Standard Logic Levels
Ensure hardware compatibility between different logic families. This reference table defines the voltage thresholds for High (1) and Low (0) signals across common 5V TTL, 3.3V CMOS, and 1.8V systems.
| Logic Family | Supply (Vcc) | Low (Max) | High (Min) |
|---|---|---|---|
| 5V TTL | 5.0V | 0.8V | 2.0V |
| 5V CMOS | 5.0V | 1.5V | 3.5V |
| 3.3V CMOS | 3.3V | 0.8V | 2.0V |
| 2.5V CMOS | 2.5V | 0.7V | 1.7V |
| 1.8V CMOS | 1.8V | 0.6V | 1.2V |
Input vs. Output: The table shows the minimum “High” voltage a pin must receive to be recognized as a logic 1.
Compatibility Tip: A 3.3V CMOS output can usually drive a 5V TTL input because 3.3V is higher than the 2.0V threshold required by TTL. However, a 5V output into a 3.3V pin usually requires a Level Shifter or a Voltage Divider to prevent overvoltage damage.