AN2433 Freescale Semiconductor / Motorola, AN2433 Datasheet - Page 9

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

5V CMOS Output to
3V MCU Inputs
MOTOROLA
V
V
DD
DD
CMOS
CMOS
Circuit
Circuit
5.0 V
5.0 V
N
N
P
P
V
V
The typical CMOS output swings fully between GND and V
millivolts) at no-load or slight-load condition. So the maximum V
CMOS output can be 5.5V for V
differential voltage for logic high in the worst-case, which is for V
with V
It is possible to use a series resistor with a larger value, but this may lead to
higher propagation delay. A better solution is to use a resistor divider network
between the CMOS output and the MCU input such that a signal voltage level
of logic high appears at the input at approximately 3.0V, see
Another level conversion from a 5V output (TTL or CMOS) to a 3.3V MCU input
circuit can employ a device from the LCX logic family powered at 3.3V, which
has 5V tolerant inputs (V
V
For example, such a circuit is suitable for connecting a JTAG interface to a PC
printer port that has a 5V level. Also, by using a MC74LCX244 logic device
instead of a simple resistor circuit, the MCU inputs are more protected against
Output
Output
SS
SS
Imax
Figure 6. 5V CMOS output to 3.3V MCU input
Freescale Semiconductor, Inc.
=7V.
For More Information On This Product,
Imax
12 k
12 k
R
R
(which is 3.5V – see previous paragraphs) is 2V.
Figure 7
5V to 3V Design Considerations
1
1
Go to: www.freescale.com
15 k
15 k
R
R
2
2
shows such a solution with the MC74LCX244 buffer.
Imax
V
V
SS
SS
=5.5V), or one from the LVX family, which has
Input
Input
DD5max
MCU
MCU
V
V
DD
DD
= 5.5V at no-load. The maximum
3.3 V
3.3 V
V
V
SS
SS
DD
Figure 6
Microcontroller Inputs
(within tens of
P
P
N
N
OH
DD5max
level of a
.
AN2433/D
and
9