The Features of 8051 Microcontroller family are as follows.
1. Single-supply +5 volt operation using HMOS technology.
2. 4096 bytes program memory on-chip (not on 8031).
3. 128 bytes data memory on-chip.
4. Four register banks.
5. 128 User-defined software flags.
6. 64 Kilobytes each program and external RAM addressability.
7. One microsecond instruction cycle with 12 MHz crystal.
8. 32 bidirectional I/O lines organized as four 8-bit ports (16 lines on 8031).
9. Multiple mode, high-speed programmable serial port.
10. Two multiple mode, 16-bit Timers/Counters.
11. Two-level prioritized interrupt structure.
12. Full depth stack for subroutine return linkage and data storage.
13. Direct Byte and Bit addressability.
14. Binary or Decimal arithmetic.
15. Signed-overflow detection and parity computation.
16. Hardware Multiple and Divide in 4 µ sec.
17. Integrated Boolean Processor for control applications.
18. Upwardly compatible with existing 8084 software.
All three devices 8031, 8051 and 8751 come in a standard 40-pin Dual In-Line Package with the same pin-out, the same timing and the same electrical characteristics. The primary difference between the three is the on-chip program memory-different types are offered to satisfy different user requirements
The 8751 provides 4 Kbytes of Ultraviolet-Erasable, Programmable Read Only Memory (EPROM) for program development, prototyping. As we know, in development and prototying stages due to change in design specification or software bugs, it is necessary to change or Erase the software. With EPROM it is a matter of few minutes, by exposure to ultraviolet light and reprogrammed with the modified code.
The Features of 8051 Microcontroller has 4 Kbytes of ROM which is programmed by manufacturer when the chip is built. This part is considerably less expensive, but cannot be erased or altered after fabrication. This is used to store final version of the program.
The 8031 does not have any program memory on chip, but may be used with upto 64 Kbytes of external standard or multiplexed ROMs, PROMs or EPROMs. The 8031 fit well in applications requiring significantly larger or smaller amounts of memory that the 4 Kbytes provided by 8051 and 8751.
NOTE : The 8051 and 8751 automatically access external program memory for all addresses greater than 4096 bytes on chip.