Q7Microprocessor

The 8051 microcontroller is packaged in a 40-pin DIP (Dual In-line Package), with pins including: Port 0 (P0.0-P0.7, pins 32-39, a dual-purpose port serving as a general I/O port or, when using external memory, as the multiplexed low-order address/data bus); Port 1 (P1.0-P1.7, pins 1-8, a dedicated general-purpose I/O port); Port 2 (P2.0-P2.7, pins 21-28, serving as general I/O or as the high-order address bus when accessing external memory); Port 3 (P3.0-P3.7, pins 10-17, a dual-purpose port whose pins double as special alternate functions including INT0, INT1, T0, T1, RxD, TxD, RD and WR); RST (pin 9, active-high reset input); XTAL1 and XTAL2 (pins 19, 18, for connecting the external crystal oscillator); Vcc (pin 40, +5V supply) and Vss (pin 20, ground); PSEN (Program Store Enable, pin 29, active-low output strobe for reading external program memory/ROM); ALE (Address Latch Enable, pin 30, used to latch the low-order address byte from the multiplexed Port 0 during external memory access); and EA (External Access, pin 31, which when tied low forces the 8051 to fetch all program code exclusively from external memory, and when tied high allows it to use internal ROM for lower addresses).