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SA9903BSA

Single phase power/energy metering IC with SPI interface

South African Micro Electronic Systems 

SA9903B

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.

Register wrap around

Positive energy flow

Register values

H7FFFFF H800000

HFFFFFF

0 ................ (8388607) (8388608) ................ (16777215)

Negative energy flow

DR-01590

Register wrap around

Figure 7: Register increment / decrement showing the

register wrap around

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:

Description

Valiable Decimal

Hex

Present register value new_val 16767215 0x00FFD8EF

Previous register value old_val 16744955 0x00FF81FB

new_val - old_val = delta_val 22260 0x000056F4

The register now wraps around so after the next read

the values are as follows:

Present register value new_val

12260 0x00002FE4

Previous register value old_val 16767215 0x00FFD8EF

new_val - old_val = delta_val -16754955 0x00FFA90B

Computing this delta value will result in incorrect calculations.

sames

USING THE REGISTER VALUES

ACTIVE ENERGY REGISTER

The active energy measured by the SA9903B is

calculated as follows:

Energy = VRATED x IRATED x N / INTTIME / 320000

VRATED

IRATED

N

Rated mains voltage of meter

Rated mains current of meter

Difference in register values between

successive reads (delta value)

INTTIME Time difference between successive register

reads (in seconds)

REACTIVE ENERGY REGISTER

The reactive energy measured by the SA9903B is

calculated as follows:

Reactive = VRATED x IRATED x N / INTTIME / 320000

VRATED

IRATED

N

Rated mains voltage of meter

Rated mains current of meter

Difference in register values between

successive reads (delta value)

INTTIME Time difference between successive register

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 = VRATED x VREGISTER VALUE / 700

VRATED

Rated mains voltage of meter

VREGISTER VALUE

Voltage 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 = FCRYSTAL / 256 / FREGISTER VALUE

FCRYSTAL

The external crystal frequency.

FREGISTER VALUE Bits D9 to D0 of the frequency register.

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

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

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