AP_BoardConfig: added BRD_SD_SLOWDOWN parameter

allows for reduction in microSD clock speed

libraries/AP_BoardConfig/AP_BoardConfig.cpp

@@ -223,6 +223,16 @@ const AP_Param::GroupInfo AP_BoardConfig::var_info[] = {
     // @Path: ../AP_RTC/AP_RTC.cpp
     AP_SUBGROUPINFO(rtc, "RTC", 14, AP_BoardConfig, AP_RTC),
 
+#if CONFIG_HAL_BOARD == HAL_BOARD_CHIBIOS
+    // @Param: SD_SLOWDOWN
+    // @DisplayName: microSD slowdown
+    // @Description: This is a scaling factor to slow down microSD operation. It can be used on flight board and microSD card combinations where full speed is not reliable. For normal full speed operation a value of 0 should be used.
+    // @Range: 0 32
+    // @Increment: 1
+    // @User: Advanced
+    AP_GROUPINFO("SD_SLOWDOWN",  17,     AP_BoardConfig, _sdcard_slowdown,  0),
+#endif
+
     AP_GROUPEND
 };
 
@@ -238,6 +248,23 @@ void AP_BoardConfig::init()
 #endif
 
     AP::rtc().set_utc_usec(hal.util->get_hw_rtc(), AP_RTC::SOURCE_HW);
+
+#if CONFIG_HAL_BOARD == HAL_BOARD_CHIBIOS && defined(USE_POSIX)
+    uint8_t slowdown = constrain_int16(_sdcard_slowdown.get(), 0, 32);
+    const uint8_t max_slowdown = 8;
+    do {
+        if (hal.util->fs_init()) {
+            break;
+        }
+        slowdown++;
+        hal.scheduler->delay(5);
+    } while (slowdown < max_slowdown);
+    if (slowdown < max_slowdown) {
+        _sdcard_slowdown.set(slowdown);
+    } else {
+        printf("SDCard failed to start\n");
+    }
+#endif
 }
 
 // set default value for BRD_SAFETY_MASK

libraries/AP_BoardConfig/AP_BoardConfig.h

@@ -149,7 +149,12 @@ public:
 #endif
     }
 
-    
+#if CONFIG_HAL_BOARD == HAL_BOARD_CHIBIOS
+    static uint8_t get_sdcard_slowdown(void) {
+        return instance?instance->_sdcard_slowdown.get():0;
+    }
+#endif
+
 private:
     static AP_BoardConfig *instance;
     
@@ -216,4 +221,8 @@ private:
 
     // real-time-clock; private because access is via the singleton
     AP_RTC rtc;
+
+#if CONFIG_HAL_BOARD == HAL_BOARD_CHIBIOS
+    AP_Int8 _sdcard_slowdown;
+#endif
 };