diff -r 0c9f5038fc63 -r f3070bd842cd content/hp34970a_6.rst
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+==========================================
+ HP 34970A Data Acquisition Unit - part 6
+==========================================
+
+
+:Author: David Douard
+:Category: Electronics
+:Tags: HP, 34970A, HP34970A, DMM, repair, test equipment
+:series: HP 34970A repair
+:series_index: 6
+
+
+Quick follow up; I recently got back on this project to try to complete it and fix
+remaining problems.
+
+The source code for the firmware is available here:
+
+  https://hg.sdfa3.org/hp34970-firmware
+
+The Kicad project is available here:
+
+  https://hg.sdfa3.org/hp34970-pcb
+
+
+Making the USBDevice work
+=========================
+
+As described in the `previous episode <{filename}/hp34970a_5.rst>`_, I could not make
+the USBDevice of the STM32F303RD work with `mbed-os <https://os.mbed.com/>`_ framework I
+use to write the firmware.
+
+The `STM32F303RD
+<https://www.st.com/en/microcontrollers-microprocessors/stm32f303rd.html>`_ is not yet
+officially supported by mbed-os (mainl because there is no `NUCLEO
+<https://www.st.com/en/evaluation-tools/stm32-nucleo-boards.html>`_ board with
+this CPU.
+
+However the STM32F303RE is supported, at least partially. For example, the USBDevice is
+not working at all in the latest version of mbed-os (6.15.0). But some `work has been
+done recently <https://github.com/ARMmbed/mbed-os/pull/15116>`_ to enable USB_DEVICE on
+CPUs of the STM32F3 family. But this last PR was not enoght to make the USB device work;
+I have submitted a `PR <https://github.com/ARMmbed/mbed-os/pull/15132>`_ that needs to
+be merged to make the USB device actually work.
+
+Also, the difference between the 2 being (as far as I know) only the size of the flash size
+(384KB vs. 512 KB).
+
+So I had 3 problems to solve:
+
+- compile the code for a custom board using the mbed framework in platformio,
+- use a custom CPU decription,
+- use a patched version of the latest version of mbed-os in platformio (while the
+  officially supported version at this moment is only the 6.9.0).
+
+
+Custom version of mbed in platformio
+------------------------------------
+
+Using a custom version of a framework (here mbed) in platformio is not properly
+documented. But thanks to `the response
+<https://community.platformio.org/t/support-for-mbed-os-6-stable-and-mature-apis-cloud-services-support-enhancements-to-the-bare-metal-profile/15079/9>`_
+to my question on the `platformio forum <https://community.platformio.org/>`_, I could
+make it work. Obviously, I submitted `a PR
+<https://github.com/platformio/platformio-docs/pull/214>`_ to `platformio-docs
+<https://github.com/platformio/platformio-docs>`_.
+
+So now I have my (more or less) `up to date repository
+<https://github.com/douardda/platformio-framework-mbed>`_ usable as `platform_packages`
+config argument.
+
+Custom board and CPU description
+--------------------------------
+
+This part I am not completely sure how it should be done. There are actually 2 parts:
+- tell mbed about the CPU description,
+- tell platformio about is and use it.
+
+The platformio part is `documented here
+<https://docs.platformio.org/en/latest/platforms/creating_board.html>`_
+
+It mostly consists in creating a json file in the `boards/` directory of the platformio
+project directory.
+
+For this project, I used the description of the NUCLEO_F303RE as starting point. The
+file is `boards/hp34970_fp_f303rd.json
+<https://hg.sdfa3.org/hp34970-firmware/file/tip/boards/hp34970_fp_f303rd.json>`_ in the
+`firmware source code repository <https://hg.sdfa3.org/hp34970-firmware/>`_.
+
+Once the board is described, it can be used in the `platformio.ini` file:
+
+.. code-block:: bash
+
+   ~/e/hp34970-firmware$ pio boards | grep hp34
+   hp34970_fp_f303rd          STM32F303RDT7    72MHz        384KB     64KB      HP34970 Replace Front Panel Board (STM32F303RD, 64k RAM, 384k Flash)
+
+
+
+Then we need to tell mbed about this board. This is `documented there
+<https://os.mbed.com/docs/mbed-os/v6.15/porting/porting-custom-boards.html>`_
+
+It is a matter of creating a `custom_targets.json` file with the board description.
+Again, I used the existing NUCLEO_F303RE as starting point, then added/removed what I
+needed for this project:
+
+.. code-block:: json
+
+   {
+     "HP34970_FP_F303RD": {
+       "inherits": [
+         "MCU_STM32F303xE"
+       ],
+       "overrides": {
+         "clock_source": "USE_PLL_HSE_XTAL"
+       },
+       "device_has_add": [
+         "USBDEVICE"
+       ],
+       "mbed_rom_size": "0x60000",
+       "bootloader_supported": true,
+       "device_name": "STM32F303RE"
+     }
+   }
+
+Then the actual description (pins, memory config, etc.) are C files put in a
+`TARGET_HP34970_FP_F303RD` directory. Here again, started from files copied from the
+`STM32F303xE target
+<https://github.com/ARMmbed/mbed-os/tree/master/targets/TARGET_STM/TARGET_STM32F3/TARGET_STM32F303xE>`_.
+
+
+End result
+==========
+
+It took me some time, but in the end, I finally got a firmware that works OK, including
+a working USB serial port. For now, this serial connection is still used only for
+debugging purpose (and firmware upload in DFU mode), but it may be improved in the
+future, if needs be.
+
+So I could put the HP34970A back together (at last). I drilled a couple of small holes
+for the Reset and DFU buttons and a (not so) square hole for the USB plug in the blue
+glass.
+
+The result looks very decent to me:
+
+.. image:: {static}/images/hp34970a/finished.jpg
+   :alt: finished look of the device with the replacement front panel
+   :class: image-process-large-photo
+
+.. peertube:: 14d0e7e2-74d5-44fe-90e9-d6b102ae01a1