April 2015
26
S
MART
M
ETERING
An innovative approach to
a single-chip power meter design
By Graeme Clark,
Renesas
This article describes
the RX21A MCU which
has been designed from the bottom
up to meet the requirements for the
next generation of solid state power
meters, and also provides an solution
for many other applications.
e market for solid state electricity meters
has to be one of the most dynamic markets
at the moment, with a number of di er-
ent factors driving systems and architecture
changes, all around the globe. ese rapidly
changing market conditions, caused mainly
by the demand for smart and green energy as
well as by the current global recession, make
the job of the designer of the now ubiquitous
solid state electricity meter ever more di cult.
ese demands, when combined with the con-
stant requirement for new and innovative fea-
tures in all our products constantly add to the
pressure and complexity of designing any new
product.
Today electricity meters have to be designed
to be ever more accurate and take less power,
while adding more and more features, includ-
ing the ability to not only meter power, but to
log data and to communicate via many dif-
ferent media, both wired and wireless. Any
solution also has to provide the exibility to
allow the developer to react quickly to the
changing market conditions. ese require-
ments automatically lead to more and more
demands to be made on the central microcon-
troller that can be found at the heart of any
solid state power meter. e microcontroller
today requires more on-chip memory than
ever before, along with higher processing per-
formance, more and more advanced periph-
erals as well as supreme low power operation,
quite a di cult combination to achieve. e
new 32-bit RX21A microcontroller has been
designed to provide a true single-chip elec-
tricity meter solution, while meeting the per-
formance and cost requirements of the next
generation of electricity meters. As a semicon-
ductor company, Renesas has looked at the
many con icting demands placed upon the
next generation of microcontrollers that will
be at the heart of the next generation of solid
state electricity meters and asked how we can
best provide a solution to these requirements.
e RX21A uses the popular RX 32-bit CPU
core which includes an on-chip DSP function
with a 48-bit multiply-accumulate register,
and can operate at up to 50 MHz while still
o ering low power consumption. e RX21A
is designed on a low power 130µm CMOS
process, providing multiple high resolution
analogue front ends, while o ering signi -
cantly more additional functionality than
existing devices at a signi cantly lower cost.
is functionality includes much larger
on-chip memory, with up to 512 kBytes of
on-chip ash and 64 kBytes of on-chip SRAM
to allow for the much more complex so -
ware that will be required in future. e use
of Renesas latest single cycle access on-chip
F-MONOs ash technology allows single-cy-
cle memory access at speeds up to 50 MHz as
well as the ability to support dynamic repro-
gramming of the application or con gura-
tion data. 8 kBytes on on-chip E2DataFlash
is also supported which allows background
operation, where the E2DataFlash can be pro-
grammed while code is executing from the
on-chip program ash memory.
e wealth of analog functions inside the
RX21A is one of the key advantages of this
device, a powerful high resolution ADC sub-
system, which we will cover later in this arti-
cle, which has 7 separate 24-bit ADC modules,
is supported by a 7-channel 10-bit ADC and
2- channel 10-bit DACs. e RX21A also adds
a huge range of additional peripheral func-
tionality including up to 9 serial interfaces
(SCI), a number of powerful timers, including
a powerful real time clock calendar system, as
well as a wealth of safety and reliability fea-
tures. A block diagram of the RX21A can be
found in gure 1.
e key function of a power meter is of course
to accurately measure the voltage and current,
and hence calculate the power in real time.
During the development process of the RX21A
and based on feedback from previous Rene-
sas power meter products, it quickly became
very clear that there was strong requirement
for making the power measurement system as
