27
April 2015
S
MART
M
ETERING
exible as possible. Many of the solutions for
electrical power measurement today rely on
either hardwired DSP engine functionality or
something similar to produce the nal result,
meaning that it’s much harder to quickly fol-
low new market requirements, such as the
requirement to measure factors such as power
quality.
A key design criterion of this device was to
allow the power measurement to be under-
taken as easily as possible, but to make the
nal result as exible as possible, allowing
the user to use the data as they please. To
achieve this requirement, a clever combina-
tion of hardware features was developed to
automate the basic process of acquiring the
data and presenting the data in the SRAM of
the microcontroller. e 24-bit analog-to-dig-
ital converter (ADC) module on the RX21A
has been especially designed for power meter
applications, and is able to support a variety
of sensor technologies including, shunt and
current transformer as well as Rogowski coil
sensors.
e 24-bit ADC uses a second order sig-
ma-delta converter technology, with built-in
programmable gain ampli ers. Four out of
the seven channels use di erential inputs with
gains of up to 64 and the remaining three
channels use single-ended inputs with gains
of up to four. e ADC system also has an
internal band gap reference circuit, (BGR)
which is capable of operating with an accu-
racy of better than 30 ppm/°C. ere is also an
option to provide the voltage reference exter-
nally to the device.
e RX21A is available in two versions, the
standard version and the enhanced G ver-
sion, the G version includes so ware calibra-
tion factors stored in the memory during the
manufacturing process. is allows the user
to develop meters with accuracies of better
than Class 0.2. ree di erent versions of the
RX21A are available, supporting single-phase
meters, with three or four 24-bit ADC mod-
ules, and a version supporting 3-phase meters
with 7 separate ADC modules. Seven separate
ADC modules allow the user complete exi-
bility in designing their meter and maximises
the sample rate on each channel.
Each channel sampling time can be inde-
pendently controlled by an on-chip 16-bit
timer ( e multi-function timer pulse unit
(MTU2), a 6-channel 16-bit timer module)
either free running or in synchronisation
with the zero crossing of the mains supply.
is technique allows the phase di erence
caused by various sensor technologies to be
removed in hardware, and this can be pro-
grammable if required depending on the sen-
sor type used. Each of the seven channels can
be controlled in synchronisation or individu-
ally, giving the designer maximum exibility
in their design, and minimises the need for
additional so ware.
e RX21A ADC system is also capable of
automating the process of moving this data
from the ADC result registers into bu ers
held in SRAM. e device has an on-chip data
transfer controller (DTC) which like a simple
DMA controller can be used to transfer auto-
matically data between the ADCs (or any other
peripheral) and SRAM without CPU inter-
vention. e on-chip data transfer controller
(DTC) can easily automate this process, allow-
ing the user to build up bu ers representing a
half or whole cycle of the mains, which once
collected, can then be presented to the CPU
for processing. e DTC is also much quicker
at transferring the data, an example of this is
shown in gure 2 where it can be seen that the
transfer of four 32-bit values from the 24-bit
sigma-delta ADC is almost 4 times faster with
the DTC. is leaves the CPU available for
other house keeping tasks while the data is
being accumulated.
Once the metering data for a complete cycle
or half-cycle is available in the SRAM, the
CPU can be interrupted from it’s other tasks
and can then make the necessary calcula-
tions. is is made easier with the high-speed
on-chip multiplier and DSP functions which
allow the device to rapidly make the necessary
calculations, for active and reactive power as
well as other parameters. e great exibility
of this solution is that the raw data is always
available to the so ware, so later versions
can add new features and measurements as
required.
An additional 7 channels of 10-bit ADC are
also available on the device for a wide variety
of uses if required. e 10-bit ADC can also
be controlled by the on-chip timers if required.
Figure 1. Rx21A block diagram
