SA9903B
Voltage Registers
REGISTER DESCRIPTION
The voltage register contains the RMS voltage measured by
the device. This measurement is a true RMS measurement
which is accurate to 1% for a range of 50% to 115% of the
rated mains voltage. The RMS mains voltage measured by
the SA9903B is calculated as follows:
Active and Reactive Registers
The active and reactive energy measured by the SA9903B is
accumulated in 2 distinct registers. These registers are 24 bit
up/down counters, that increment or decrement at a rate of
320000 samples per second at rated conditions (nominal
mains voltage VNOM and maximum rated mains current IMAX).
The register values will increment for positive energy flow and
decrement for negative energy flow.
푉푁푂푀 × 푉ꢉ퐸퐺
ꢉꢊꢋ ꢊ푎푖푛푠 푉표푙푡푎푔푒 =
700
where
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 represents the measured energy
consumption since the previous register access. At rated
conditions, the active and reactive registers will wrap around
every 52 seconds. The microcontroller software needs to take
this condition into account when calculating the difference
between register values. The register difference is always
computed correctly if 24 bit arithmetic is used, regardless if a
wrap-around has occurred or not. If the controller software
uses 32 bit arithmetic, the 24 bit register readings should be
sign extended to 32 bits. This ensures that the difference is
computed correctly even if a register wrap-around has
occurred.
VREG is the voltage register value.
The value of the voltage register will default to zero when the
RMS measurement produces a register value of less than 64.
This occurs at a mains voltage of just below 10% VNOM. The
settling time of the RMS measurement algorithm is in the
order of 200 mains cycles.
Frequency register
The frequency register contains the measured mains
frequency information. Only bits D0 to D9 are used for the
mains frequency calculation result. Bit D23 is a voltage zero
crossover bit. It changes state with each rising edge of the
mains voltage. Bits D22 to D10 are unused and default to
zero.
The mains frequency measured by the SA9903B is
calculated as follows:
The active and reactive energy measured per register count
can be calculated by applying the following formula:
퐹퐶푅푌푆ꢄ퐴퐿 × 퐹ꢉ퐸퐺
ꢊ푎푖푛푠 퐹푟푒푞푢푒푛푐푦 =
ꢃ56
푉푁푂푀 × 퐼푀퐴푋
퐸푛푒푟푔푦 푝푒푟 푐표푢푛푡 =
3ꢃ0000
where
FREG is frequency register value in bits D9 to D0 and
FCRYSTAL is the frequency of the external crystal.
where
VNOM is the nominal rated mains voltage of meter and
IMAX is the maximum rated mains current of meter.
The result is watt seconds or var seconds.
OSCILLATOR
The SA9903B contains a crystal oscillator driver circuit
requiring only an external crystal to be connected between
OSC1 and OSC2. All other components are integrated on the
device. The recommended crystal is a TV colour burst crystal
(3.5795MHz).
The active and reactive power measured by the SA9903B is
calculated as follows:
푉푁푂푀 × 퐼푀퐴푋 × ꢈ
푃표푤푒푟 =
3ꢃ0000 × 푇
ꢂ푁ꢄ
where
N is the difference in register values between successive
register reads and
TINT is the time difference between successive register
reads.
SPEC-0051 (REV. 5)
29-09-2017
12/17
SAMES [ SAMES ]
