Logic Levels
Overview
Logic levels define the voltage ranges that represent binary 0s and 1s in digital circuits. These standardized voltage ranges ensure reliable operation and communication between different digital components. The specific voltage values depend on the technology being used (TTL, CMOS, etc.), but the principle remains the same: a lower voltage range represents logic 0, and a higher voltage range represents logic 1, with a forbidden zone in between to prevent ambiguity.
Detailed Explanation
Standard Logic Levels
Voltage
^
5V |------------ MAX
| LOG 1
3.3V|------------ VIH min
|
2V |............. Undefined
|
0.8V|------------ VIL max
| LOG 0
0V |------------ MIN
+------------->Voltage Ranges by Technology
| Technology | VIL (max) | VIH (min) | VOL (max) | VOH (min) |
|---|---|---|---|---|
| TTL | 0.8V | 2.0V | 0.4V | 2.4V |
| CMOS (5V) | 1.5V | 3.5V | 0.1V | 4.9V |
| CMOS (3.3V) | 0.8V | 2.0V | 0.4V | 2.4V |
Important Terms
-
Noise Margin
- LOW Noise Margin = VIL - VOL
- HIGH Noise Margin = VOH - VIH
- Provides safety against signal interference
-
Fan-out
- Number of inputs one output can drive
- Limited by current capability
- Affects signal integrity
Signal Integrity
Good Signal: Poor Signal:
________ _/\___
| | _/ \_
__| |__ / \__Practice Problems
-
Calculate noise margins for:
- TTL gates
- 5V CMOS gates
-
What happens when:
- Input voltage is 1.5V in TTL?
- Output drives too many inputs?
References
- Digital Electronics by Morris Mano
- TTL Cookbook by Don Lancaster
- Logic Levels Tutorial