鈩?/div>
ER microcontrollers feature 5-V tolerant signal operation with
a 3.3-V power supply. The move to 3.3 V is driven by industry requirements for higher performance
and lower cost through smaller geometry process technology. This application note discusses con-
siderations for designing 5-V systems with the 3.3-V Am186ER microcontroller. All references in
the document to the Am186ER apply equally to the Am188ER microcontroller.
BACKGROUND
The use of a 3.3-V device in a 5-V system has become
a common theme with today鈥檚 denser, faster microcon-
trollers. Using these processors in a 5-V design, with-
out redesigning the end system for 3.3 V, has
considerations.
The Am186ER microcontroller features 5-V tolerant in-
puts; the device can interface to both 3.3-V and 5-V sig-
nal inputs, even though it operates from a 3.3-V supply.
The input circuit provides protection from a 5-V signal
level, but utilizes the same TTL-compatible thresholds
that are familiar from existing 5-V technology devices.
Ostensibly, a designer need not be concerned about
the 5-V-to-3-V interface, thanks to this input circuit.
In systems without an additional 3.3-V power supply
tap, 3.3 V for the microcontroller must be synthesized
from the existing 5-V supply. This can be accomplished
in a number of different ways, discussed below.
Regulator Types
Zener Diode Regulators
The zener diode is the simplest
5V
3.3 V
and least expensive solution,
but has a slower response than
an active supply. It also re-
quires strong current biasing, and may dissipate a
good deal of power, as the difference voltage is
dropped across the diode and bias resistor.
The nature of this solution is that the current supplied
by the 5-V power supply is nearly constant; this means
that as the load current decreases the current is
shunted through the zener diode. The increased cur-
rent through the zener diode causes a corresponding
increase in the voltage across the zener diode. Be-
cause of this, when using this solution, care must be
given to insure that variations in the load do not result
in excessive variation in the regulated voltage. A com-
plete analysis of system current requirements must be
completed to properly specify the bias resistor and
zener diode. Care must be taken to include all devices
that draw current from the 3.3-V power plane. It is also
very important that the zener diode be able to handle
the maximum current shunted from the load. If the
zener diode is over driven to the point of failure, the
only protection left for the microcontroller is the series
bias resistor which will typically result in the microcon-
troller being driven with a voltage over the maximum
rated V
cc
of 3.6 V.
This solution is not recommended for use in designs
that operate at greater than 25 Mhz due to the higher
current requirements of such designs. This solution is
also inappropriate for designs that use power save
mode since this results in a very large variation in the
current drawn from 3.3-V power plane and causes ex-
cessive variation in the regulated voltage and potential
damage to the microcontroller.
SUPPLY VOLTAGE CONSIDERATIONS
In recent years there have been numerous voltage-reg-
ulation solutions introduced specifically to generate
3.3-V supply power from 5-V supplies. These are bro-
ken into three main groups: passive zener diode, active
switching and active linear regulators.
The preliminary recommendation for total provided
supply current for the Am186ER microcontroller (with-
out termination) is approximately 5 mA/Mhz, or 200 mA
at 40 Mhz and 125 mA at 25 Mhz. Regardless of the
method of power supply, the designer should be sure
that the solution provides for peak current require-
ments, not only of the processor operation, but of the
current sourcing on the I/O pins.
This document contains information on a product under development at Advanced Micro Devices. The information
is intended to help you evaluate this product. AMD reserves the right to change or discontinue work on this proposed
product without notice.
Publication#
21045
Rev:
A
Amendment/0
Issue Date:
September 1996
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