Compatibility between logic families

The table shows six common logic types and which versions can be interconnected without conflict.

		Input type being driven
		3.3V CMOS	3.3V but 5V tolerant	5V HCT, NMOS	5V LS	5V HC	5V 4000 CMOS
O u t p u t t y p e
	3.3V CMOS	YES	YES	YES	YES	Out of specification	Out of specification
	3.3V but 5V tolerant	YES	YES	YES	YES	Open drain with resistor	Open drain with resistor
	5V HCT	Limiter needed	YES	YES	YES	YES	YES
	5V NMOS	Limiter needed	YES	YES	YES	NEED PULL-UP RESISTOR	NEED PULL-UP RESISTOR
	5V LS	Your mileage may vary	YES	YES	YES	NEED PULL-UP RESISTOR	NEED PULL-UP RESISTOR
	5V HC	Limiter needed	YES	YES	YES	YES	YES
	5V 4000 CMOS	Limiter needed	YES	YES	Too weak	YES	YES

Logic types

3.3V CMOS

Many microcontrollers are 3.3V CMOS devices, for example the LPC microcontrollers used in the MBED development tool.

HCMOS parts can be powered from 3.3V for compatibility with 3.3V microcontrollers. Generally a 5V signal should not be connected to 3.3V logic.

3.3V but 5V tolerant

Some 3.3V microcontrollers feature pins that can accept 5V signals safely. There are limitations, typically the pin must be an input or an open-drain output requiring an external pull-up resistor.

Some specific HCMOS parts can accept 5V inputs when powered from 3.3V

5V HCT

A variant of HCMOS with lowered input threshold for compatibility with LSTTL and NMOS.

5V NMOS

NMOS devices such as early 1980s microprocessors and EPROMs. NMOS was largely superseded by CMOS.

5V LS (LSTTL)

The dominant logic family prior to HCMOS, common on "legacy" hardware. An enhanced version of TTL.

5V HC (CMOS)

HCMOS powered from 5V

5V 4000 CMOS

4000 series CMOS when powered from a 5V supply. This doesn't have as much output drive as HCMOS.

Conditions

Out of specification

This combination will typically work but does not meet published requirements for reliable operation.

Operating at low speed a 3.3V powered device will output almost exactly 3.3V and a 5V powered device will have an input threshold of 2.5V. At low speed this will work. At high speeds the device output may not reach 3.3V within the given transition time as the given timings are based on reaching 90% or 3.0V. The 5V device input may need to reach 70% or 3.5V to meet the worst case input threshold.

In other words it will work on the test bench but do not count on it working in production.

Open drain with resistor

Some 3.3v devices have open-drain outputs that can be pulled up to 5V by an external resistor, allowing for direct connection to 5V circuits

Limiter needed

This combination will overdrive the input unless some means is found to restrict it. At low speeds a series resistor or voltage divider may suffice. At higher speeds a pass-transistor circuit could be used to limit the voltage applied to the 3.3V device

Your mileage may vary

In principle the LSTTL gate should not be able to drive the CMOS input above 3.3V but I do not believe this behaviour is guaranteed. VOH (output high) has a typical value of 3.4V but doesn't have an upper limit. In practice the LSTTL output has to pass through two diode junctions setting a practical upper limit of 3.8V. This is higher than 3.3V but the difference is less than 0.6V so it should be insufficient to cause the CMOS protection diode to conduct.

NEED PULL-UP RESISTOR

The output is not guaranteed to reach the levels of the input being driven unless pulled up to 5V.

Too Weak

The output of a 4000-series CMOS gate isn't strong enough to reliably drive a LSTTL input. Exceptions are the 4049 and 4050 buffers which have a stronger output than other devices. HCMOS has a higher drive capability than 4000 series.