AN2433 Freescale Semiconductor / Motorola, AN2433 Datasheet - Page 5

• Manufacturer Part Number: AN2433
• Description: 5V to 3V Design Considerations
• Manufacturer: Freescale Semiconductor / Motorola

Microcontroller Inputs
Input Levels to Port
Pins
MOTOROLA
E x t e r n a l
P in
Microcontrollers with 3V supplies require care when interfacing to the external
control circuitry. There is a wide range of 3V compatible components and ICs
in the market so replacing 5V digital control circuitry by 3V control circuitry
represents no significant complication in interfacing. The easiest way of
interfacing 3V and 5V devices happens when 3V devices have 5V tolerant
inputs and 5V devices have TTL compatible input levels. In such cases, simple
direct connections without any level translators and current limit resistors are
possible.
All Motorola MCUs contain some form of ESD protection circuitry between the
physical input pin and the active circuit. There are two basic types of input pins:
input-only and input / output pins.
All the pins have an internal diode clamp to V
a diode clamp to V
than V
normal n-channel transistor acting as a protection device whose gate terminal
is hard wired to V
V
First, let us examine in more detail the CMOS circuitry of an MCU pin capable
of operating as an input-only pin, see
IN
Freescale Semiconductor, Inc.
higher than V
For More Information On This Product,
I n t e r n a l
DD
, so the maximum voltage is not limited by V
Figure 3. Digital Input-Only Pin
N
5V to 3V Design Considerations
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DD
SS
DD
V
, i.e. only negative V
.
S S
. Input pins like IRQ are designed to take higher voltages
P r o t e c t i o n
T h ic k
F ie ld
Figure 3
IN
is clamped to V
SS
.
, but some of the pins include
DD
. IRQ pins have a
SS
P
N
B u f f e r
– 0.6V and not
V
In p u t
Microcontroller Inputs
V
D D
S S
AN2433/D
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