the Black via 
P9.7/P9.8.  It works I guess but I wouldn't think this is good practice 
since you are connecting to the output of the PMIC (and back-feeding it).  
I think the Black with USB peripherals is intended to be powered with a 
high-current 5V supply through the barrel jack input since this is the 
high-current path through the PMIC.

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