Hi,
what follows are a few questions/remarks/misunderstandings regarding the UART pins during early boot.
In the EOMA68 specification, the section "Start-up procedure"[1] specifies that "It is required that all pins be disabled (floating tri-state) with the exception of the I2C Bus, the 5.0v Power and the Ground Pins.", which would mean that *any software* running on a CPU card is required to check the housing board's EEPROM before it may output any data on the UART, if I read it correctly. This would make early debugging harder, because EEPROM detection has to be integrated in very early code, and early code can't simply use the UART as an unconditional debug channel, that's guaranteed to reach the outside of the card, anymore.
The section "Requirements for UART"[2], however, states: "As this problem is to be taken care of on the I/O Board[3] it is worth observing that CPU Cards do not require UART buffering.", suggesting that the "Start-up procedure" section simply left out UART as one of the interfaces that do not need to be tri-stated by mistake.
But I am unclear about the part "CPU Cards do not require UART buffering": - Does it mean that CPU cards do not require the housing board to perform UART buffering, because they do it themselves? - Or does it mean that CPU cards are not required to perform UART buffering?
So, can a CPU card assume that it may use the UART right after reset, without first consulting the EEPROM?
(I understand that if a housing board has, for example, a Bluetooth module connected to the UART, something must make sure that none of that early debug output reaches the BT module, but I guess one could easily enough have a GPIO pin that controls whether the UART is cut off from the BT module, and the OS could toggle this pin once it knows that it won't send debug output anymore. The same GPIO pin might also control if the Bluetooth module is powered up at all.)
Best regards, Jonathan Neuschäfer
[1]: https://elinux.org/Embedded_Open_Modular_Architecture/EOMA68/Hardware#Start-... [2]: https://elinux.org/Embedded_Open_Modular_Architecture/EOMA68/Hardware#Requir... [3]: As a side-note, the term "I/O Board" is a little bit unclear to me: Both CPU cards/boards and housing boards perform I/O. I figured that "I/O board" means "housing board", but I (personally) think the spec would be clearer if it talked about "housings" or "housing boards" in the first place.