PhilE
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 2571
Joined: Mon Sep 29, 2014 1:07 pm
Location: Cambridge

Re: Pi-4 Activating additional UART ports

Tue Jan 14, 2020 11:05 am

Did I understand right, that without a customized dt-blob, (I will not yet go into the experience of compiling a new one yet), the HAT probing will occur anyway?
No, not if you have used the gpio directive to change to some pin function other than input.

And if you haven't used either a custom dt-blob or gpio directive then the pin is an input.

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rin67630
Posts: 847
Joined: Fri Mar 04, 2016 10:15 am

Re: Pi-4 Activating additional UART ports

Tue Jan 14, 2020 11:36 am

PhilE wrote:
Tue Jan 14, 2020 11:05 am
Did I understand right, that without a customized dt-blob, (I will not yet go into the experience of compiling a new one yet), the HAT probing will occur anyway?

No, not if you have used the gpio directive to change to some pin function other than input.
And if you haven't used either a custom dt-blob or gpio directive then the pin is an input.
Sorry, I am now completely confused...
I have been speaking about the short time during HAT probing.
You wrote it would happen at boot time before the GPIO directive.
During that short time GPIO1 will be an output, providing the system clock to sense the EEPROM. Right?
After that it turns back to an unused input until being set to RX2 by the GPIO directive ALT4. Right again?

PhilE
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 2571
Joined: Mon Sep 29, 2014 1:07 pm
Location: Cambridge

Re: Pi-4 Activating additional UART ports

Tue Jan 14, 2020 11:38 am

I wrote:
(at the point of probing, which is after the "gpio ..." handling)

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rin67630
Posts: 847
Joined: Fri Mar 04, 2016 10:15 am

Re: Pi-4 Activating additional UART ports

Tue Jan 14, 2020 1:46 pm

PhilE wrote:
Tue Jan 14, 2020 11:38 am
I wrote:
(at the point of probing, which is after the "gpio ..." handling)
No pin function can be set (until the kernel boots, at which point the HAT probing is long since finished) except via the firmware.
What is the right statement?
Could you recommend a lecture about the corresponding steps of the boot process?
I could not find the info in https://www.raspberrypi.org/documentati ... ce-tree.md

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