STM32U5: RNG hangs when using HSE clock source

[mubes]

A bit confused by this one TBH. I initialise my HSE as follows;

  {
        use embassy_stm32::rcc::*;
        use embassy_stm32::time::Hertz;
        config.rcc.hse = Some(Hse {
            freq: Hertz(16_000_000),
            mode: HseMode::Oscillator,
        });
        config.rcc.pll1 = Some(Pll {
            source: PllSource::HSE,
            prediv: PllPreDiv::DIV2,   // HSE / 2 = 8MHz
            mul: PllMul::MUL60,        // 8MHz * 60 = 480MHz
            divr: Some(PllDiv::DIV3),  // 480MHz / 3 = 160MHz (sys_ck)
            divq: Some(PllDiv::DIV10), // 480MHz / 10 = 48MHz (USB)
            divp: Some(PllDiv::DIV15), // 480MHz / 15 = 32MHz (USBOTG)
        });
        config.rcc.mux.otghssel = mux::Otghssel::PLL1_P;
        config.rcc.hsi = true;
        config.rcc.mux.rngsel = mux::Rngsel::HSI48;
        config.rcc.voltage_range = VoltageScale::RANGE1;
        config.rcc.sys = Sysclk::PLL1_R;
    }

This causes the line let mut rng = Rng::new(p.RNG, Irqs); to hang and eventually time out.

If I comment out the HSE configuration, the above Rng::new works correctly. It looks for all the world like a missing clock, but both HSI and HSI48 are enabled and show up in the trace;

0.000000 INFO  Hello World!
└─ ironage::____embassy_main_task::{async_fn#0} @ src/main.rs:47  
0.000000 TRACE BDCR ok: 0c00c200
└─ embassy_stm32::rcc::bd::{impl#3}::init @ embassy/embassy-stm32/src/rcc/bd.rs:216 
0.000000 DEBUG rcc: Clocks { audioclk: MaybeHertz(0), hclk1: MaybeHertz(160000000), hclk2: MaybeHertz(160000000), hclk3: MaybeHertz(160000000), hse: MaybeHertz(16000000), hse_div_2: MaybeHertz(8000000), hsi: MaybeHertz(16000000), hsi48: MaybeHertz(48000000), hsi48_div_2: MaybeHertz(0), lse: MaybeHertz(0), lsi: MaybeHertz(32000), msik: MaybeHertz(4000000), pclk1: MaybeHertz(160000000), pclk1_tim: MaybeHertz(160000000), pclk2: MaybeHertz(160000000), pclk2_tim: MaybeHertz(160000000), pclk3: MaybeHertz(160000000), pll1_p: MaybeHertz(32000000), pll1_p_div_2: MaybeHertz(16000000), pll1_q: MaybeHertz(48000000), pll2_p: MaybeHertz(0), pll2_q: MaybeHertz(0), pll2_r: MaybeHertz(0), pll3_p: MaybeHertz(0), pll3_q: MaybeHertz(0), rtc: MaybeHertz(32000), shsi: MaybeHertz(0), shsi_div_2: MaybeHertz(0), sys: MaybeHertz(160000000) }
└─ embassy_stm32::rcc::set_freqs @ embassy/embassy-stm32/src/rcc/mod.rs:71  

I did force-enable both hsi and hsi48 in the rcc driver just to be sure it wasn't that, but to no avail. Any advice what to poke next?

Regards

DAVE