diff --git a/libraries/SD_MMC/src/SD_MMC.cpp b/libraries/SD_MMC/src/SD_MMC.cpp
index 0c95fb9dece..450657cfa6f 100644
--- a/libraries/SD_MMC/src/SD_MMC.cpp
+++ b/libraries/SD_MMC/src/SD_MMC.cpp
@@ -36,7 +36,7 @@ SDMMCFS::SDMMCFS(FSImplPtr impl)
     : FS(impl), _card(NULL)
 {}
 
-bool SDMMCFS::begin(const char * mountpoint, bool mode1bit, bool format_if_mount_failed)
+bool SDMMCFS::begin(const char * mountpoint, bool mode1bit, bool format_if_mount_failed, int sdmmc_frequency)
 {
     if(_card) {
         return true;
@@ -46,7 +46,7 @@ bool SDMMCFS::begin(const char * mountpoint, bool mode1bit, bool format_if_mount
     sdmmc_host_t host;
     host.flags = SDMMC_HOST_FLAG_4BIT;
     host.slot = SDMMC_HOST_SLOT_1;
-    host.max_freq_khz = SDMMC_FREQ_HIGHSPEED;
+    host.max_freq_khz = sdmmc_frequency;
     host.io_voltage = 3.3f;
     host.init = &sdmmc_host_init;
     host.set_bus_width = &sdmmc_host_set_bus_width;
diff --git a/libraries/SD_MMC/src/SD_MMC.h b/libraries/SD_MMC/src/SD_MMC.h
index 6e40fd4540c..e8540b32fc4 100644
--- a/libraries/SD_MMC/src/SD_MMC.h
+++ b/libraries/SD_MMC/src/SD_MMC.h
@@ -21,6 +21,13 @@
 #include "driver/sdmmc_types.h"
 #include "sd_defines.h"
 
+// If reading/writing to the SD card is unstable, 
+// you can define BOARD_MAX_SDMMC_FREQ with lower value (Ex. SDMMC_FREQ_DEFAULT) 
+// in pins_arduino.h for your board variant.
+#ifndef BOARD_MAX_SDMMC_FREQ
+#define BOARD_MAX_SDMMC_FREQ SDMMC_FREQ_HIGHSPEED
+#endif
+
 namespace fs
 {
 
@@ -31,7 +38,7 @@ class SDMMCFS : public FS
 
 public:
     SDMMCFS(FSImplPtr impl);
-    bool begin(const char * mountpoint="/sdcard", bool mode1bit=false, bool format_if_mount_failed=false);
+    bool begin(const char * mountpoint="/sdcard", bool mode1bit=false, bool format_if_mount_failed=false, int sdmmc_frequency=BOARD_MAX_SDMMC_FREQ);
     void end();
     sdcard_type_t cardType();
     uint64_t cardSize();
diff --git a/variants/esp32-evb/pins_arduino.h b/variants/esp32-evb/pins_arduino.h
index 15fa98e2496..a2e02dc71a6 100644
--- a/variants/esp32-evb/pins_arduino.h
+++ b/variants/esp32-evb/pins_arduino.h
@@ -29,5 +29,6 @@ static const uint8_t MISO  = 15;
 static const uint8_t SCK   = 14;
 
 #define BOARD_HAS_1BIT_SDMMC
+#define BOARD_MAX_SDMMC_FREQ SDMMC_FREQ_DEFAULT
 
 #endif /* Pins_Arduino_h */