[BL] Updating project documentation

Author: aethaniel

bootloaders/zero/README.md

@@ -1,21 +1,47 @@
-1- Prerequisites
+# Arduino Zero Bootloader
+# -----------------------
 
-IAR Embedded Workbench for ARM 7.30
+## 1- Prerequisites
 
-2- Selecting between USB and UART interface
+The project build is based on Makefile system.
+Makefile is present at project root and try to handle multi-platform cases.
+
+Multi-plaform GCC is provided by ARM here: https://launchpad.net/gcc-arm-embedded/+download
+
+### Windows
+
+Native command line:
+Make binary can be obtained here: http://gnuwin32.sourceforge.net/packages/make.htm
+
+* Cygwin/MSys/MSys2/Babun/etc...:
+It is available natively in all distributions.
+
+* Atmel Studio:
+An Atmel Studio Makefile-based project is present at project root, just open samd21_sam_ba.atsln file.
+
+### Linux
+
+Make is usually available by default.
+
+### OS X
+
+Make is available through XCode package.
+
+
+## 2- Selecting available SAM-BA interfaces
+
+By default both USB and UART are made available, but this parameter can be modified in sam_ba_monitor.h, line 31:
 
 Set the define SAM_BA_INTERFACE to
-SAM_BA_UART_ONLY for only UART interface
-SAM_BA_USBCDC_ONLY for only USB CDC interface
-SAM_BA_BOTH_INTERFACES for enabling both the interfaces
+* SAM_BA_UART_ONLY for only UART interface
+* SAM_BA_USBCDC_ONLY for only USB CDC interface
+* SAM_BA_BOTH_INTERFACES for enabling both the interfaces
 
-SAM_BA_INTERFACE value should be modified in
-Project Options -> C/C++ Compiler -> Preprocessor -> Defined symbols
-Project Options -> Assembler -> Preprocessor -> Defined symbols
 
-3- Start application check
+## 3- Behaviour
 
-Bootloader checks for the state of BOOT_LOAD_PIN (configurable by the user from main.h). If BOOT_LOAD_PIN is pulled low, bootloader execution is resumed.
-Else, the first location of application is fetched and checked. If it is empty (0xFFFFFFFF), then bootloader execution is resumed. Else it jumps to application and starts execution from there.
+This bootloader implements the double-tap on Reset button.
+By quickly pressing this button two times, the board will reset and stay in bootloader, waiting for communication on either USB or USART.
 
-Currently, BOOT_LOAD_PIN is PA15 of SAMD21G18A, pin 5 of Arduino Zero board.
+The USB port in use is the USB Native port, close to the Reset button.
+The USART in use is the one available on pins D0/D1, labelled respectively RX/TX.