Recipe

Connect a 3.3 V part and a 5 V part

Use when: two chips that run at different supply voltages share one or more signal wires

Goal: every signal read reliably in both states, with no input pushed past its absolute maximum

Taught in 3.3 V vs 5 V and level shifting

1. 1

   Note which chip drives each wire, and whether any wire is driven from both ends.

   Example

   Uno R4 TX → ESP32-S3 RX (down); ESP32-S3 TX → Uno R4 RX (up); a shared I²C data line (both).

   Why: down and up need different fixes, and a both-ways wire needs a third.

2. 2

   For each direction compare the driver's highest HIGH with the input's maximum, then V_{OH} with V_{IH} and V_{OL} with V_{IL}.

   • over the maximum — shift down.
   • a negative HIGH margin — shift up.
   • both margins positive and under the maximum — connect directly.

   Why: the maximum protects the chip; the margins make the reading reliable.

3. 3

   Down only: a divider for slow signals, a 5 V-tolerant buffer for fast ones. Up: a buffer powered from 5 V, or a MOSFET shifter. Both ways: a MOSFET shifter or an auto-direction chip.

   Why: a divider can only lower a voltage, and pull-up shifters are slower than driven ones.

4. 4

   Recompute the HIGH, the LOW and the worst case at the receiving pin.

   Why: a shifter that lands below the HIGH threshold is a new fault, not a fix.