diff --git a/bluepill-rs/src/main.rs b/bluepill-rs/src/main.rs
index 96f9233..957221b 100644
--- a/bluepill-rs/src/main.rs
+++ b/bluepill-rs/src/main.rs
@@ -9,6 +9,7 @@ use panic_halt as _;
 
 #[rtic::app(device = stm32f1::stm32f103, peripherals = true)]
 mod app {
+    use cortex_m::singleton;
     use stm32f1xx_hal::{dma, gpio, pac, prelude::*, serial, timer};
     use systick_monotonic::Systick;
 
@@ -18,8 +19,7 @@ mod app {
 
     #[shared]
     struct Shared {
-        #[lock_free]
-        buffer: [u8; 256],
+        buffer: Option<&'static mut [u8; 512]>,
     }
 
     #[local]
@@ -64,16 +64,13 @@ mod app {
                 gpioa.pa10, //.into_pull_up_input(&mut gpioa.crh),
             ),
             &mut afio.mapr,
-            serial::Config {
-                baudrate: 250_000.bps(),
-                ..Default::default()
-            },
+            250_000.bps(),
             &clocks,
         );
 
         (
             Shared {
-                buffer: [0b01010101; 256],
+                buffer: Some(singleton!(: [u8; 512] = [0b01010101; 512]).unwrap()),
             },
             Local {
                 tx: Some(serial.tx.with_dma(dma1.4)),
@@ -89,20 +86,25 @@ mod app {
         )
     }
 
-    #[idle(local = [tx, delay_us, led], shared = [&buffer])]
-    fn idle(cx: idle::Context) -> ! {
+    #[idle(local = [tx, delay_us, led], shared = [buffer])]
+    fn idle(mut cx: idle::Context) -> ! {
         let mut tx = cx.local.tx.take().unwrap();
 
         loop {
-            let xfer = tx.write(cx.shared.buffer);
+            tx = (cx.shared.buffer).lock(|buffer| {
+                let buf = buffer.take().unwrap();
+                let xfer = tx.write(buf);
 
-            cx.local.delay_us.delay(1.secs());
-            cx.local.led.set_high();
+                cx.local.delay_us.delay(1.secs());
+                cx.local.led.set_high();
 
-            cx.local.delay_us.delay(1.secs());
-            cx.local.led.set_low();
+                cx.local.delay_us.delay(1.secs());
+                cx.local.led.set_low();
 
-            (_, tx) = xfer.wait();
+                let (buf, tx) = xfer.wait();
+                buffer.replace(buf);
+                tx
+            });
         }
     }
 }