AN3254

Converting readings into measured values

Doc ID 17783 Rev 1

17/24

This also means that the STPMC1 energy counters hold a very small energy value (in the

example above, when LSB represents 15.258 µWh, the whole register stores 16 Wh), and

further energy integration must be performed inside the application.

To accomplish this task, the procedure below 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. It is suggested 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

is 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 the 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

old and e

value in order to enable a correct

e

When

e

bit and the

e

In this way,

e

integ variable holds accumulated energy of which the least significant bit represents 10 Wh.

Considering an active energy meter where P = 64000 imp/kWh, for a step of 0.01 KWh = 10

Wh, as each bit of

e

energy counter, because

e

In a microcontroller based application, a high priority timer interrupt should be set to perform

measuring tasks every 1/512 s. Within this interrupt service 16 different subtasks could be

established in order to break the whole meter task into 16 shorter consecutive subtasks

(reading of device register, checking the data read, and, if ok, computing the value of

e

→

quant). In this way, the main program and other interrupt services are not blocked for more

than a few 100 us every 2 ms, and the meter task is completed in 16 steps - that is in 1/32 s.