this is the right code:

```rust
    p.SEC_RCC.ahb1enr().modify(|_r, w| {
        w.gtzc1en().set_bit();
        w
    });
```

We're in secure mode! We have to use the secure-mode alias to the RCC peripheral, over at 0x5nnn_nnnn, not the non-secure alias at 0x4nnn_nnnn.