rpi pico: reading VSYS with ADC gives incorrect values

[soyflourbread]

Hi,
I'm trying to read VSYS voltage using Raspberry Pi Pico's ADC using steps described here:
https://forums.raspberrypi.com/viewtopic.php?t=301152

However, reading it from PIN_29 using ADC gives incorrect values.
It should be around 4 volts, yet the ADC measured ~0.15 volts.

Is this a bug in embassy's ADC implementation?
The sample code I used is below:

#![no_std]
#![no_main]

use defmt::*;
use embassy_executor::Spawner;
use embassy_rp::{gpio, pwm, uart, adc};
use embassy_time::Duration;

use {defmt_rtt as _, panic_probe as _};

embassy_rp::bind_interrupts!(struct Irqs {
    ADC_IRQ_FIFO => adc::InterruptHandler;
});

fn to_voltage(raw_voltage: u16) -> f32 {
    let mut ret = raw_voltage as f32;
    ret *= 3.23;
    ret *= 3.0;
    ret /= 65535.0;
    ret
}

#[embassy_executor::main]
async fn main(spawner: Spawner) {
    let p = embassy_rp::init(Default::default());

    let mut adc_0 = adc::Adc::new(p.ADC, Irqs, adc::Config::default());

    let mut p_voltage = adc::Channel::new_pin(p.PIN_29, gpio::Pull::None);
    let voltage = adc_0.read(&mut p_voltage).await.unwrap();
    let voltage = to_voltage(voltage);
    info!("input voltage: {}", voltage);
}