April 2015
28
S
MART
M
ETERING
Many of the other peripherals implemented on
this device have also been implemented with
the exact requirements of the power meter in
mind. e RX21A includes many peripherals
which have been speci cally shaped by the
special requirements of a power meter.
For instance today many devices are required
to operate in low power mode, but still allow
for low power reading. e MCU is designed
to work in conjunction with an external low
power LCD driver, and so has multiple I²C
and SPI interfaces to make interfacing to such
LCD drivers as easy as possible. e RX21A is
also capable of operating in a number of very
low power modes, both from an external 32
kHz oscillator or from the integrated 125 kHz
on-chip oscillator. e device also supports
deep standby, where the RTC and a few sim-
ple peripherals still operate, while the device
is only consuming 1µA. Another enhanced
feature is the on-chip real-time clock calen-
dar (RTC) , this has been designed to be easily
used in a power meter environment, allow-
ing for easy calibration in production with
programmable outputs, and an on-chip cali-
bration function to allow for the adjustment
of the 32 kHz clock used for the RTC, and
of course also includes leap year support. In
addition the on-chip temperature sensor can
be used to allow even greater calibration for
the temperature deviation of the external 32
kHz clock. e real-time clock also supports
a binary count mode, which can be extremely
useful to allow the support of all the RTC
functions while using no Gregorian calendars.
e RTC also supports a programmable alarm
function which allows the device to wake up
at a programmable time in the future, as well
as a system of regular wake-up interrupts.
Finally the RTC supports a tamper system,
where a change of state of an external pin can
be time-stamped inside the RTC to detect
when an external event, such as the opening
of the meter case occurred.
Communications is also becoming a more
and more important feature of many power
meters, whether for automatic meter reading,
for remote calibration, for so ware updates
or even for use as a home gateway. e MCU
supports up to nine serial interfaces (SCI) ,
ve of these serial interfaces are capable of a
variety of synchronous or asynchronous com-
munications functions including high-speed
USART, I2C and SPI as well as two providing
dedicated multi-master I2C interfaces and
two more providing dedicated high-speed SPI
interfaces, as well as an on-chip IrDA inter-
face. is huge number of serial interfaces
allows the complete integration of all the
metering functions into one device, a typical
example of the total solution using the RX21A
is shown in gure 3.
System reliability is also a key feature, and
with this in mind the MCU integrates a num-
ber of features to improve reliability, including
both high- and low-speed on-chip oscilla-
tors, a comprehensive low voltage detection
subsystem (brown out detection), power on
reset circuit, a tamper detection function,
and a watchdog timer with its own dedicated
on-chip oscillator. Memory reliability check-
ing is also supported with a CRC function to
support the checking of the program memory
and a built-in function to support the check-
ing of the on-chip SRAM reliability.
Most importantly the RX21A also integrates
a memory protection unit, which allows the
user to control access by the CU to various
parts of the memory, allowing the partition of
the application into various secure and inse-
cure portions, which cannot interfere with
each other without generating a system excep-
tion. is provides a solution to many of the
more di cult design decisions forced upon
the designer by some current legislation.
Renesas has developed a power meter refer-
ence platform based on the RX21A, which has
been designed to help users evaluate the per-
formance of the MCU. e reference design
shows a typical use of the RX21A in a 3-phase
meter capable of supporting single point cal-
ibration, while achieving accuracies of better
than Class 0.2S active. e meter so ware
provides for measurement of the voltage,
current, active and reactive power and the
harmonic power on each phase.
Temperature compensation is provided for
both the on-chip gain ampli ers, the on-chip
voltage reference and for the on-chip RTC.
e reference design supports a current range
from 100mA to 100A with a 10mA starting
current, with a meter accuracy equivalent to
IEC 62053-22 (and ANSI C12.20), to at least
0.2s active and compliant to IEC 62053-23,
class 2 reactive.
Figure 3. Typical system con guration
Figure 2. RX21A ADC system design using DTC
