Converting readings into measured values

AN2159

16/23

Doc ID 11400 Rev 3

6

Converting readings into measured values

6.1

Energies

The first four registers contain 20 bit value of internal energy up/down counters.

The value of least significant bit of every energy counter is related to power meter constant

P, which is the number of pulses per kWh that the meter, through calibration, is configured to

provide to LED pin.

This means that this value changes with the application and relative calibration.

Given P, the value of the LSB of the source energy registers is indicated below:

For example, if P = 64000 imp/kWh:

This also means that the STPMxx energy counters hold a very small energy value (in the

example above, the active energy register stores about 8 Wh), and further energy

integration has to be performed inside the application.

To accomplish this task, the below procedure should be followed.

Because all energy counters rollover in approximately 1 s when they are integrating maximal

power, the reading must be done frequently enough. Our suggestion is to read the registers

at least 32 times per second.

For each energy type a variable

e should be allocated, having the following structure (below,

the variable definition for an ST7 microcontroller):

typedef struct energ {

unsigned long old;

/* previous energy value - 32 bits */

unsigned int quot;

/* quant/16 - 16 bits */

signed int quant;

/* new - old, measure of power - 16 bits */

signed long frac;

/* fractional part of energy integrator - 32 bits */

signed long integ;

/* integer part of energy integrator - 32 bits */

} ENERG;

The application should keep previous value of each energy counter in order to evaluate the

difference of readings, from which also a direction of energy flow can be obtained. This

value should be stored in

e

register reading should be stored in

e

To calculate consummated energy the software should implement a 32-bit integrator. The

suggested integrator is two stages, with

e

variables. Into

e