diff --git a/src/system.c b/src/system.c
new file mode 100644
--- /dev/null
+++ b/src/system.c
@@ -0,0 +1,61 @@
+//
+// System: basic low-level system configuration
+//
+
+#include "stm32f0xx_hal.h"
+#include "wspr.h"
+
+void enter_sleep(void)
+{
+    //HAL_SuspendTick();
+    wspr_sleep();
+    HAL_PWR_EnterSLEEPMode(PWR_MAINREGULATOR_ON, PWR_SLEEPENTRY_WFI);
+    wspr_wakeup();
+    //HAL_ResumeTick();
+}
+
+
+void enter_deepsleep(void)
+{
+    // Request to enter STOP mode with regulator in low power mode
+    HAL_PWR_EnterSTOPMode(PWR_LOWPOWERREGULATOR_ON, PWR_STOPENTRY_WFI);
+    // After wake-up from STOP reconfigure the PLL
+    sysclk_init();
+}
+
+
+// Initialize system clocks
+void sysclk_init(void)
+{
+    RCC_OscInitTypeDef RCC_OscInitStruct;
+    RCC_ClkInitTypeDef RCC_ClkInitStruct;
+    RCC_PeriphCLKInitTypeDef PeriphClkInit;
+
+    RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI|RCC_OSCILLATORTYPE_HSI14;
+    RCC_OscInitStruct.HSIState = RCC_HSI_ON;
+    RCC_OscInitStruct.HSI14State = RCC_HSI14_ON;
+    RCC_OscInitStruct.HSICalibrationValue = 16;
+    RCC_OscInitStruct.HSI14CalibrationValue = 16;
+    RCC_OscInitStruct.PLL.PLLState = RCC_PLL_NONE;
+    HAL_RCC_OscConfig(&RCC_OscInitStruct);
+
+    RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK|RCC_CLOCKTYPE_SYSCLK
+                                |RCC_CLOCKTYPE_PCLK1;
+    RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_HSI;
+    RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
+    RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV1;
+    HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_0);
+
+    PeriphClkInit.PeriphClockSelection = RCC_PERIPHCLK_USART1|RCC_PERIPHCLK_I2C1;
+    PeriphClkInit.Usart1ClockSelection = RCC_USART1CLKSOURCE_SYSCLK; //RCC_USART1CLKSOURCE_PCLK1;
+    PeriphClkInit.I2c1ClockSelection = RCC_I2C1CLKSOURCE_SYSCLK;
+    HAL_RCCEx_PeriphCLKConfig(&PeriphClkInit);
+
+    HAL_SYSTICK_Config(HAL_RCC_GetHCLKFreq()/1000);
+
+    HAL_SYSTICK_CLKSourceConfig(SYSTICK_CLKSOURCE_HCLK);
+
+    __SYSCFG_CLK_ENABLE();
+    // SysTick_IRQn interrupt configuration
+    HAL_NVIC_SetPriority(SysTick_IRQn, 0, 0);
+}