sames

sames

SA9903B

7/12

http://www.sames.co.za

ACTIVE AND REACTIVE REGISTER VALUES

The active and reactive registers are 24 bit up/down counters,

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

second at rated conditions. The register values will increment

for positive energy flow and decrement for negative energy

flow as indicated in figure 7. 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:

Figure 7: 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

Present register value

Previous register value

new_val - old_val =

The register now wraps around so after the next read

the values are as follows:

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

12260

16767215

-16754955

0x00FFD8EF

0x00FF81FB

0x000056F4

0x00002FE4

0x00FFD8EF

0x00FFA90B

Decimal

Hex

Description

Valiable

Computing this delta value will result in incorrect calculations.

USING THE REGISTER VALUES

ACTIVE ENERGY REGISTER

The active energy measured by the SA9903B is

calculated as follows:

Energy = V

x I

x N / INT

/ 320000

RATED

RATED

TIME

V

Rated mains voltage of meter

RATED

I

Rated mains current of meter

RATED

N

Difference in register values between

successive reads (delta value)

INT

Time difference between successive register

TIME

reads

(in seconds)

REACTIVE ENERGY REGISTER

The reactive energy measured by the SA9903B is

calculated as follows:

Reactive = V

x I

x N / INT

/ 320000

RATED

RATED

TIME

V

Rated mains voltage of meter

RATED

I

Rated mains current of meter

RATED

N

Difference in register values between

successive reads (delta value)

INT

Time difference between successive register

TIME

reads (in seconds)

MAINS VOLTAGE REGISTER

The RMS voltage measurement is accurate to 1% in a

range of 50% to 115% of rated mains voltage. The RMS

mains voltage measured by the SA9903B is calculated as

follows:

Voltage = V

x V

/ 700

RATED

REGISTER VALUE

V

Rated mains voltage of meter

RATED

V

Voltage register value

REGISTER VALUE

MAINS FREQUENCY REGISTER

Bits D0 to D9 represents a counter value that is scaleable

to the mains frequency measured.

The mains frequency measured by the SA9903B is

calculated as follows:

Frequency = F

/ 256 / F

CRYSTAL

REGISTER VALUE

F

The external crystal frequency.

CRYSTAL

F

Bits D9 to D0 of the frequency register.

REGISTER VALUE

Bits D10 to D22 are not used in the frequency register.

Bit D23 is set with the same status as the FMO output.