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Digital IC signal levels and Gray code

Short answer

Digital IC signal levels define voltage ranges for logic 0 and 1; TTL example: LOW input ≤0.8 V, HIGH input ≥2.0 V. Gray code: successive codes differ in one bit; binary 1001 → Gray 1101, reducing errors in transitions.

Define digital IC signal levels. What is Gray Code? Explain with example.

Digital IC signal levels

Digital IC signal levels are the voltage ranges that an IC family accepts or produces as logic 0 (LOW) and logic 1 (HIGH). Any voltage in the forbidden band between them is not a valid logic level. For standard TTL (5 V supply):

ParameterMeaningTTL value
VIL(max)V_{IL(max)}Highest input read as 00.8 V
VIH(min)V_{IH(min)}Lowest input read as 12.0 V
VOL(max)V_{OL(max)}Highest output for 00.4 V
VOH(min)V_{OH(min)}Lowest output for 12.4 V
 5.0 V +--------------+
       |  logic 1     |  input HIGH: 2.0 to 5 V
 2.0 V +--------------+
       |  forbidden   |
 0.8 V +--------------+
       |  logic 0     |  input LOW: 0 to 0.8 V
 0   V +--------------+

Noise margin = VOH(min)−VIH(min)=2.4−2.0=0.4V_{OH(min)} - V_{IH(min)} = 2.4 - 2.0 = 0.4 V (same for LOW: 0.8−0.4=0.40.8 - 0.4 = 0.4 V). For 5 V CMOS, typical levels are VIL=1.5V_{IL} = 1.5 V and VIH=3.5V_{IH} = 3.5 V.

Gray code

Gray code is an unweighted, reflected binary code in which successive numbers differ in only one bit.

DecimalBinaryGray
0000000
1001001
2010011
3011010
4100110
5101111
6110101
7111100

Binary to Gray: keep MSB, then Gi=Bi+1⊕BiG_i = B_{i+1} \oplus B_i. Example: (1001)2(1001)_2 → 1, 1⊕0 = 1, 0⊕0 = 0, 0⊕1 = 1 → (1101)Gray(1101)_{Gray}. It is used in shaft encoders and K-maps because only one bit changes per step.

✓ Answer checked by Claude Opus 5.5 (high effort)

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