diff --git a/components/freemodbus/port/portserial.c b/components/freemodbus/port/portserial.c
index ce808e8..57d107d 100644
--- a/components/freemodbus/port/portserial.c
+++ b/components/freemodbus/port/portserial.c
@@ -238,6 +238,7 @@ BOOL xMBPortSerialInit(UCHAR ucPORT, ULONG ulBaudRate,
         .stop_bits = UART_STOP_BITS_1,
         .flow_ctrl = UART_HW_FLOWCTRL_DISABLE,
         .rx_flow_ctrl_thresh = 2,
+        .use_ref_tick = UART_SCLK_APB,
     };
     // Set UART config
     xErr = uart_param_config(ucUartNumber, &xUartConfig);
diff --git a/components/freemodbus/port/portserial_m.c b/components/freemodbus/port/portserial_m.c
index 175d2a6..e6566be 100644
--- a/components/freemodbus/port/portserial_m.c
+++ b/components/freemodbus/port/portserial_m.c
@@ -227,6 +227,7 @@ BOOL xMBMasterPortSerialInit( UCHAR ucPORT, ULONG ulBaudRate, UCHAR ucDataBits,
         .stop_bits = UART_STOP_BITS_1,
         .flow_ctrl = UART_HW_FLOWCTRL_DISABLE,
         .rx_flow_ctrl_thresh = 2,
+        .source_clk = UART_SCLK_APB,
     };
     // Set UART config
     xErr = uart_param_config(ucUartNumber, &xUartConfig);