got to idle state waiting for key

src/main.rs

@@ -8,7 +8,10 @@ mod keyboard;
 mod panic;
 mod timer;
 
-use avr_device::interrupt::{CriticalSection, Mutex};
+use avr_device::{
+    asm::sleep,
+    interrupt::{CriticalSection, Mutex},
+};
 use calc_math::{
     calc::{StackCalc, StackCalcError},
     Decimal,
@@ -544,6 +547,7 @@ fn main() -> ! {
         });
 
         if let TransientState::Done = state.transient {
+            // No transient state - handle key or wait for key
             if let Some(key) = key {
                 ufmt::uwriteln!(
                     &mut serial,
@@ -578,9 +582,14 @@ fn main() -> ! {
                 for (i, dec) in calc.iter().enumerate() {
                     ufmt::uwriteln!(&mut serial, "[{}] {}", i, dec).ok();
                 }
+            } else {
+                // Wait for key
+                sleep(); // Enter idle mode; wake up on interrupt
             }
         } else {
-            state.transient.on_frame(key)
+            // In transient state - handle animation
+            arduino_hal::delay_ms(10);
+            state.transient.on_frame(key);
         }
 
         match &state.transient {
@@ -603,7 +612,5 @@ fn main() -> ! {
                 io_loop.update_display(&display);
             });
         });
-
-        arduino_hal::delay_ms(10);
     }
 }