diff --git a/firmware/config/boards/STM32F407VET6_Mini.bat b/firmware/config/boards/STM32F407VET6_Mini.bat
index 43c23076f6..34bd32ee8c 100644
--- a/firmware/config/boards/STM32F407VET6_Mini.bat
+++ b/firmware/config/boards/STM32F407VET6_Mini.bat
@@ -5,7 +5,7 @@ rem TODO: somehow this -DDUMMY is helping us to not mess up the parameters, why?
 rem https://github.com/rusefi/rusefi/issues/684
 rem this board has only 512K flash so using custom FLASH_ADDR
 rem You probably want "flash0  : org = 0x08000000, len = 450K" in the .ld file
-set EXTRA_PARAMS="-DDUMMY -DEFI_COMMUNICATION_PIN=GPIOB_9 -DSTM32_PLLM_VALUE=25 -DDEFAULT_ENGINE_TYPE=MINIMAL_PINS -DFLASH_ADDR=0x08075000"
+set EXTRA_PARAMS="-DDUMMY -DEFI_COMMUNICATION_PIN=GPIOB_9 -DSTM32_PLLM_VALUE=25 -DSTM32_RTCPRE_VALUE=25 -DDEFAULT_ENGINE_TYPE=MINIMAL_PINS -DFLASH_ADDR=0x08075000"
 make -j8 clean
 make -j8
 rm -f build/rusefi.dfu
diff --git a/firmware/config/stm32f4ems/mcuconf.h b/firmware/config/stm32f4ems/mcuconf.h
index d5e08e2aa9..bd94e4f9a5 100644
--- a/firmware/config/stm32f4ems/mcuconf.h
+++ b/firmware/config/stm32f4ems/mcuconf.h
@@ -91,7 +91,9 @@
 #else
  #define STM32_RTCSEL                        STM32_RTCSEL_LSI
 #endif
+#ifndef STM32_RTCPRE_VALUE
 #define STM32_RTCPRE_VALUE                  8
+#endif
 #define STM32_MCO1SEL                       STM32_MCO1SEL_HSI
 #define STM32_MCO1PRE                       STM32_MCO1PRE_DIV1
 #define STM32_MCO2SEL                       STM32_MCO2SEL_SYSCLK