rhythm16 / rpi4-bare-metal Goto Github PK

The baudrate setting of the PL011 UART was copied from the exercise solutions, I don't know how it's calculated and is surprised that the value also works for the RPI4.

The hint is in this file:

for calculation hint see PrimeCell ® UART (PL011) Technical Reference Manual

Said reference manual is here: https://developer.arm.com/documentation/ddi0183/g/; section 3.3.6 example 3-1 contains an example for how to calculate the baudrate setting (the formulas in section 3.3.4 and 3.3.6 are pretty obscure):

If the required baud rate is 230400 and UARTCLK = 4MHz then:
Baud Rate Divisor = (4×10^6)/(16×230400) = 1.085
This means BRDI = 1 and BRDF = 0.085.
Therefore, fractional part, m = integer((0.085×64)+0.5) = 5
Generated baud rate divider = 1+5/64 = 1.078
Generated baud rate = (4×10^6)/(16×1.078) = 231911
Error = (231911-230400)/230400 × 100 = 0.656%
The maximum error using a 6-bit UARTFBRD Register = 1/64×100 = 1.56%. This occurs when m = 1, and the error is cumulative over 64 clock ticks.

There's also table 3-8 with "typical bit rates and their corresponding divisors when UARTCLK is 7.3728MHz" and table 3-9 with "required bit rates and their corresponding integer and fractional divisor values and generated bit rates when UARTCLK is 4MHz".

As both the RPi3 and the RPi4 have a PL011 UART, these calculations work for both models.

(this is a bit like a detective following clues...)

Hey nice work, I'm trying to get this working as well so I can startup the MMU on rpi4, it doesn't seem to startup for me though.

I tried downloading the firmware from today and from ~6 months ago but neither will boot up for me.

I'm able to get the MMU working on the rpi 3 but not the 4, so was hoping to be able to boot this up and use some of your code.

Low level devel said the RPI4's system clock frequency is 500MHz, this is correct in my testing, but I can't find this anywhere, in fact the bcm2711 (the soc on the PI4) peripheral datasheet actually says the system clock frequency is 250MHz on page 10.

The document on overclocking contains the default frequencies:

This table gives the default values for the options on various Raspberry Pi Models, all frequencies are stated in MHz
...
core_freq... Pi4... 500

(yeah, the documentation is all over the place)