I'm beginning bare metal programming on the Pi4. As a simple 'hello world' equivalent I'm trying to get the ACT led to blink. I have seen references that it is tied to the BCM2711 GPIO Controller pin 16. However, looking at the BCM2711 datasheet I can't find any references to this. Is this information available in a datasheet or an official documentation source somewhere?
The BCM2711 peripherals documentation can be found on the Foundation website.
The GPIO is a superset of the earlier GPIO i.e. it is identical EXCEPT some pins have additional modes and the peripheral address differs between models. See https://www.raspberrypi.org/documentation/hardware/raspberrypi/peripheral_addresses.md
AFAIK there is no public documentation for the on-board interfaces e.g. ACT LED.
Exploring the gpiochip implementation in latest Raspberry Pi OS gives some hints about the on-board GPIO functions.
Update: The following may be of interest re the expansion GPIO.
gpioinfo raspberrypi-exp-gpio gpiochip1 - 8 lines: line 0: "BT_ON" unused output active-high line 1: "WL_ON" unused output active-high line 2: "PWR_LED_OFF" "led1" output active-low [used] line 3: "GLOBAL_RESET" unused output active-high line 4: "VDD_SD_IO_SEL" "vdd-sd-io" output active-high [used] line 5: "CAM_GPIO" unused output active-high line 6: "SD_PWR_ON" "sd_vcc_reg" output active-high [used] line 7: "SD_OC_N" unused input active-high