SA9603B
ssaammeess
Synchronised Reading of Registers
The voltage sense zero crossing (approx. 50% duty cycle
pulse) is available on Fm0, (Pin 15). This information allows a
supervisor (ie: micro-controller) to monitor when the next read
operation should be performed. Another approach could be to
monitor the upper bits of the frequency register (18, 19). A
measurement cycle is completed when these bits change to
the same state again (00..00 or 01..01 or 10..10 or 11..11).
The SA9603B integrated circuit updates the registers on a
continual basis. The SA9603B register content in latched onto
the SPI interface as soon as a read command has been
detected or the next register is addressed during continual
access. The registers can be accessed at any time however
for maximum stability the time between readings must be in
multiples of 8 mains cycles. The internal offset cancellation
procedure requires 8 mains cycles to complete. The 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. This methodology holds true for Active,
Reactive and Voltage registers. The data read from the
registers represents the active power, reactive power and
voltage integrated over time. The increase of decrease
between readings is the energy consumption. The registers
are not affected during access. No error is possible during
read, because all control signals are generated on chip. The
registers can be accessed in any sequence at any time without
problems.
Register Values
The 24 bit registers are up/down counters, which increment or
decrement at a rate of 640k*2/PI for reactive) at rated
conditions. The energy register values will increment for
positive energy flow and decrement for negative energy flow
as can be seen in the following diagram:
At power-up the register values are underfined and for this
reason the msb of the delta value (delta value = present
register value - previous register value) should be regarded as
an indication of the measured energy direction. (0 = positive, 1
= negative).
register wrap around
positive energy flow
Register values
...............
0
HFFFFFF
(16777215)
H7FFFFF
(8388607)
H800000
(8388608)
...............
negative energy flow
register wrap around
Voltage
FM0
+5V
0V (Vss)
Dr-01498
Figure 5: Fm0 Output
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