AN2159 datasheet(17/23 Pages) STMICROELECTRONICS

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AN2159

Converting readings into measured values

Doc ID 11400 Rev 3

17/23

old and e

→ new energy values; then e → old value should be rewritten with e → new

value in order to enable a correct

→ quant computation next time.

When

→ frac would collect a certain amount of energy, let say 10 Wh for active energy

(corresponding to a certain threshold value according to KAW), e → integ should change for

1 bit and the

→ frac should change by the threshold value.

This way

→ frac stores 0.01 kWh, after which e → integ is increased by one, and e →

integ variable will hold accumulated energy of which the least significant bit will represent

10 Wh.

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

Wh, since each bit of e'quant represents KAW Wh (is the same resolution of internal energy

counter, because

→ quant is calculated as a difference of two energy counter values),

the threshold value will be 10 / KAW = 10 * 2^17 = 0 x 140000.

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 broke the whole meter task into 16 shorter consecutive subtasks

(reading of device's register, checking the data read and if OK, computing the value of

→

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

more than few 100 µs every 2 ms, and the meter task will be completed in 16 steps - that is

in 1/32 s.

The interrupt service should do the following:

●

update

→ frac and e → integ of energy variable using e → quot = e → quant / 16

●

generate output pulses (if needed) from

→ frac

●

call the next subtask

●

perform other tasks (if needed)

In this way the addition of

→ quant is split in 16 times. This generates a microcontroller

output pulse that has a 16 times better accuracy of position in time. In fact the period of

reading would be 1/32 s = 31.25 ms. If the whole value of

→ quant would be added to the

final energy register

→ frac, only 31.25 ms resolution of output pulse position would be

possible, which would be seen as a jitter just by eye looking to the LED. Using suggested

method the resolution of output pulse position would be 1.95 ms, which is short jitter enough

that nobody would see it.

Below an example of subtasks organization is given:

subtask_0: latch the values in the STPMxx

subtask_1: read the STPMxx

subtask_2: repeat the reading of STPMxx (without latching again) and stop SPI

communication

subtask_3: verify the parity codes of registers and equality of both readings, result is flag OK

subtask_4: if OK unpack values of registers read from STPMxx

subtask_5: if OK process STPMxx status

subtask_6: if OK compute

→ quant and update e → old of active energy

subtask_7: if OK compute

→ quant and update e → old of reactive energy

subtask_8: if OK compute

→ quant and update e → old of apparent energy

subtask_9: if OK calculate Vrms and Irms,

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