Moved version definition in Makefile

The expected version can now be set in the Makefile via USR_CXXFLAGS.
Additionally, the README.md has received documentation regarding the
version check. Lastly, the version check can now be disabled by omitting
the flags or setting one of them to a negative value.

README.md

@@ -66,6 +66,32 @@ dbLoadRecords("$(masterMacs_DB)/asynRecord.db","P=$(INSTR)$(NAME),PORT=$(ASYN_PO
 
 Please see the documentation for the module sinqMotor: https://git.psi.ch/sinq-epics-modules/sinqmotor/-/blob/main/README.md.
 
+### Firmware version checking
+
+This driver expects a certain version of the firmware running on the controller itself.
+This is checked at IOC startup by reading the version directly from the hardware.
+If the firmware version is incompatible to the driver, the IOC will be shut down.
+If the firmware version cannot be read (e.g. because the variable used to do so
+does not exist yet on old firmware versions), the firmware is assumed to be compatible
+to the driver.
+
+The version check is separated into a check of the major and the minor firmware
+version against expected values. The firmware is seen as compatible if the following conditions hold:
+- Read-out major version == Expected major version
+- Read-out read major version >= Expected minor version
+
+The expected versions are defined via compiler flags in `Makefile`:
+
+```
+USR_CXXFLAGS += -DFIRMWARE_MAJOR_VERSION=1 -DFIRMWARE_MINOR_VERSION=0
+```
+Be aware that these flags are only used to compile C++-files (.cpp, .cxx) and not
+C-files (.c). For C-files, the Makefile variable `USR_CFLAGS` must be used.
+
+In order to disable the checks, the flags can be set to -1 or just be removed
+entirely. If one of the flags is not given, both the major and the minor version 
+checks are deactivated.
+
 ### How to build it
 
 Please see the documentation for the module sinqMotor: https://git.psi.ch/sinq-epics-modules/sinqmotor/-/blob/main/README.md.