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Document how to implement platform specifics (#1262)

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Some interfaces need to be implemented in every project
or platform using the ETL:

* etl_get_high_resolution_clock
* etl_get_system_clock
* etl_get_steady_clock
* etl_putchar

Co-authored-by: John Wellbelove <jwellbelove@users.noreply.github.com>
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README.md
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@ -184,6 +184,84 @@ add_executable(foo main.cpp)
target_link_libraries(foo PRIVATE etl::etl)
```
## Profile definition
When using ETL in a project, there is typically an `etl_profile.h` defined to
adjust ETL to the project needs. ETL will automatically find `etl_profile.h`
if it is available in the include path(s). If it's not available, ETL will
work with default values.
### Example
```
#ifndef __ETL_PROFILE_H__
#define __ETL_PROFILE_H__
#define ETL_TARGET_DEVICE_GENERIC
#define ETL_TARGET_OS_NONE
#define ETL_NO_STL
#endif
```
## Platform specific implementation
Although ETL is generally a self-contained header-only library, some interfaces need to be
implemented in every project or platform, at least if those interfaces are actually being
used, due to project specifics:
| ETL header | Platform specific API to be implemented | Needed when using |
|------------|-----------------------------------------|-------------------------------------|
| `chrono.h` | `etl_get_high_resolution_clock()` | `etl::high_resolution_clock::now()` |
| | `etl_get_system_clock()` | `etl::system_clock::now()` |
| | `etl_get_steady_clock()` | `etl::steady_clock::now()` |
| `print.h` | `etl_putchar()` | `etl::print()` |
| | | `etl::println()` |
### Example
```
#include <etl/chrono.h>
#include <etl/print.h>
extern "C"
{
etl::chrono::high_resolution_clock::rep etl_get_high_resolution_clock()
{
etl::chrono::high_resolution_clock::rep(static_cast<int64_t>(getSystemTimeNs()));
}
etl::chrono::system_clock::rep etl_get_system_clock()
{
return etl::chrono::system_clock::rep(static_cast<int64_t>(getSystemTimeNs()));
}
etl::chrono::system_clock::rep etl_get_steady_clock()
{
return etl::chrono::system_clock::rep(static_cast<int64_t>(getSystemTimeNs()));
}
void etl_putchar(int c)
{
putByteToStdout(static_cast<uint8_t>(c));
}
}
```
The following default values apply if the respective macros are not defined
(e.g. in `etl_profile.h`):
| Macro | Default |
|-----------------------------------------------|----------------------------|
| `ETL_CHRONO_SYSTEM_CLOCK_DURATION` | `etl::chrono::nanoseconds` |
| `ETL_CHRONO_SYSTEM_CLOCK_IS_STEADY` | `true` |
| `ETL_CHRONO_HIGH_RESOLUTION_CLOCK_DURATION` | `etl::chrono::nanoseconds` |
| `ETL_CHRONO_HIGH_RESOLUTION_CLOCK_IS_STEADY` | `true` |
| `ETL_CHRONO_STEADY_CLOCK_DURATION` | `etl::chrono::nanoseconds` |
## Arduino library
The content of this repo is available as a library in the Arduino IDE (search for the "Embedded Template Library" in the IDE library manager). The Arduino library repository is available at ```https://github.com/ETLCPP/etl-arduino```, see there for more details.