Have been using USB Pocket AVR Programmer and Logic Level Converter - Bi-Directional to run avrdude on Windows. Target is circuit with ATMega328P operating at 3.3v. This setup seems to work fine (for flashing, fuse bits). However, I noticed when I turned off bench power to the 328P, one of its connected LEDs stayed lit until I powered down the USB. I thought LV was an input only as a reference level. Obviously it cannot produce enough current to run a 328P. I wonder if there’s any risk of LV and bench power fighting each other. Should I be adding in a serial diode such that power flows only from the target to LV? The Hookup Guide says nothing about any of this.
If both supplies are the same voltage and within tolerance (both ~3.3 V), the risk is low but not zero. You can still get currents circulating through protection diodes, MOSFETs, or pull-ups which can cause unexpected behaviour, inaccurate voltage sensing, or stress on components.
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The probability of major damage is low if both are regulated 3.3 V supplies within tolerance. However, the possibility of subtle issues or long-term stress is still there. Especially during power-up, power-down, or transient events.
I wasn’t too worried but the hookup guide should have said something about it. Your response implies that adding a protection diode is unnecessary. Thanks.