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electronica Nov 2014
M
ICROCONTROLLERS
The CPU architecture of the RX family is
extremely compute-efficient, achieving the
highest possible number of instructions
per mW. Interrupt latency is only 5 cycles
and processing performance is rated at 1.54
DMIPS/MHz and 3.08 Coremarks/MHz. The
large number of parallel buses in its architec-
ture makes possible simultaneous movements
of data between the CPU core, flash, SRAM
and peripherals. This design feature ensures
that no bottlenecks are present when the
CPU wakes up from a power-down mode. By
maintaining direct control of all the elements
of MCU development and manufacture, the
semiconductor technology experts of the
company enable the production of high-qual-
ity, optimised system design solutions that
customers can apply to implement ultra-low-
power products and systems.
As already mentioned, the CPU has three pow-
er-controlled run modes: high speed, middle
speed and low speed. Each of these modes
makes available a different set of on-chip
peripheral modules. However, some restric-
tions apply. The availability of some oscillators,
the PLL, flash memory programming and cer-
tain peripheral clock frequencies depends on
the run mode selected. By contrast, the MCU
supply voltage requirements aren’t affected by
the power-controller run modes. Operation
is always allowed over the full 1.8V to 3.6V
range of the device. However, the clock fre-
quencies usable in the high, middle, and low
speed modes do depend on the supply voltage
(table 1).
Besides the three power-controlled operating
modes, RX100MCUs also offer thementioned
low-power operating modes: sleep, deep sleep
and software standby. In each of them, differ-
ent MCU functions are stopped and/or pow-
ered down, saving various amounts of current.
Here are the details: in sleep mode the CPU is
stopped with data retained. This reduces the
CPU dynamic current consumption, which is
a significant contributor to the overall operat-
ing current of the MCU. The CPU wakes up
from sleep mode into the run mode in only
0.21μs at 32MHz. In deep sleep mode the
CPU, RAM and flash memory are stopped
with data retained. At 32MHz with multiple
peripherals active, the typical operating cur-
rent is only 4.6mA. It takes just 2.24μs for the
CPU to wake up from deep sleep mode and
enter run mode. In software standby mode
the PLL and all the oscillators, except the
sub-clock and IWDT, are stopped. Almost all
of the RX100 modules – CPU, SRAM, flash,
DTC and peripheral blocks – are stopped,
with data retained. The power-on-reset (POR)
circuit remains operational though, and if
necessary, the iWDT, RTC, and LVD modules
can be operated. Current consumption in this
mode is from 350nA to 790nA, depending on
whether or not the LVD and RTC functions
are used. When waking up in the 4MHz run
mode, CPU operation begins after a 4.8μs
delay. When waking up in the fast 32MHz run
mode, the wait time extends to 40μs.
Although the sleep, deep sleep and software
standby modes of RX100 MCUs are very
helpful for decreasing the current the chips
consume, system engineers can use other
techniques to achieve further power reduc-
tions. For instance, they can set various
clock-signal frequency-division ratios indi-
vidually. This capability applies to the system
Table 1. Operating modes
Figure 2. Flow meter example
