external interface of the Standard 8051

 Small 8051
Low-cost members of the 8051 family with reduced number of port pins, and no support for off-chip memory. Typical application: Low-cost consumer goods.
 Standard 8051: The Small 8051s and the Extended 8051s are derived
 Extended 8051:Members of the 8051 family with extended range of no-chip facilities (e.g. CAN controllers, ADC, DAC,etc), large numbers of port pins, and - in recent devices - support for large amounts of off-chip memory. Typical applications: Industrial and automotive systems.
Clock frequency and performance
All digital computer systems are driven by some form of oscillator circuit: the 8051 is certainly
no exception. If the oscillator fails, the system will not function at all; if the oscillator runs
irregularly, any timing calculations performed by the system will be inaccurate.

Memory Issues
A. Types of memory
 Dynamic RAM (DRAM)
 Static RAM (SRAM)
 Mask Read-Only Memory (ROM)
 Programmable Read-Only Memory (PROM)
 UV Erasable Programmable Read-Only Memory (UV EPROM)
 EEPROM and Flash ROM
Dynamic RAM Vs Static RAM
Dynamic RAM
Static RAM
Dynamic RAM is a read-write memory technology that uses a small capacitor to store information.
Static RAM is a read-write memory technology that uses a form of electronic flip-flop to store the information
It must be frequently refreshed to maintain the required information
No refreshing is required
Less complex and least cost
More complex and costs can be several times that of the corresponding size of DRAM
Access time is large compared to SRAM
Access times may be one-third those of DRAM
 Mask Read-Only Memory (ROM):
Mask ROM is – from the software developer’s perspective – read only; A ‘mask’ is provided by the company for which the chips are being produced. Such devices are therefore sometimes referred to as ‘factory programmed”.
 Programmable Read-Only Memory (PROM)
PROM is a form of Write-Once, Read-Many (WORM) or ‘One-Time Programmable’ (OTP) memory. Basically, we use a PROM programmer to blow tiny ‘fuses’ in the device.
 UV Erasable Programmable Read-Only Memory (UV EPROM) :
UV EPROMs are programmed electrically. Unlike PROMs, they also have a quartz window which allows the memory to be erased by exposing the internals of the device to UV light.
 EEPROM and Flash ROM
Electrically-Erasable Programmable Read-Only Memory (EEPROMs) and ‘Flash’ ROMs are a more user-friendly form of ROM that can be both programmed and erased electrically. EEPROM and Flash ROM are very similar. EEPROMs can usually be reprogrammed on a byte-by-byte basis, and are often used to store passwords or other ‘persistent’ user data. Flash ROMs generally require a block-sized ‘erase’ operation before they can be programmed.