Using 74HC04N with Fez Panda II Safe?

I just wanted to make sure I would be able to use a 74HC04H Hex Inverter IC with my Panda II.

I would use the 5 volt output from the Fez, using a 200 Ohm resistor to give 25mA of current to the 74HC04H Vin pin and attach the ground to the Fez ground.

Would it be okay to connect an output from the inverter directly to an input pin on the Fez?

welcome to forum.

The USBIzi chip pins are 5V compatible. you can connect 5v TTL signals directly to the Panda.

The only limitation is an analog input signal may not be more than 3.3v.

Thanks for the welcome and your reply.

Sorry, I’m being a little dumb here…

Is the resistor necessary to limit the supply current? Or could I attach the Logic IC direct to the +5V? I’m assuming the IC would only draw what it needs? Also the hex inverter datasheet says the output pins might supply as much as 25mA - will this be okay?

Documentation for the Fez Panda II says it can tolerate 5 volts on the input pins, I’m only wanting to use them in a digital fashion, but naturally I’m dealing with analogue signals.

you can connect the TTL chip directly to the panda 5v pin. a resistor is not required.

there will be no current issues with the output of the TTL chip when connected to panda.

wire it up and have fun

Thanks for the help Mike.

I wanted to use an inverter because I have a microswitch which is normally closed, and rather than keeping a fez pin high all the time, I’d rather just know when the switch is open. I thought it would be a good opportunity to use a not gate.

Do you think this a sensible approach?

I would suggest an alternative which does not require an inverter.

Create an InterruptPort with a pull up resistor and glitch filter.
Connect the NC switch with one end to the pin and the other end to ground.
Register for a rising edge interrupt with the InterruptPort, which will occur when the NC button is depressed.
You could register for a falling edge interrupt if you are interested in knowing when the button has been pressed and then released.

Read Home - GHI Electronics

I was reading a little about this before, and assumed I’d have to put the resistor in myself, but now I’m thinking that’s not required. The resistor pulling the signal ‘up’ is actually on the fez board?

The pull up/down resistors are inside the processor chip. The only exception is the SPI pins, which have resistors on the board.

Fantastic, that makes the project much easier. Thanks for your advice.