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+# Getting started with bootloader on mbed OS
+
+This example is a clone of [mbed-os-example-bootloader](https://github.com/ARMmbed/mbed-os-example-bootloader)
+with SD card in SPI mode replaced with SD mode.
+For bootloader flow details, please refer to the [original](https://github.com/ARMmbed/mbed-os-example-bootloader) one.
+
+## Supported platforms
+- [NuMaker-PFM-NUC472](https://developer.mbed.org/platforms/Nuvoton-NUC472/)
+- [NuMaker-PFM-M487](https://developer.mbed.org/platforms/NUMAKER-PFM-M487/)
+
+For targets which don't support SD card in SD mode, please refer back to the [original](https://github.com/ARMmbed/mbed-os-example-bootloader) one.
+- [NuMaker-PFM-M453](https://developer.mbed.org/platforms/Nuvoton-M453/)
+- [NuMaker-PFM-NANO130](https://os.mbed.com/platforms/NUMAKER-PFM-NANO130/)
+
+## Configuration for a new target
+To support a new target, we need to modify `mbed_app.json` for the new target:
+1. Specify bootloader image size after padding. Here it is 0x20000 for both **NUMAKER_FPM_NUC472** and **NUMAKER_FPM_M487**.
+1. Specify application image name which would be saved in SD card.
+   Same as the [original](https://github.com/ARMmbed/mbed-os-example-bootloader) one,
+   the application image is built from [mbed-os-example-bootloader-blinky](https://github.com/ARMmbed/mbed-os-example-bootloader-blinky).
+1. The SPI pins `sd_card_mosi`/`sd_card_miso`/`sd_card_sck`/`sd_card_cs` are for SD card in SPI mode.
+   They are for information and are not used here. 
+```
+        "NUMAKER_PFM_NUC472": {
+            "target.restrict_size": "0x20000",
+            "update_file": "\"mbed-os-example-bootloader-blinky_application.bin\"",
+            "sd_card_mosi": "PF_0",
+            "sd_card_miso": "PD_15",
+            "sd_card_sck":  "PD_14",
+            "sd_card_cs":   "PD_13"
+        },
+        "NUMAKER_PFM_M487": {
+            "target.restrict_size": "0x20000",
+            "update_file": "\"mbed-os-example-bootloader-blinky_application.bin\"",
+            "sd_card_mosi": "D11",
+            "sd_card_miso": "D12",
+            "sd_card_sck":  "D13",
+            "sd_card_cs":   "D10"
+        }
+```
+
+## SD card in SD mode
+In `main.cpp`, include header file for block device of SD card in SD mode:
+```
+#include "mbed.h"
+#if defined(TARGET_NUMAKER_PFM_NUC472) || defined(TARGET_NUMAKER_PFM_M487)
+#include "NuSDBlockDevice.h"
+#else
+#include "SDBlockDevice.h"
+#endif
+#include "FATFileSystem.h"
+```
+In `main.cpp`, instantiate block device of SD card in SD mode:
+```
+#if defined(TARGET_NUMAKER_PFM_NUC472) || defined(TARGET_NUMAKER_PFM_M487)
+NuSDBlockDevice sd;
+#else
+//Pin order: MOSI, MISO, SCK, CS
+SDBlockDevice sd(MBED_CONF_APP_SD_CARD_MOSI, MBED_CONF_APP_SD_CARD_MISO,
+                 MBED_CONF_APP_SD_CARD_SCK, MBED_CONF_APP_SD_CARD_CS);
+#endif
+```
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