Source for addresses and protocols

[yanniknelson]

Hi, I've been reading through these tutorials ensuring that i understand every bit of logic and for the most part i'm doing fine.
I've reached 03_uart1 and i've reached a bit of a block. Where did you find the memory addresses for the MMIO_BASE and all the offsets (all the AUX offsets for example).

I've been looking at all the documentation for the board and they hint that the MMIO_BASE should be 0x7E..... https://www.raspberrypi.com/documentation/computers/processors.html#bcm2837 says that the 2837 has the same archicture as the 2836 which has the same architecture as 2835, only the 2835 has documentation that (i can find) that mentions the AUX adresses https://datasheets.raspberrypi.com/bcm2835/bcm2835-peripherals.pdf (page 8) (the 2836 docs https://datasheets.raspberrypi.com/bcm2836/bcm2836-peripherals.pdf don't mention those addresses). The os dev wiki does agree with your MMIO_BASE address https://wiki.osdev.org/Detecting_Raspberry_Pi_Board but doesn't then provide the offsets for the AUX and other addresses, also i'd still be interested in the documentation they found these values from.

Additionally in https://github.com/bztsrc/raspi3-tutorial/blob/master/03_uart1/uart.c I'd like to understand the documentation for how uart_init was written, e.g. why does it busy wait for 150 cycles twice, is that in the specification or is that a random large enough value you chose. Where is it specified which register does what, where are the flag definitions? I have a feeling the flag values come from https://datasheets.raspberrypi.com/bcm2835/bcm2835-peripherals.pdf page 15 where it describes the AUX_MU_LSR_REG Register's bits.

(I've also checked https://github.com/raspberrypi/firmware/wiki and don't see it in there)

[yanniknelson]

I believe i have found the correct reference datasheet https://cs140e.sergio.bz/docs/BCM2837-ARM-Peripherals.pdf
This provides addresses consistent with what you've used. However it does not describe the protocols for initializing the UART so i'm still wondering where you found that procedure.

That doc simply says the initialize the GPIO before using the UART, it doesn't (in its GPIO section) describe how to initialize the GPIO pins, e.g. it tells you how to select a function mode for a pin and set/clear etc... but it doesn't give any indication as to how to initialize.

1.    r=*GPFSEL1;
2.    r&=~((7<<12)|(7<<15)); // gpio14, gpio15
3.    r|=(2<<12)|(2<<15);    // alt5
4.    *GPFSEL1 = r;
5.    *GPPUD = 0;            // enable pins 14 and 15
6.    r=150; while(r--) { asm volatile("nop"); }
7.    *GPPUDCLK0 = (1<<14)|(1<<15);
8.    r=150; while(r--) { asm volatile("nop"); }
9.    *GPPUDCLK0 = 0;        // flush GPIO setup

As i understand it:

1. Gets the current GPIO state
2. clears the function select for pins 14 and 15
3. sets the function select for pins 14 and 15 to function 5 which sets pin 14 to UART 1 Transmit Data and pin 15 to UART 1 Recieve Data
4. Writes this into the register
5. Actually disables the pins' pull-up/down, (Where does it say this is necessary? https://cs140e.sergio.bz/docs/BCM2837-ARM-Peripherals.pdf#page=101 describes the procedure for changing pull-up/down states)
6. Waits 150 cycles
7. Enables the GPIO pull-up/down clocks for pins 14 and 15
8. Waits 150 cycles
9. Removes the GPIO pull-up/down clocks for all pins

• Why did you pick pins 14 and 15? (I know if using real hardware the choice is arbitrary, but does it matter for QEMU)
• Should you not have ensured none of the other pins where operating in UART1 modes?
• How did you connect QEMU to pins 14 and 15 for the purposes of reading the UART or does it skip the pins and read the UART1 directly somehow?