--- title: "8051 pin diagram; ports 1 and 2" subject: "Embedded System" chapter: 9 key: "semester_6/embedded_system:8051-pin-diagram-ports-1-2" source: https://www.bctengineer.com/q/semester_6/embedded_system/8051-pin-diagram-ports-1-2 site: BCT Engineering --- # 8051 pin diagram; ports 1 and 2 Embedded System · Chapter 9: Microcontrollers in Embedded Systems - Asked in: 2072 Asoj, 3 marks - Video: [8051 pin diagram | micro controller | | MPMC | Lec-55 | Bhanu Priya](https://www.youtube.com/watch?v=5fESTph5gA8) — Education 4u - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/embedded_system/8051-pin-diagram-ports-1-2 - Whole chapter: https://www.bctengineer.com/md/semester_6/embedded_system/chapter-9 ## Question Draw the pin diagram of 8051 microcontroller and explain ports 1 and 2 only. ## Short answer Port 1 is an 8-bit bidirectional I/O port with internal pull-ups, always available for general I/O. Port 2 is also 8-bit bidirectional with pull-ups; it functions as I/O when no external memory is used, but outputs high address byte A8–A15 if external memory is attached. ## Answer #### Pin diagram (40-pin DIP) ```text +-----\_/-----+ P1.0 1 | | 40 VCC P1.1 2 | | 39 P0.0 AD0 P1.2 3 | | 38 P0.1 AD1 P1.3 4 | | 37 P0.2 AD2 P1.4 5 | | 36 P0.3 AD3 P1.5 6 | | 35 P0.4 AD4 P1.6 7 | 8051 | 34 P0.5 AD5 P1.7 8 | | 33 P0.6 AD6 RST 9 | | 32 P0.7 AD7 P3.0 RXD 10 | | 31 EA/VPP P3.1 TXD 11 | | 30 ALE/PROG P3.2 INT0 12 | | 29 PSEN P3.3 INT1 13 | | 28 P2.7 A15 P3.4 T0 14 | | 27 P2.6 A14 P3.5 T1 15 | | 26 P2.5 A13 P3.6 WR 16 | | 25 P2.4 A12 P3.7 RD 17 | | 24 P2.3 A11 XTAL2 18 | | 23 P2.2 A10 XTAL1 19 | | 22 P2.1 A9 GND 20 | | 21 P2.0 A8 +-------------+ ``` #### Port 1 (pins 1-8) An 8-bit bidirectional I/O port with internal pull-ups. It has no alternate function in the basic 8051 (P1.0 and P1.1 are T2 and T2EX in the 8052), so it is always available for general I/O such as LEDs and switches. Writing 1 to a pin makes it an input. #### Port 2 (pins 21-28) An 8-bit bidirectional I/O port with internal pull-ups. It is used as plain I/O when no external memory is attached; with external memory it outputs the high-order address byte A8-A15 and then cannot be used for I/O. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which pins of the 40-pin 8051 form Port 1, and what is it used for? A. Pins 21-28, address lines B. Pins 10-17, special functions C. Pins 1-8, general-purpose I/O D. Pins 32-39, multiplexed address/data Answer: C. Port 1 is pins 1-8 and has no alternate function in the basic 8051, so it is always free for I/O. (Key point: Port 1 pins) 2. What does Port 2 do when external memory is attached? A. Outputs the low-order address byte A0-A7 B. Outputs the high-order address byte A8-A15 and cannot be used for I/O C. Carries the data bus D0-D7 D. Works as serial port Answer: B. With external memory Port 2 supplies A8-A15, so it is no longer available for ordinary I/O. (Key point: Port 2 dual role) 3. Which pins supply power to the 40-pin 8051? A. VCC at pin 40 and GND at pin 20 B. VCC at pin 20 and GND at pin 40 C. VCC at pin 1 and GND at pin 21 D. VCC at pin 31 and GND at pin 9 Answer: A. VCC is pin 40 and GND is pin 20. (Key point: Power pins) 4. How is a Port 1 pin made an input? A. By writing 0 to it B. By clearing the PSW C. By writing 1 to it D. By pulling EA low Answer: C. Port pins have internal pull-ups; writing 1 lets an external device drive the pin as an input. (Key point: Input configuration) ## Related in Embedded System - [Microcontroller vs microprocessor; 8051 features](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Amicro-vs-microprocessor-8051-features) — Embedded System, chapter 9, asked 2× - [Microprocessor vs microcontroller; 8051 addressing](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3A8051-addressing-modes-micro-vs-processor) — Embedded System, chapter 9 - [8051 minimum circuit and 75% duty pulse](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3A8051-min-config-pwm-75) — Embedded System, chapter 9 - [Addressing modes of 8051](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3A8051-addressing-modes) — Embedded System, chapter 9, asked 4× - [Why 8051 / its operational features](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Awhy-8051-features) — Embedded System, chapter 9, asked 2× - [Purpose of port 3 and port 2 of 8051](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3A8051-port3-port2-purpose) — Embedded System, chapter 9, asked 2× ## Related in other subjects - [Equivalent Y-parameter of parallel two-ports](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_circuit_theory%3Aequivalent-y-parameter-parallel-two-ports) — Electrical Circuit Theory, chapter 7, asked 3× - [Equivalent Z-parameter of three series two-ports](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_circuit_theory%3Aequivalent-z-parameter-three-series-two) — Electrical Circuit Theory, chapter 7, asked 3×