Unit Validation

OSynaptic-TX validates unit strings at encode time using the OSTX_UNIT() macro. Pass an invalid unit and the encode function returns 0 (no packet emitted).

OSTX_UNIT() Macro

/* Will not compile if "xyz" is not a recognised unit code */
#define MY_UNIT OSTX_UNIT("Cel")

/* Use in pack calls */
ostx_sensor_pack(aid, "T1", OSTX_UNIT("Cel"), tid, ts, scaled, out);

On compilers that support _Static_assert (C11) the validation is a compile-time error. On C89 targets the check falls back to a runtime guard that returns 0 on failure.

Recognised Unit Codes (subset)

The full list is in ostx_unit_table.h. Common categories:

Temperature

Code	Full name
Cel	Degree Celsius
K	Kelvin
[degF]	Degree Fahrenheit

Humidity & Pressure

Code	Full name
Pct	Percentage (0–100)
kPa	Kilopascal
hPa	Hectopascal (millibar)
Pa	Pascal
mm[Hg]	Millimetres of mercury

Concentration & Air Quality

Code	Full name
ppm	Parts per million
ppb	Parts per billion
mg/m3	Milligrams per cubic metre
ug/m3	Micrograms per cubic metre

Electrical

Code	Full name
V	Volt
mV	Millivolt
A	Ampere
mA	Milliampere
W	Watt
Wh	Watt-hour
kWh	Kilowatt-hour
Ohm	Ohm
Hz	Hertz
kHz	Kilohertz

Length, Speed, Acceleration

Code	Full name
m	Metre
cm	Centimetre
mm	Millimetre
m/s	Metre per second
km/h	Kilometre per hour
m/s2	Metre per second squared

Light, Radiation

Code	Full name
lx	Lux
uW/cm2	Microwatt per square centimetre
mSv	Millisievert

Count & Dimensionless

Code	Full name
1	Dimensionless (ratio)
{count}	Raw count
{rpm}	Revolutions per minute

SI Prefix Rules

All UCUM SI prefixes are accepted:

Prefix	Symbol	Factor
peta	P	10¹⁵
tera	T	10¹²
giga	G	10⁹
mega	M	10⁶
kilo	k	10³
hecto	h	10²
deca	da	10¹
deci	d	10⁻¹
centi	c	10⁻²
milli	m	10⁻³
micro	u	10⁻⁶
nano	n	10⁻⁹
pico	p	10⁻¹²

Example: km, mg, uV, nA are all valid by prefix expansion.

Bypassing Validation

If you have a custom unit string not covered by the table, define OSTX_ALLOW_CUSTOM_UNITS=1 and the validation is skipped. Receivers will still accept the packet; they simply forward the raw unit string as-is.