5-V supply, then the 5-V circuitry in the system may
start driving the processor’s inputs above the maxi-
mum levels (VCC + 2.6 V). The system design
should ensure that the 5-V supply does not exceed
2.6 V above the 3.3-V supply during a power-on se-
quence.
LOW-VOLTAGE OPERATION
The low-voltage operation of the Am186ER and
Am188ER microcontrollers is an enabling technology
for the design of portable systems with long battery life.
This capability, combined with CPU clock management,
enables design of very low-power computing systems.
n Preferably, all inputs will be driven by sources that
can be three-stated during a system reset condition.
The system reset condition should persist until sta-
ble VCC conditions are met. This should help ensure
that the maximum input levels are not exceeded
during power-up conditions.
Low-Voltage Standard
Industry standards for low-voltage operation are
emerging to facilitate the design of components that
will make up a complete low-voltage system. As a
guideline, the Am186ER and Am188ER microcontrol-
ler specifications follow the first article or regulated ver-
sion of the JEDEC 8.0 low-voltage proposal. This
standard proposal calls for a VCC range of 3.3 V ± 10%.
n Preferably, all pullup resistors will be tied to the
3.3-V supply, which will ensure that inputs requiring
pullups are not over stressed during power-up.
Power Savings
CMOS dynamic power consumption is proportional to
the square of the operating voltage multiplied by capac-
itance and operating frequency. Static CPU operation
can reduce power consumption by enabling the system
designer to reduce operating frequency when possible.
However, operating voltage is always the dominant fac-
tor in power consumption. By reducing the operating
voltage from 5 V to 3.3 V for any device, the power
consumed is reduced by 56%.
Reduction of CPU and core logic operating voltage dra-
matically reduces overall system power consumption.
Additional power savings can be realized as low-voltage
mass storage and peripheral devices become available.
Two basic strategies exist in designing systems con-
taining the Am186ER and Am188ER microcontrollers.
The first strategy is to design a homogenous system in
which all logic components operate at 3.3 V. This pro-
vides the lowest overall power consumption. However,
system designers may need to include devices for
which 3.3-V versions are not available. In the second
strategy, the system designer must then design a
mixed 5-V/3.3-V system. This compromise enables the
system designer to minimize the core logic power con-
sumption while still including functionality of the 5-V
features. The choice of a mixed voltage system design
also involves balancing design complexity with the
need for the additional features.
Input/Output Circuitry
To accommodate current 5-V systems, the Am186ER
and Am188ER microcontrollers have 5-V tolerant I/O
drivers. The drivers produce TTL-compatible drive out-
put (minimum 2.4-V logic High) and receive TTL and
CMOS levels (up to VCC + 2.6 V). The following are
some design issues that should be considered when
upgrading an Am186ER microcontroller 5-V design:
n During power-up, if the 3.3-V supply has a signifi-
cant delay in achieving stable operation relative to
Am186TMER and Am188TMER Microcontrollers Data Sheet
59
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