diff --git a/isr.c b/isr.c
index 142ba3ce2bedb19d54330232271cc2be7daeed52..f541c37610d3cae26ece38f4b45c7ccd07ca8d4b 100644
--- a/isr.c
+++ b/isr.c
@@ -10,7 +10,7 @@ uint32_t irq_unhandled_source = 0;
 uint8_t irq_unhandled_vector_valid = 0;
 uint8_t irq_unhandled_source_valid = 0;
 
-uint64_t uptime_counter_ms = 0;
+volatile uint64_t uptime_counter_ms = 0;
 
 void isr(uint64_t vec);
 void isr_dec(void);
diff --git a/main.c b/main.c
index a56f16b2dea66c8a1cfbc6a359f158cc933992f2..edd0190293a209b1b08d3378f39a49c24f6b0415 100644
--- a/main.c
+++ b/main.c
@@ -112,7 +112,7 @@
 
 uint8_t host_flash_write_buffer[FLASH_MAX_WR_WINDOW_BYTES];
 
-extern uint64_t uptime_counter_ms;
+extern volatile uint64_t uptime_counter_ms;
 
 extern uint32_t irq_unhandled_vector;
 extern uint32_t irq_unhandled_source;