Technical tips and hints

To level shift down from a 5V system you can use a simple open drain buffer as a solution. The 74LVC06A is an open drain inverting buffer, and the 74LVC07A is and open drain non-inverting buffer. These devices are 3.3V counterparts to the popular 74F06 and 74F07 5V parts.

The LVC inputs are 5V tolerant and can connect to 5V devices. Since the outputs are open drain, they will shift the output VOH level to the chosen pull-up voltage (e.g., 3.3V or 2.5V). At V_CC=3.3V, the V_OL level is still guaranteed to be 0.55V maximum. JEDEC standards specify V_IH and V_IL levels of 2.0V and 0.8V for 3.3V systems and V_IH and V_IL levels of 1.7V and 0.7V for 2.5V systems. The LVC06A and LVC07A can be used to level shift down from a 5V system to a 3V or 2.5V system and still meet the input threshold voltage requirements.

To level shift up from a 3.3V system to drive 5V CMOS input levels, simply connect the LVC output to a 5V termination voltage through a pullup resistor as shown below. The outputs of these devices are 5V tolerant and provide a simple solution to drive 5V CMOS input levels.