A beginner style question.
I'm attempting to perform some logic level conversions on 3.3V signals to 5V using the 74HC4050D powered at 3.3V and 74HC541 powered at 5V. Is it OK to directly connect the outputs of the 74HC4050 to the inputs of the 74HC541 ? or do I need pull up/Pull down resistors from the output of the 74HC4050D to the input of the 74HC541 for each pin?
What should I be looking for in terms of the datasheets when determining these things (in my case I don't really aim to source or sink current to other devices.I just aim to do the voltage conversion. Is it only leakage current that would be output in this instance?
If one didn't add a pull up/down resistors or a series resistor would the only risk be that of an unkown logic state if VCC were to change abruptly or signal noise was present?
PS I'm aware of that the maximum current into these devices Icc is about 50ma and a max output of a particular pin Io is around 4-5ma.
Best Answer
Whether a 3.3 V signal can reliably drive a 5 V logic input depends on the minimum guaranteed logic high level of the 5 V logic input. Whether that is sufficiently below 3.3 V for reliable operation varies with logic families and manufacturers, so consulting the datasheet is essential. For example, Microchip PICs with Schmitt trigger inputs usually require 80% Vdd for guaranteed logic high. That would be 4.0 V, which is too high to be driven from 3.3 V logic.
A easy hack is to use logic with "T" in its name, like 74HCT as apposed to 74HC. The T stands for TTL, and means the logic input levels are compatible with the old TTL logic. This had a finite guaranteed logic high threshold sufficiently below 3.3 V so that driving one of these from 3.3 V logic is fine, although its a good idea to check the specs in any one instant. So if you need a logic gate in there anyway, make it a HCT type and you get the 3.3 V to 5 V conversion for free.