Bare metal, Assembly language • Booting, UART and clock speed

I'm a bloody beginner on bare-metal PI, however, I do have experience on the Arduino. I'm walking through the demos and learned a lot but I'm still a bit confused with clock speed. I'm using a PI4-B, so from the specs it's a BCM 2711 with 1,5 GHz CPU speed. And that's exactly how I calculated the register value for AUX_MU_BAUD_REG just to see garbage - then I took the suggested value from a Youtube video (based on a 500 MHz speed) and voila. All right, maybe CPU speed and board speed are not the same. Strange, because my application requires somewhat real-time capabilities, so I have to rely on the statements from the data sheets and I've never seen anything the let me assume the PI4 is running at 500 MHz, but ok, be that as it may.

All right then - lets get rid of the tons of stuff on the SD Card - so that is my SD Boot media:

fixup4.dat
start4.elf
#miniuart-bt.dtbo
bcm2711-rpi-4-b.dtb
config.txt
kernel.img

That is what's in the config.txt:

arm_64bit=1
uart_2ndstage=1
#dtoverlay=miniuart-bt
kernel=kernel.img

I see the kernel running (just reading from the UART and sending the character back) but again, garbage. Having the miniuart-bt in the config does not make a change.

Can anyone please give me a hint and could explain at what clock speed the board is running and would I have found this information?

Thank you very much,
Christian.

Statistics: Posted by jasnevo — Sun Jul 28, 2024 6:25 pm

---