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+# Cmake Usage
+
+## Introduction
+
+This guide documents inav usage of the `cmake` build tool.
+
+## Target Defintion
+
+A target requires `CMakeLists.txt` file. This file contains one of more lines of the form:
+
+```
+target_hardware_definition(name optional_parameters)
+```
+
+For example:
+
+```
+target_stm32f405xg(QUARKVISION HSE_MHZ 16)
+```
+
+* The hardware is `stm32f405xg`, a standard F405
+* The name is `QUARKVISION` (a board that never reached production)
+* The optional parameter is `HSE_MHZ 16` defining the non-standard high-speed external (HSE) oscillator clock.
+
+## Hardware names
+
+As of inav 2.6, the following target hardware platforms are recognised:
+
+* stm32f303xc
+* stm32f405xg
+* stm32f411xe
+* stm32f427xg
+* stm32f722xe
+* stm32f745xg
+* stm32f765xg
+* stm32f765xi
+
+## Optional Parameters
+
+The following optional parameters are recognised:
+
+| Paramater | Usage |
+| --------- | ----- |
+| `SKIP_RELEASES` | The target is disabled |
+| `COMPILE_DEFINITIONS "VAR=[value]"` | Sets a preprocessor define. |
+| `HSE_MZ value` | The target uses an non-standard non-standard high-speed external (HSE) oscillator clock. The `value` is the desired clock, for example `HSE_MHZ 24` |
+
+Multiple optional parameters may be specified, for example `HSE_MHZ 16 SKIP_RELEASES`.
+
+## Target variations
+
+A number of targets support multiple variants, either successive versions of the same hardware, or varations that enable different resources (soft serial, leds etc.) This is defined by adding additional `target_` lines to `CMakeLists.txt`. For example, the OMNIBUSF4 and its multiple clones / variations, `src/main/target/OMNIBUSF4/CMakeLists.txt`:
+
+```
+target_stm32f405xg(DYSF4PRO)
+target_stm32f405xg(DYSF4PROV2)
+target_stm32f405xg(OMNIBUSF4)
+# the OMNIBUSF4SD has an SDCARD instead of flash, a BMP280 baro and therefore a slightly different ppm/pwm and SPI mapping
+target_stm32f405xg(OMNIBUSF4PRO)
+target_stm32f405xg(OMNIBUSF4PRO_LEDSTRIPM5)
+target_stm32f405xg(OMNIBUSF4V3_S5_S6_2SS)
+target_stm32f405xg(OMNIBUSF4V3_S5S6_SS)
+target_stm32f405xg(OMNIBUSF4V3_S6_SS)
+# OMNIBUSF4V3 is a (almost identical) variant of OMNIBUSF4PRO target,
+# except for an inverter on UART6.
+target_stm32f405xg(OMNIBUSF4V3)
+```