update defmt deps to v0.3 (#56)

* bump defmt to v0.3

remove Cargo features which are no longer needed

* remove `timestamp!`

it's no longer mandatory and a monotonic counter does not add much value given that `defmt-rtt`
works by disabling interrupts so one won't see logs out of order anyways (which would the case for a
lock-free multi-channel `Logger`)

* use println! in most the examples

so that all examples print something when `DEFMT_LOG` is unset

* make overflow example terminate in less steps

also include an estimate of the stack pointer to show the stack grows downwards towards the RAM
boundary

src/bin/bitfield.rs

@@ -7,7 +7,7 @@ use {{crate_name}} as _; // global logger + panicking-behavior + memory layout
 fn main() -> ! {
     // value of the FREQUENCY register (nRF52840 device; RADIO peripheral)
     let frequency: u32 = 276;
-    defmt::debug!("FREQUENCY: {0=0..7}, MAP: {0=8..9}", frequency);
+    defmt::println!("FREQUENCY: {0=0..7}, MAP: {0=8..9}", frequency);
 
     {{crate_name}}::exit()
 }

src/bin/format.rs

@@ -17,10 +17,13 @@ struct S2 {
 
 #[cortex_m_rt::entry]
 fn main() -> ! {
-    let s = S1 { x: 42, y: S2 { z: 43 } };
-    defmt::info!("s={:?}", s);
+    let s = S1 {
+        x: 42,
+        y: S2 { z: 43 },
+    };
+    defmt::println!("s={:?}", s);
     let x = 42;
-    defmt::info!("x={=u8}", x);
+    defmt::println!("x={=u8}", x);
 
     {{crate_name}}::exit()
 }

src/bin/hello.rs

@@ -5,7 +5,7 @@ use {{crate_name}} as _; // global logger + panicking-behavior + memory layout
 
 #[cortex_m_rt::entry]
 fn main() -> ! {
-    defmt::info!("Hello, world!");
+    defmt::println!("Hello, world!");
 
     {{crate_name}}::exit()
 }

src/bin/levels.rs

@@ -5,6 +5,8 @@ use {{crate_name}} as _; // global logger + panicking-behavior + memory layout
 
 #[cortex_m_rt::entry]
 fn main() -> ! {
+    // try setting the DEFMT_LOG environment variable
+    // e.g. `export DEFMT_LOG=info` or `DEFMT_LOG=trace cargo rb levels`
     defmt::info!("info");
     defmt::trace!("trace");
     defmt::warn!("warn");

src/bin/overflow.rs

@@ -10,12 +10,15 @@ fn main() -> ! {
 }
 
 fn ack(m: u32, n: u32) -> u32 {
-    defmt::info!("ack(m={=u32}, n={=u32})", m, n);
-    let mut big = [2; 512];
+    // waste stack space to trigger a stack overflow
+    let mut buffer = [0u8; 16 * 1024];
+    // estimate of the Stack Pointer register
+    let sp = buffer.as_mut_ptr();
+    defmt::println!("ack(m={=u32}, n={=u32}, SP={:x})", m, n, sp);
+
     if m == 0 {
         n + 1
     } else {
-        big[100] += 1;
         if n == 0 {
             ack(m - 1, 1)
         } else {

src/bin/panic.rs

@@ -5,7 +5,7 @@ use {{crate_name}} as _; // global logger + panicking-behavior + memory layout
 
 #[cortex_m_rt::entry]
 fn main() -> ! {
-    defmt::info!("main");
+    defmt::println!("main");
 
     defmt::panic!()
 }