diff --git a/src/fan_driver.h b/src/fan_driver.h
index e22958d..df1f02c 100644
--- a/src/fan_driver.h
+++ b/src/fan_driver.h
@@ -13,10 +13,10 @@ volatile uint32_t _delta_idx = 0;
 static const uint16_t pwm_timer_period = 2048;
 static const uint8_t rpm_buf_size = sizeof(_fan_driver_delta) / sizeof(_fan_driver_delta[0]);
 
-uint16_t fan_driver_target_rpm = 0;
-uint16_t fan_driver_current_rpm = 0;
-bool fan_driver_is_stalled = false;
-bool fan_driver_is_steady = false;
+volatile uint16_t fan_driver_target_rpm = 0;
+volatile uint16_t fan_driver_current_rpm = 0;
+volatile bool fan_driver_is_stalled = false;
+volatile bool fan_driver_is_steady = false;
 volatile bool fan_driver_powered_down = false;
 volatile bool fan_driver_constant_duty = false; // true = constant rpm mode