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
0
H7FFFFF
................ (8388607)
H800000
(8388608)
HFFFFFF
................ (16777215)
sames
USING THE REGISTER VALUES
ACTIVE ENERGY REGISTER
The active energy measured by the SA9903B is
calculated as follows:
Energy = V
RATED
x I
RATED
x N / INT
TIME
/ 320000
V
RATED
Rated mains voltage of meter
I
RATED
Rated mains current of meter
N
Difference in register values between
successive reads (delta value)
INT
TIME
Time difference between successive register
reads (in seconds)
REACTIVE ENERGY REGISTER
The reactive energy measured by the SA9903B is
calculated as follows:
Reactive = V
RATED
x I
RATED
x N / INT
TIME
/ 320000
V
RATED
Rated mains voltage of meter
I
RATED
Rated mains current of meter
N
Difference in register values between
successive reads (delta value)
INT
TIME
Time difference between successive register
reads (in seconds)
Negative energy flow
Register wrap around
DR-01590
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
Present register value
Valiable Decimal
Hex
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
RATED
x V
REGISTER VALUE
/ 700
V
RATED
Rated mains voltage of meter
V
REGISTER 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 = F
CRYSTAL
/ 256 / F
REGISTER VALUE
F
CRYSTAL
The external crystal frequency.
F
REGISTER 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.
new_val 16767215
0x00FFD8EF
16744955
0x00FF81FB
22260
0x000056F4
Previous register value old_val
new_val - old_val =
delta_val
The register now wraps around so after the next read
the values are as follows:
Present register value
new_val
12260
0x00002FE4
16767215
0x00FFD8EF
Previous register value old_val
new_val - old_val =
delta_val -16754955
0x00FFA90B
Computing this delta value will result in incorrect calculations.
7/12
SAMES [ SAMES ]
