1
\$\begingroup\$

I have a power-hungry 74ls digital logic project that I want to connect a USB-connected Arduino Nano to, but I'm not sure how design the interface between them safely.

The project has an external 5V power supply (a USB wall-socket power adapter,) and I'm concerned about what may happen if USB power is supplied to the Nano without the project's power supply turned on. I'm much less concerned about the Nano getting fried (which probably has all kinds of protection circuitry) than I am about my (poorly-designed) project, which is composed of ~50 chips.

The Nano needs to drive a 74ls245 (bus transceiver, including its direction and enable pins), the inputs of a 74ls173 (4-Bit D-Type Register), and an input of a 74ls04 (inverter). The Nano needs to accept input from the bus transceiver and 2 TTL-compatible CMOS EEPROM output pins.

My first thought was to connect the project's ground to the Nano's ground, but leave the Nano's 5V and VIN pins disconnected. However, the Nano will be outputting digital high to the unpowered chips above, and I'm concerned that that may damage the chips, and/or cause those chips to malfunction and damage the 50 other chips they are connected to.

If I connect the project's +5V to the Nano's VIN, I imagine that the Nano would be safe when the project is powered and the Nano's USB not plugged in.

How can I keep my project safe when the Nano's USB is plugged in but the project's power supply is off?

I have everything wired up, but am terrified of powering it up.

\$\endgroup\$
1
  • 1
    \$\begingroup\$ If your signals are not very fast, you can put some series resistors in between to limit the issue. And then ideally, give the Arduino an input to detect the external circuit's power state, and drive all its outputs low when that is off. Arduino's typically get an explicit reset on USB serial connection opening anyway. What you are building sounds like more of a "science fair project" type of setup than an engineered product (as the obsolete components chosen would make little sense for that) so you may be able to get by with a solution that is "good enough" even if not fully sound. \$\endgroup\$ Mar 17 '19 at 19:06
1
\$\begingroup\$

I would not worry about damaging the inputs by applying voltage from your Nano when the LS chips are not powered. It would be good practice to configure the Nano's outputs to inputs whenever you power down the LS circuitry to save power. To learn about the input and output circuitry of LS logic, you can read about it at http://www.ti.com/lit/an/sdya009c/sdya009c.pdf and http://ecee.colorado.edu/~mcclurel/ON_Semiconductor_LSTTL_Data_DL121-D.pdf . Good luck!

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.