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This README describes the layout of system files and headers for
microcontrollers supported by tinyusb from the vendor [ST](www.st.com).
The CMSIS by ARM is an attempt to unify how system initialization and headers
for ARM microcontrollers are handled across vendors. A vendor is expected to
provide a number of files, including vendor-specific device headers,
vendor-specific system init code (in C), vendor-specific startup code (coded in
assembly _and_ meant to make writing C interrupt handlers easy), and ARM
architecture-specific headers. In practice, it's actually difficult to find all
these files in one place.
* stm32cube generates a self-contained application. All the relevant headers,
system init, and startup code are under `Drivers/CMSIS`. The linker script is
stored at the root of your project, _along_ with a copy of the startup code.
Relative to `Drivers/CMSIS`:
* Device header files are stored under `Device/ST/$family/Include`.
* Arm-provided CMSIS headers are stored under `Include`.
* The portable C portion of the startup code, called `system_$family.c`, is
stored under `Device/ST/$family/Source/Templates`. This "system init" code
is required by CMSIS.
* The assembly portion of the startup code, called `startup_$device.c`, is
stored under `Device\ST\STM32F4xx\Source\Templates\gcc`.
* Micropython maintains [a repo](https://github.com/micropython/stm32lib)
with _only_ the device header files. We use that
repo and provide the remaining files ourselves.
* Micropython keeps the linker script and assembly portion of the startup
code under the
[boards](https://github.com/micropython/micropython/tree/master/ports/stm32/boards)
directory in the stm32 port (but the code is board agnostic).
* Micropython also embeds its own
[copy](https://github.com/micropython/micropython/tree/master/lib/cmsis/inc)
of CMSIS headers for all ARM targets into the repo.
* Micropython indeed _does_ use their own custom C startup code (CMSIS system
init) for
[stm32](https://github.com/micropython/micropython/blob/master/ports/stm32/system_stm32.c)
targets. It is
[missing](https://github.com/micropython/micropython/commit/fdbc22e4d27b9d1c686647c9c67a32206f7bc83a)
the `SystemCoreClockUpdate` function.
* TinyUSB uses Micropython's [stm32lib](stm32lib) as a submodule to provide
device headers. The remaining system files can be found in the following
directories:
* C system init code is in the [system-init](system-init) directory.
* Assembly startup code is in the [startup](startup) directory. Each family
gets its own subdirectory under startup (e.g. [stm32f4](startup/stm32f4)).
* ARM-provided headers are stored in the [cmsis](cmsis) directory.
* The linker script for the STM32F4-DISCOVERY board demo is supplied as
part of TinyUSB's
[Board Support Packages](../../bsp/stm32f407g_disc1/STM32F407VGTx_FLASH.ld).
The above files were extracted from a dummy stm32cube project in February
2019.
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