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hw_voltage

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As it was stated in The Affine Space chapter, every measurement can (and probably should) be modelled as a quantity_point. This is a perfect example of such a use case.

This example implements a simplified scenario of measuring voltage read from hardware through a mapped 16-bits register. The actual voltage range of [-10 V, 10 V] is mapped to [-32767, 32767] on hardware. Translation of the value requires not only scaling of the value but also applying of an offset.

First we include all the dependencies:

hw_voltage.cpp
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#include <mp-units/compat_macros.h>
#include <mp-units/ext/format.h>
#ifdef MP_UNITS_IMPORT_STD
import std;
#else
#include <iostream>
#include <optional>
#endif
#ifdef MP_UNITS_MODULES
import mp_units;
#else
#include <mp-units/format.h>
#include <mp-units/systems/isq.h>
#include <mp-units/systems/si.h>
#endif

using namespace mp_units;

Next, we specify the real measurement voltage range to be in the range of [-10, 10]:

hw_voltage.cpp
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// real voltage range
inline constexpr int min_voltage = -10;
inline constexpr int max_voltage = 10;
inline constexpr int voltage_range = max_voltage - min_voltage;

and provide a storage type and special values for the hardware representation:

hw_voltage.cpp
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// hardware encoding of voltage
using voltage_hw_t = std::uint16_t;
inline constexpr voltage_hw_t voltage_hw_error = std::numeric_limits<voltage_hw_t>::max();
inline constexpr voltage_hw_t voltage_hw_min = 0;
inline constexpr voltage_hw_t voltage_hw_max = voltage_hw_error - 1;
inline constexpr voltage_hw_t voltage_hw_range = voltage_hw_max - voltage_hw_min;
inline constexpr voltage_hw_t voltage_hw_zero = voltage_hw_range / 2;

Finally, we define a quantity point origin, an offset unit that scales the value and uses this origin to offset the zero of the sale, and a dedicated quantity point alias using those:

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inline constexpr struct hw_voltage_origin final :
  relative_point_origin<absolute<si::volt>(min_voltage)> {} hw_voltage_origin;

inline constexpr struct hw_voltage_unit final :
  named_unit<"hwV", mag_ratio<voltage_range, voltage_hw_range> * si::volt, hw_voltage_origin> {} hw_voltage_unit;

using hw_voltage_quantity_point = quantity_point<hw_voltage_unit, hw_voltage_origin, voltage_hw_t>;

Now, when everything is ready, we can simulate mapping of our hardware register, and provide a helper function that will read the value and construct a quantity point from the obtained copy:

hw_voltage.cpp
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// mapped HW register
volatile voltage_hw_t hw_voltage_value;

std::optional<hw_voltage_quantity_point> read_hw_voltage()
{
  voltage_hw_t local_copy = hw_voltage_value;
  if (local_copy == voltage_hw_error) return std::nullopt;
  return absolute<hw_voltage_unit>(local_copy);
}

We also provide a simple print helper for our quantity points:

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void print(QuantityPoint auto qp)
{
  std::cout << MP_UNITS_STD_FMT::format("{:10} ({:5})", qp.quantity_from_zero(),
                                        value_cast<double, si::volt>(qp).quantity_from_zero());
}

In the main function we simulate setting of 3 values by our hardware. Each of them is read and printed in the voltage unit used on the hardware as well as in the standard SI unit:

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int main()
{
  // simulate reading of 3 values from the hardware
  hw_voltage_value = voltage_hw_min;
  quantity_point qp1 = read_hw_voltage().value();
  hw_voltage_value = voltage_hw_zero;
  quantity_point qp2 = read_hw_voltage().value();
  hw_voltage_value = voltage_hw_max;
  quantity_point qp3 = read_hw_voltage().value();

  print(qp1);
  print(qp2);
  print(qp3);
}

The above program results with the following text output:

     0 hwV (-10 V)
 32767 hwV (  0 V)
 65534 hwV ( 10 V)