I'm using `embedded-alloc` for my heap and it is setup here https://github.com/bsodmike/arduino-giga-r1-wifi-stm32h747xi-async-quickstart/blob/master/src/mem/mod.rs#L1-L5 and the FMC (aka SDRAM) is setup here https://github.com/bsodmike/arduino-giga-r1-wifi-stm32h747xi-async-quickstart/blob/master/src/main.rs#L107-L249.

Notice how I take a `ptr` to just a slice of data within the SDRAM https://github.com/bsodmike/arduino-giga-r1-wifi-stm32h747xi-async-quickstart/blob/master/src/main.rs#L241-L249.

This is my log, notice I can step through the main loop once. However, 

```
0.000000 INFO  Booting up...
└─ stm32h747_async_quickstart::____embassy_main_task::{async_fn#0} @ src/main.rs:89  
0.000000 INFO  ADC frequency set to 25000000 Hz
└─ embassy_stm32::adc::_version::{impl#7}::new @ /Users/mdesilva/.cargo/git/checkouts/embassy-9312dcb0ed774b29/4c7ed5e/embassy-stm32/src/fmt.rs:143 
0.017211 INFO  Got RTC! 1589538615
└─ stm32h747_async_quickstart::____embassy_main_task::{async_fn#0} @ src/main.rs:104 
0.017272 INFO  SDRAM Memory Size 0x18
└─ stm32h747_async_quickstart::____embassy_main_task::{async_fn#0}::log2minus1 @ src/main.rs:162 
0.017517 INFO  RAM contents before writing: [2000000, 0, 3, 4, 0, 0, 0, 0, 0, 0]
└─ stm32h747_async_quickstart::____embassy_main_task::{async_fn#0}::log2minus1 @ src/main.rs:254 
0.017517 INFO  RAM contents after writing: [1, 2, 3, 4, 0, 0, 0, 0, 0, 0]
└─ stm32h747_async_quickstart::____embassy_main_task::{async_fn#0}::log2minus1 @ src/main.rs:261 
0.017517 INFO  Assertions succeeded.
└─ stm32h747_async_quickstart::____embassy_main_task::{async_fn#0}::log2minus1 @ src/main.rs:268 
1.742309 INFO  high
└─ stm32h747_async_quickstart::____embassy_main_task::{async_fn#0}::log2minus1 @ src/main.rs:271 
4.042327 INFO  low
└─ stm32h747_async_quickstart::____embassy_main_task::{async_fn#0}::log2minus1 @ src/main.rs:275 
4.342346 INFO  vrefint: 24575
└─ stm32h747_async_quickstart::____embassy_main_task::{async_fn#0}::log2minus1 @ src/main.rs:280 
4.342346 INFO  measured: 14974
└─ stm32h747_async_quickstart::____embassy_main_task::{async_fn#0}::log2minus1 @ src/main.rs:282 
4.842346 INFO  loop: Got RTC! Fri, 15 May 2020 10:30:19 +0000
└─ stm32h747_async_quickstart::____embassy_main_task::{async_fn#0}::log2minus1 @ src/main.rs:291 

// This ends up in a HardFault.
```

Notice the code here:

```rust
let now: NaiveDateTime = rtc.now().unwrap().into();
let dt_opt = chrono::DateTime::<chrono::Utc>::from_timestamp(now.timestamp() as i64, 0);
if let Some(dt) = dt_opt {
    // NOTE: this only works due to alloc.  It works though as we can see it in the first `loop()` iteration of `main()`.
    let dt_rfc2822 = dt.to_rfc2822();
    let now = dt_rfc2822.as_str();
    // let now = dt.timestamp();
    info!("loop: Got RTC! {=str}", now);

    // <---- This is where the drop takes place
}
```

The call to `dealloc` gets handled here:
https://github.com/rust-embedded/embedded-alloc/blob/master/src/llff.rs#L82

The dealloc tries to enter a critical section.  The call to `unsafe { f(CriticalSection::new()) }` proceeds into a [HardFault](https://github.com/rust-embedded/cortex-m/blob/6df9a38585a9552970523c26ea5c7f3eccfe2742/cortex-m-rt/src/lib.rs#L1053). See:
https://github.com/rust-embedded/critical-section/blob/main/src/lib.rs#L239

Any ideas re. the dealloc here?