diff --git a/kw11-l.cpp b/kw11-l.cpp
index f044f84..863438e 100644
--- a/kw11-l.cpp
+++ b/kw11-l.cpp
@@ -181,6 +181,11 @@ void kw11_l::write_byte(const uint16_t addr, const uint8_t value)
 		return;
 	}
 
+#if defined(BUILD_FOR_RP2040)
+	xSemaphoreTake(lf_csr_lock, portMAX_DELAY);
+#else
+	std::unique_lock<std::mutex> lck(lf_csr_lock);
+#endif
 	uint16_t vtemp = lf_csr;
 	
 	if (addr & 1) {