Hey Microcontroller and Hardware Fans 
I have an issue with a 74HCT04D,653. If I connect 5V to VCC and GND to GND and don't connect any other of the pins, I can measure 5 V on the Input pins.
That was not the behavior I was expecting. I thought I need to supply 5V to an input pin to pull the corresponding output pin low and vice versa. But why does the input pins supply voltage? The should't supply, anything right?
Can someone clarify and help me to understand this beahvior?
Thanks in advance!
Possibly from the overvoltage protection. The data sheet shows up to 1µA input leakage current.
Why do you worry about unconnected input pins? Just in this case it's the best solution if the pins are clamped to a stable voltage to reduce power consumption.
Also the 74HC04 should be used in CMOS envirornment, The 74HCT family is designed for legacy TTL compatibility.
Well it is the behaviour that you get with 74xxXX devices.
Originally these were TTL (transistor transistor logic) devices where inputs float high and to get them low you have to sink current through them. The amount of current depended on the technology, given by the letters between the 74 and the function number.
The original TTL would just be a 7404 and would take 4mA sink current to put a logic zero on the input. Then the 74LS04 (Low Power Schottky) would take 1mA, and so on through all the 74 technology types.
Original 74 family took 1.6mA for logic LOW input and 0.4mA for logic HIGH. The L/LS families took 1/10 of the I/O currents. Fan-out was 10 so that one output could drive 10 inputs.
Thank you guys, for coming back so fast to me. I'm experimenting with those kind of chips for the first time. Coming from simple simple discrete parts like resistors, transistors, diodes and so on, but not in a single package.
My next step would have been to connect Output Pins of a 3.3V Microcontroller to the Input Pins of the 74HCT04D (5V) to use it like a level shifter in combination with the NOR behavior. But I was afraid that the Output Pins of the Microcontroller gets damaged because of the 5V of the Input Pins of the 74HCT04D. But I seem to not fully understand the concept here.
So how do I need to handle the Inputs of the 74HCT04D to get the Outputs high or low? I thought I supply a voltage to the input to get a low on the output and vice versa.
Measure the current out of the input pins. If it's the 1µA or less leakage current then it won't damage 3.3V inputs.
You cannot get no voltage on an output pin. In the worst case at about halfways between LOW and HIGH the output drivers may consume internal current as well. TTL inputs defaulted to a logic HIGH level if disconnected.
Correct.
But you need to ground the input to get a high out and then you can leave the input to get the low output, or connect the input to the 5V. In fact for best stability you should connect it high through a resistor. So if you are driving it form a 3V3 computer, just wire the output of the processor to the input of the chip. The current is so low that this is easily caught by the static protection diodes, with no damage.
I would use the 74HCT14 instead because that gives you the right sort of threshold voltages for driving the chip from your processor.
You are right. It is only a few µA.
Understood, I think now I'm back on track. I will also check the 74HCT14.
Thanks for your help
This topic was automatically closed 180 days after the last reply. New replies are no longer allowed.