sames

SA9904B

7/12

http://www.sames.co.za

Figure 10: Register increment / decrement showing the

register wrap around

Register values

Positive energy flow

Negative energy flow

Register wrap around

Register wrap around

H7FFFFF

(8388607)

H800000

(8388608)

HFFFFFF

(16777215)

0

................

................

DR-01590

Using this delta value will result in incorrect calculations.

The RMS voltage measurement is accurate to 1% for a range

of 50% to 115% of the rated mains voltage.

Voltage registers

The three voltage registers contain the RMS voltage measured

for each phase.

Frequency register

The single frequency register contains the measured mains

frequency information for a valid phase. Internal logic ensures

that the frequency information is generated from the same

phase being used for the F50 output. Only bits D0 to D9 are

used for the frequency calculation however the remaining bits

must still be clocked out as additional information can be

derived from these data bits.

REGISTER DESCRIPTION

Active and reactive registers

The active and reactive power is accumulated in 24 bit registers

for each phase. These registers are 24 bit up/down counters,

that increment or decrement at a rate of 320k samples per

second at rated conditions.

Active or Reactive Energy Register

23 22 21 20 19

10 987654321 0

The register values will increment for positive energy flow and

decrement for negative energy flow as indicated in figure 10.

The active and reactive registers are not reset after access, so

in order to determine the correct register value, the previous

value read must be subtracted from the current reading. The

data read from the registers represents the active or reactive

power integrated over time. The increase or decrease between

readings represent the measured energy consumption. At rated

conditions, the active and reactive registers will wrap around

every 52 seconds. The micro controller program needs to take

this condition into account when calculating the difference

between register values.

As an example lets assume that with a constant load connected,

the delta value (delta value = present register - previous register

value) is 22260. Because of the constant load, the delta value

should always be 22260 every time the register is read and the

previous value subtracted (assuming the same time period

between reads). However this will not be true when a wrap

around occurs, as the following example will demonstrate:

Voltage Register

23 22 21 20 19

10 987654321 0

Frequency Register

23 22 21 20 19

10

11

12

13

14

15

16

17

18

987654321 0

Mains Frequency

Not used

Missing phase

Phase sequence

error

Voltage zero

crossover

Bit location

Description

0to9

10 to 17

18,19,20

These bits represent a value that is used

in the frequency calculation

Not used

Missing phase. These bits indicate which

phase is missing during a lost phase

condition.

D18

1

D19

X

D20

X

Missing phase

Phase 1

X

1

X

Phase 2

X

X

1

Phase 3

21,22

The phase error status can be ascertained

from these two bits.

D21

0

D22

0

Missing phase

No phase error

0

1

Phase sequence error.

1

X

Missing phase

23

Voltage zero crossover. This bit changes

state with the rising edge of the mains

voltage.

Present register value

Previous register value

new_val - old_val =

Present register value

Previous register value

new_val - old_val =

new_val

old_val

delta_val

new_val

old_val

delta_val

16767215

16744955

22260

12259

16767215

-16754956

0x00FFD8EF

0x00FF81FB

0x000056F4

0x00002FE4

0x00FFD8EF

0x00FFA90B

Decimal

Hex

Description

Variable

The register now wraps around so after the next read

the values are as follows: