Specifications

case of H8/300H clock gearing
effects the entire chip,so reduction
will be almost proportional,in case of
H8S clock gearing effects only bus
masters (CPU, DMA, DTC) to avoid
reprogramming of e.g. timers. The
effect is shown in Figure 9.
• Some H8S derivatives also feature a
32KHz subclock, which allows to
run the CPU, the 8-bit timers and the
watchdogs at 32KHz (‘subactive’),or
just the 8-bit timers and watchdogs
(‘subsleep’) or finally the watchdogs
only (‘watch mode’). In watch mode
a periodical interrupt, which is
generated from the subclock at a rate
of once per second for example,
wakes up the CPU. The CPU then
checks system status (e. g. key
pressed?) and then activates other
modules or returns to sleep,as
8
In today’s electronics industry, low power
consumption is an ever increasing
concern for many engineers. Beyond
obvious reasons like longer battery life
for example in mobile phones,notebooks
and electronic cameras, there are many
more reasons why low power is
paramount:
• Some applications have legal
restrictions on power consumption
like gas meters (to avoid a spark
causing an explosion),electricity
metering to avoid unnecessary loading
of the mains network or fixed
telecommunication equipment that is
powered off the line
• other applications can draw much
current under normal conditions but
have to run of a battery when mains
fails, e.g. security systems and cash
registers
• Some car electronics must keep up a
minimum level of activity while a car
is parked, nevertheless the car battery
must not be flat when the car is to be
used again
• and last but not least,low power goes
hand in hand with low radiation!
Numerous measures have been taken to
allow H8/300H and H8S to meet all
these demands:
• All derivatives are available in low
voltage versions as well (except
H8/3039). Low voltage operation not
only significantly reduces power
consumption,but also allows battery
operation with a smaller number of
cells and reduces radiation drastically,
especially where external busses have to
be used.
• All derivatives can be run off a slower
clock,down to 2MHz. Power
consumption will drop almost in
Low Power
proportion. A H8S/2246 for example
will only draw appr. 4mA (typical) at
3V and at 2MHz with all the modules
operating.
• The CPU can be stopped (sleep
mode),with all the registers and
memory contents being retained.
Peripherals continue to operate and
any interrupt will wake up the CPU
instantly. Sleep mode reduces power
consumption by about one third.
• Just like the CPU can be stopped at
times of low activity, some H8/300H
and all H8S derivatives allow to stop
peripheral modules. For example,
while the ADC is not needed the
software simply clears the
corresponding bit in the module stop
register. Figure 8 shows how much
current is saved per module for a
H8S/2355 and also shows a curve for
H8S/2246 with one module after the
other being turned off. The lower
curve shows the same,if the CPU is
brought into sleep as well.
• Beyond the possibility to run
H8/H8S off a slower clock statically,
some H8/300H and all H8S offer the
ability to dynamically reduce the
clock internally (‘clock gearing’). In
DTC stop 0.3mA
TPU stop 9mA
TMR stop 2mA
DAC stop 2. 2mA
ADC stop 2mA
each SCI stop 2mA
Sample data (not guaranteed), typical, at 5V /20MHz
for a H8S/2355
All
module
Stop
Current
dissipation (mA)
15
10
5
0
Conditions:
HD6472246
typ. Sample
10Mhz/3.0V
Ta=25°C
Singlechip mode
Values based on sample
measurements, values not
guaranteed
Icc-ope.
All
module
Sleep
Mode
DTC
Stop
TPU
Stop
TMR
Stop
A/D
Stop
SCi0
Stop
SCi1
Stop
SCi2
Stop
TMR
only
Operate
Active mode
Sleep mode
Figure 8