# SI Units¶

lbuild module: `modm:math:units`

modm uses a couple of common SI units for configuration of peripherals:

• `frequency_t` in Hertz: `Hz`, `kHz` and `MHz`.
• `baudrate_t` in Baud: `Bd`, `kBd`, `MBd`.
• `bitrate_t` in bit/s, `bps`.

These are integral units, so 1 Hz/Bd/bps cannot be split further, and are cast directly to `uint32_t` type, so they can be used as a non-type template argument.

Conversion can be done via constexpr functions from any numerical type:

• `modm::Hz(T value)`, `modm::kHz(T value)`, `modm::MHz(T value)`.
• `modm::Bd(T value)`, `modm::kBd(T value)`, `modm::MBd(T value)`.
• `modm::bps(T value)`, `modm::kbps(T value)`, `modm::Mbps(T value)`.

In addition, user-defined literals are provided in the `modm::literals` namespace:

``````using namespace modm::literals;

frequency_t frequency = modm::Mhz(10.5);
frequency = 10.5_MHz;
baudrate_t baudrate = 115.2_kBd;
baudrate = modm::kBd(115.2);
bitrate_t bitrate = modm::kbps(125);
bitrate = 125_kbps;

frequency = 4295_MHz; // OVERFLOW at 2^32 units!
``````

## Percentages¶

Percentages are stored as normalized floating point numbers and can be converted using these convenience constructors:

``````using namespace modm::literals;

percent_t tolerance = modm::pct(10);
tolerance = 10_pct;
tolerance = 0.1f;
``````

## Time¶

Simplified types allow passing `std::chrono::duration` values as template parameters:

``````seconds_t duration = 10s;
milliseconds_t duration = 10ms;
microseconds_t duration = 10us;

auto count = duration.count();

template< milliseconds_t period >
void setPeriod()
{
auto seconds = 1000.0 / period.count();
}
``````