RS232 Interface Circuits for 3.3V Systems – Design Note 75 Gary Maulding
The rapid, widespread use of 3.3V logic circuits complicates the selection of RS232 interface circuits. The
optimum choice of an interface circuit should be based
upon several application dependent factors:
1) Logic circuitry connected to interface chip port mouse. The LTВ®1342, LT1330, and LT1331 are all
good RS232 transceiver choices for systems with both
5V and 3V power. Typical performance waveforms for
the LT1342 operating with VCC = 5V and VL = 3.3V are
shown in Figure 2. 2) Power supply voltages available
RECEIVER
OUTPUT 3) Power consumption constraints
4) Serial interface environment
5) Mouse driving requirements
As Figure 1 illustrates, 5V interface circuits cannot be
used to directly connect to 3.3V CMOS logic circuits.
The receiver output level will forward bias the logic
circuit’s input protection diode, causing large current
flow. In the worst case the CMOS logic circuit may
latch up. Resistor voltage dividers or level shift buffers may be used to prevent forward biasing the CMOS
input diode, but an RS232 transceiver designed for 3V
logic application prevents this problem without extra
components or power dissipation.
Many of today’s systems have both 5V and 3V power …