Explain function of following pins of 8051 (i) Pin 31 (ii) Pin 29 (iii) Pin 21-28

Explain function of following pins of 8051 (i) Pin 31 (ii) Pin 29 (iii) Pin 21-28 
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Answer :

i) Pin 31-EA : It is and active low I/P to 8051 microcontroller. When (EA)= 0, then 8051 microcontroller access from external program memory (ROM) only. When (EA) = 1, then it access internal and external program memories (ROMS). 

ii) Pin 29- PSEN : This is an output pin. PSEN stands for “program store enable.” It is active low O/P signal. It is used to enable external program memory (ROM). When [PSEN(bar)]= 0, then external program memory becomes enabled and micro controller read content of external memory location. Therefore it is connected to (OE) of external ROM. 

iii) Pin 21-28: A8 – A15 : These pins are known as Port 2. It serves as I/O port. Each pin is bidirectional Input /Output with internal pull – up resistors. Besides the Input /Output, when external memory is interfaced, PORT 2 pins act as the higher-order address bus. (A8-A15)
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