diff --git a/src/main.c b/src/main.c
index 2aad978..af73129 100644
--- a/src/main.c
+++ b/src/main.c
@@ -3,6 +3,7 @@
 #include <avr/io.h>
 #include <avr/pgmspace.h>
 #include <avr/interrupt.h>
+#include <util/atomic.h>
 #include <util/delay.h>
 #include "hmi_msg.h"
 #include "uart-wrapper.h"
@@ -80,7 +81,10 @@ static inline void search_month() {
 
 static inline void heartbeat() {
     static uint32_t last_time;
-    uint32_t cur_time = time;
+    uint32_t cur_time;
+    ATOMIC_BLOCK(ATOMIC_FORCEON) {
+        cur_time = time;
+    }
     if ((last_time - cur_time) > 0) {
         // Toggle led on arduino pin 25
         PORTA ^= _BV(PORTA3);
diff --git a/src/uart-wrapper.c b/src/uart-wrapper.c
index 571e628..c086a0b 100644
--- a/src/uart-wrapper.c
+++ b/src/uart-wrapper.c
@@ -29,5 +29,5 @@ int uart3_putchar(char c, FILE *stream)
 int uart0_getchar(FILE *stream)
 {
     (void) stream;
-    return (unsigned char)uart0_getc();
+    return uart0_getc() & 0xff;
 }