--- title: "Digital IC signal levels and Gray code" subject: "Digital Logic" chapter: 1 key: "semester_3/digital_logic:ic-signal-levels-gray-code" source: https://www.bctengineer.com/q/semester_3/digital_logic/ic-signal-levels-gray-code site: BCT Engineering --- # Digital IC signal levels and Gray code Digital Logic · Chapter 1: Introduction - Asked in: 2069 Chaitra, 3+3 marks - Video: [Lec-9: Binary to Gray Code Conversion & Vice-versa](https://www.youtube.com/watch?v=R7uuIACpdGQ) — Gate Smashers - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/digital_logic/ic-signal-levels-gray-code - Whole chapter: https://www.bctengineer.com/md/semester_3/digital_logic/chapter-1 ## Question Define digital IC signal levels. What is Gray Code? Explain with example. ## 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. ## Answer #### 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): | Parameter | Meaning | TTL value | |---|---|---| | $V_{IL(max)}$ | Highest input read as 0 | 0.8 V | | $V_{IH(min)}$ | Lowest input read as 1 | 2.0 V | | $V_{OL(max)}$ | Highest output for 0 | 0.4 V | | $V_{OH(min)}$ | Lowest output for 1 | 2.4 V | ```text 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 = $V_{OH(min)} - V_{IH(min)} = 2.4 - 2.0 = 0.4$ V (same for LOW: $0.8 - 0.4 = 0.4$ V). For 5 V CMOS, typical levels are $V_{IL} = 1.5$ V and $V_{IH} = 3.5$ V. #### Gray code **Gray code** is an unweighted, reflected binary code in which successive numbers differ in only **one bit**. | Decimal | Binary | Gray | |---|---|---| | 0 | 000 | 000 | | 1 | 001 | 001 | | 2 | 010 | 011 | | 3 | 011 | 010 | | 4 | 100 | 110 | | 5 | 101 | 111 | | 6 | 110 | 101 | | 7 | 111 | 100 | Binary to Gray: keep MSB, then $G_i = B_{i+1} \oplus B_i$. Example: $(1001)_2$ → 1, 1⊕0 = 1, 0⊕0 = 0, 0⊕1 = 1 → $(1101)_{Gray}$. It is used in shaft encoders and K-maps because only one bit changes per step. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What are digital IC signal levels? A. The clock frequency B. Voltage ranges treated as logic 0 and logic 1 by an IC family C. The supply current D. The number of gates Answer: B. The band between them is forbidden. (Key point: IC signal levels) 2. For standard TTL, what are the input thresholds? A. VIL(max) = 2.0 V and VIH(min) = 0.8 V B. VIL(max) = 0.4 V and VIH(min) = 2.4 V C. VIL(max) = 0.8 V and VIH(min) = 2.0 V D. VIL(max) = 1.5 V and VIH(min) = 3.5 V Answer: C. The second pair is the TTL output pair; the last is CMOS. (Key point: TTL input levels) 3. What is the TTL high-level noise margin? A. 2.0 V B. 0.8 V C. 1.6 V D. 0.4 V (2.4 - 2.0) Answer: D. VOH(min) - VIH(min) = 2.4 - 2.0. (Key point: Noise margin) 4. What is (1001) binary in Gray? A. 1101 B. 1001 C. 1011 D. 0110 Answer: A. 1; 1^0=1; 0^0=0; 0^1=1. (Key point: Gray conversion) ## Close variants set in other exams - [Digital signal and Gray code](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Adigital-signal-gray-code) — Digital Logic, chapter 1, asked 2× ## Related in Digital Logic - [Digital/analog signals, Gray and Excess-3 codes](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Adigital-analog-gray-excess3) — Digital Logic, chapter 1 - [TTL input and output signal levels](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Attl-signal-levels) — Digital Logic, chapter 10, asked 2× - [Gray code with examples](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Agray-code-explain) — Digital Logic, chapter 1, asked 3× - [3-bit Gray code synchronous counter](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Agray-code-sync-counter-3bit) — Digital Logic, chapter 8, asked 2× - [BCD code and 3-bit binary-to-Gray converter](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Abcd-code-gray-converter) — Digital Logic, chapter 1 - [Gray code and 3-bit binary-to-Gray converter](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Agray-code-binary-gray-converter) — Digital Logic, chapter 1 ## Related in other subjects - [PAM, PWM, PPM; PCM of 4.8 MHz TV signal with 512 levels](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Apam-pwm-ppm-tv-4-8mhz) — Communication Systems, chapter 4 - [WBS with example, levels and code](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Awbs-example-levels-code) — Project Engineering, chapter 3, asked 4× - [PCM bit rate for 4.5 MHz TV signal with 1024 levels](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Apcm-tv-4-5mhz-1024) — Communication Systems, chapter 4 - [E1 hierarchy; PCM of 4.2 MHz TV signal with 512 levels](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ae1-pcm-tv-4-2mhz-512) — Communication Systems, chapter 4