Re: RPI3 serlial clock confusion?

[Michael Cheponis <michael.cheponis%gmail.com@localhost>]

Thank you.

It's completely unclear to me, though, why we'd want to use the 'mini UART' vs the Main uart, because when we use the mini uart, *we have no access to the regular uart via the rpi pins*.  https://elinux.org/RPi_BCM2835_GPIOs  see how the 40 pins on the connector are mapped, with alternates.

The effect seems to be to give us a downgraded functionality uart, and not providing a way to use the regular uart.  I'm not sure this is progress?  In defense of this change, the Broadcom manual says the intention of mini uart is specifically for console uart use.

The baudrate register to achieve 115.2K baud on mini uart is set to 650 @ 600 MHz sysclock; it has to change to 1518 when going to 1400 MHz.  (baudrate = sysclk / (8 * (BRreg+1)) )

The only potential upside I see from this change is having the main uart available on the Compute Module via J1-46, J1-48 or  J1-58, J1-60.

What am I missing?  Thanks again.

On Fri, Jul 10, 2020 at 9:19 AM Michael van Elst <mlelstv%serpens.de@localhost> wrote:

On Thu, Jul 09, 2020 at 08:53:59AM -0700, Michael Cheponis wrote:
> It must be the dtb files.
>
> Yes, the console is perfect with 9.99.69 with (ahem)  .17 good dtb files
> ... ;-)
>
> Thing is, if this is true - how does it work?  And, has this
> discussion about cpu speed coupled with UART speed been incorrect?

See

https://www.raspberrypi.org/documentation/configuration/uart.md

for a Raspbian-centric documentation.

In opposite to that description we use the mini UART as console
with the RPI3. See:

sys/external/gpl2/dts/dist/arch/arm/boot/dts/bcm2837-rpi-3-b.dts


Greetings,
--
                                Michael van Elst
Internet: mlelstv%serpens.de@localhost
                                "A potential Snark may lurk in every tree."