diff --git a/drivers/dma_mbuf.c b/drivers/dma_mbuf.c
index 68462a7..47bb2cc 100644
--- a/drivers/dma_mbuf.c
+++ b/drivers/dma_mbuf.c
@@ -49,10 +49,11 @@ uint32_t dma_mbuf_write_byte(volatile struct DmaMultiBuffer* buffer,
 	//if the current buffer is full, we need to switch it with an empty one
 	if (buffer->byte_index >= buffer->buffer_size) {
 
-		//if all buffer full, simply wait for the DMA to empty one
-		dma_exit_critical(buffer->dma, buffer->channel);
-		while (buffer->free_buffer_nb == 0) {}
-		dma_enter_critical(buffer->dma, buffer->channel);
+		//if all buffer full, give up
+		if (buffer->free_buffer_nb == 0) {
+			dma_exit_critical(buffer->dma, buffer->channel);
+			return 1;
+		}
 
 		++buffer->buffer_index;
 		if (buffer->buffer_index >= buffer->buffer_nb) {
@@ -61,8 +62,6 @@ uint32_t dma_mbuf_write_byte(volatile struct DmaMultiBuffer* buffer,
 		--buffer->free_buffer_nb;
 
 		buffer->byte_index = 0;
-	} else {
-		dma_enter_critical(buffer->dma, buffer->channel);
 	}
 
 	//write the byte
diff --git a/drivers/usart.c b/drivers/usart.c
index e5b8491..05e0cc8 100644
--- a/drivers/usart.c
+++ b/drivers/usart.c
@@ -134,9 +134,13 @@ uint32_t usart_write_byte(enum UsartPeriph periph, uint8_t byte)
 			return dma_mbuf_write_byte(buffer, byte);
 		}
 	} else {
-		while (regs->SR.TXE == 0) {}
-		reg_write(regs->DR, USART_DR_DR, byte);
-		return 0;
+		//only write data if the tx register it empty, give up otherwise
+		if (regs->SR.TXE) {
+			reg_write(regs->DR, USART_DR_DR, byte);
+			return 0;
+		} else {
+			return 1;
+		}
 	}
 }