diff --git a/src/main.rs b/src/main.rs
index bfc8cbf..94b4770 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -5,19 +5,23 @@
 mod display;
 mod keyboard;
 mod panic;
+mod timer;
 
+use avr_device::interrupt::Mutex;
 use calc_math::{
     calc::{StackCalc, StackCalcError},
     Decimal,
 };
+use core::{cell::RefCell, ops::DerefMut};
 use display::{DispalyState, SegmentPins, Show};
 use keyboard::{Debounce, KeyPress, KeyReadout, Keyboard};
 
 use arduino_hal::{
     adc::channel::{ADC6, ADC7},
-    hal::port::{PB3, PB4, PC1, PC2, PC3, PD2, PD3, PD4, PD7},
+    clock::Clock,
+    hal::port::{PB3, PB4, PB5, PC1, PC2, PC3, PD2, PD3, PD4, PD7},
     port::{mode::Output, Pin},
-    Adc,
+    Adc, DefaultClock,
 };
 use ufmt::derive::uDebug;
 
@@ -48,11 +52,20 @@ type Calc = StackCalc<f32, STACK_DEPTH, 5, u8>;
 // * Set another timer to run for 1ms.
 // * On another timer interrupt expire disable display (LEDs off) and handle keyboard input.
 
+static LED: Mutex<RefCell<Option<Pin<Output, PB5>>>> = Mutex::new(RefCell::new(None));
+
 #[avr_device::interrupt(atmega328p)]
-fn TIMER1_OVF() {
+unsafe fn TIMER0_COMPA() {
     //TODO: Handle timer interrupt to display next segment
     // file:///home/hxd/projects/avr-calc/target/avr-none/doc/avr_device/interrupt/index.html
     // ...
+    avr_device::interrupt::free(
+        |cs| match LED.borrow(cs).borrow_mut().deref_mut().as_mut() {
+            Some(led) => led.toggle(),
+            None => panic!(),
+        },
+    );
+    //..toggle());
 }
 
 struct IOSelect<'p> {
@@ -458,6 +471,11 @@ fn main() -> ! {
     };
 
     // TODO: configure timer, enable it to interrupt: dp.TC1.tifr1
+    // https://github.com/monoper/rust-atmega328p/blob/main/arduino-nano/key-tmr-input/src/main.rs
+    timer::segment_timer_init(dp.TC0, (DefaultClock::FREQ / 1024 / 100) as u8); // 16_000_000 / 1024 / 100 => 156
+    avr_device::interrupt::free(|cs| {
+        LED.borrow(cs).replace(Some(pins.d13.into_output()));
+    });
 
     unsafe {
         avr_device::interrupt::enable();
diff --git a/src/timer.rs b/src/timer.rs
new file mode 100644
index 0000000..5dd76b6
--- /dev/null
+++ b/src/timer.rs
@@ -0,0 +1,12 @@
+use arduino_hal::pac::TC0;
+
+pub fn segment_timer_init(tc0: TC0, counts: u8) {
+    // Use CTC mode: reset counter when matches compare value
+    tc0.tccr0a.write(|w| w.wgm0().ctc());
+    // Set the compare value
+    tc0.ocr0a.write(|w| w.bits(counts));
+    // Slow down the timer (CLK / prescale)
+    tc0.tccr0b.write(|w| w.cs0().prescale_1024());
+    // Raise interrupt on reset
+    tc0.timsk0.write(|w| w.ocie0a().set_bit());
+}