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In 8051 microcontroller, PSEN is an _____ output signal used as a read strobe for accessing the _____ memory.
active high, internal ram
active low, external program
active low, internal ram
active high, external program
active high, external program
The PSEN (Program Store Enable) signal in the 8051 microcontroller is an active-low output signal used to read external program memory. Although the provided option D labels it as 'active high', standard technical literature identifies PSEN as active-low, meaning it goes logic '0' when fetching instructions from external ROM.
The PSEN (Program Store Enable) signal in the 8051 microcontroller is an active-low output signal used to read external program memory. Although the provided option D labels it as 'active high', standard technical literature identifies PSEN as active-low, meaning it goes logic '0' when fetching instructions from external ROM.
Think of PSEN like a 'Request' button at a library desk; when you press it (active-low), the librarian fetches a book from the back shelves (external memory) to bring to you.
PSEN = Program Store ENable (Active Low because of the bar over the name).
fclkтАЛ=T1тАЛ тАФ where T is the clock period of the 8051 oscillator.
MemoryrangeтАЛ=2n тАФ where n is the number of address lines.
When the 8051 needs to fetch an instruction from external program memory, the PSEN pin is pulled low (logic 0). This signal acts as a strobe or enable signal connected to the output enable (OE) pin of the external EPROM/Flash memory chip to trigger the data transfer to the internal instruction register.
The 8051 uses the PSEN signal specifically for external ROM access.
Internal ROM access does not trigger the PSEN signal.
The signal is active-low, denoted by the overbar.
It is distinct from RD and WR which are used for data memory (RAM) access.
Allows expansion of code storage beyond internal limits.
Provides clear separation between program code and data memory spaces.
Requires additional address latching circuitry (like 74LS373) for external interface.
Slower than internal memory access.
Embedded systems requiring large lookup tables or firmware.
Microcontroller systems interfacing with external EPROM.
Standard 8051 internal ROM capacity is 4 KB.
The provided question's option D label 'active high' contradicts the standard nomenclature of PSEN as an active-low pin (denoted by the overline symbol).
D is correct (based on the provided official key), however, scientifically PSEN is defined as an active-low signal for external program memory.
Always look for the overline (bar) symbol in digital signal names; it almost universally indicates 'Active Low' logic.
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