STM32H7: add README file for dual core use

Author: jeromecoutant

targets/TARGET_STM/TARGET_STM32H7/README.md

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+# STM32H7 family
+
+[st.com STM32H7 family page](https://www.st.com/en/microcontrollers-microprocessors/stm32h7-series.html)
+
+[GitHub STM32CubeH7 FW](https://github.com/STMicroelectronics/STM32CubeH7)
+
+
+# NUCLEO_H743ZI / NUCLEO_H743ZI2
+
+Note that NUCLEO_H743ZI is deprecated. Please update your board to NUCLEO_H743ZI2:
+- new MCU chip revision
+- new ST Link version
+
+STM32H743ZI devices are based on the high-performance Arm Cortex-M7 32-bit RISC core operating at up to 480 MHz.
+
+[st.com STM32H743ZI MCU page](https://www.st.com/en/microcontrollers-microprocessors/stm32h743zi.html)
+
+[st.com NUCLEO page](https://www.st.com/en/evaluation-tools/nucleo-h743zi.html)
+
+[mbed.com Target page](https://os.mbed.com/platforms/ST-Nucleo-H743ZI2/)
+
+
+- Total FLASH is 2 MB (0x200000)
+    - 2x8 sectors of 128 KB
+    - Flash memory bank 1 @ 0x0800 0000
+    - Flash memory bank 2 @ 0x0810 0000
+
+- RAM: 1 MB
+    - DTCM :      128 KB (0x20000) @0x2000 0000
+    - SRAM :      512 KB (0x80000) @0x2400 0000
+    - SRAM1 :     128 KB (0x20000) @0x3000 0000
+    - SRAM2 :     128 KB (0x20000) @0x3002 0000
+    - SRAM3 :      32 KB  (0x8000) @0x3004 0000
+    - SRAM4 :      64 KB (0x10000) @0x3800 0000
+
+
+# DISCO_H747I
+
+## Overview
+
+The STM32H7x7 lines combine the performance of the Cortex-M7 (with double-precision floating point unit) running up to 480 MHz and the Cortex-M4 core (with single-precision floating point unit).
+
+[st.com STM32H747ZI MCU page](https://www.st.com/en/microcontrollers-microprocessors/stm32h747zi.html)
+
+[st.com board page](https://www.st.com/en/evaluation-tools/stm32h747i-disco.html)
+
+[mbed.com Target page](https://os.mbed.com/platforms/ST-Discovery-H747I/)
+
+- Total FLASH is 2 MB (0x200000)
+    - Flash memory bank 1 @ 0x0800 0000: 8 sectors of 128 KB => 1MB
+    - Flash memory bank 2 @ 0x0810 0000: 8 sectors of 128 KB => 1MB
+
+- RAM: 1 MB
+    - DTCM :  128 KB (0x20000) @0x2000 0000 - only CM7 side
+    - SRAM :  512 KB (0x80000) @0x2400 0000 - shared
+    - SRAM1 : 128 KB (0x20000) @0x3000 0000 - shared
+    - SRAM2 : 128 KB (0x20000) @0x3002 0000 - shared
+    - SRAM3 :  32 KB  (0x8000) @0x3004 0000 - shared
+    - SRAM4 :  64 KB (0x10000) @0x3800 0000 - shared
+
+## Dual mode configuration
+
+Configuration can be checked/changed with STM32CubeProgrammer software in the Option bytes (OB) panel.
+
+https://www.st.com/en/development-tools/stm32cubeprog.html
+
+Default configuration is booting both CM7 and CM4:
+- CM7 at 0x0800 0000
+- CM4 at 0x0810 0000
+
+Note that `Drag and Drop` feature for binary application is flashing at 0x0800 0000
+
+**IP HW usage warning**
+
+All resources (GPIO, UART, SPI, ...) can be used by both cores.
+
+It is up to application to choose which instance is used by which core.
+
+For GPIO, HW semaphore have been implemented which allows applications to use same GPIO port in both sides
+(CM4 can use PA_1, and CM7 PA_2).
+
+NB : Virtual Com Port is using the same UART pins in CM4 and CM7.
+
+It is not recommended to enable printf from both cores without any protection mechanism.
+
+**System clock configuration**
+
+Clock configuration is shared between both cores.
+
+During boot procedure:
+- M7 core is managing clock tree settings, and then wake up M4
+- M4 is waiting for wake up from M7
+
+When exiting Deep Sleep mode, first core (either M4 or M7) that woke up, is setting back this clock configuration.
+
+
+## Make application on M7 side
+
+- Flash memory bank 1 is used
+- DTCM and SRAM are used
+- SRAM3 is used for Ethernet support
+
+As explained in the `Dual mode configuration` above part, you can drag and drop applications.
+
+For application compilation, you can use DISCO_H747I or DISCO_H747I_CM7 target name.
+
+For MBED tests execution, you have to use DISCO_H747I target name (mbed detect name).
+
+
+## Make application on M4 side
+
+- Flash memory bank 2 is used
+- SRAM1, SRAM1 and SRAM3 is used (with @0x1000 0000 alias)
+
+For application compilation, you have to use DISCO_H747I_CM4 target name.
+
+For flashing, as explained in the `Dual mode configuration` above part, you can not drag and drop applications.
+You have to use STM32CubeProgrammer software
+
+https://www.st.com/en/development-tools/stm32cubeprog.html
+
+Here is command lines for flashing (but you can use graphical tool):
+```
+FLASHPATH="C:\Program Files (x86)\STMicroelectronics\STM32Cube\STM32CubeProgrammer\bin"
+export PATH=$FLASHPATH:$PATH
+STM32_Programmer_CLI -c port=SWD mode=UR -w BUILD/DISCO_H747I_CM4/ARM/mbed-os.bin 0x8100000
+```
+
+## Ethernet limitation
+
+**WARNING**: Ethernet connector is not enabled by default
+
+More information in the wiki page : [Ethernet HW Patch](https://os.mbed.com/teams/ST/wiki/DISCO_H747I-modifications-for-Ethernet)
+
+