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Module 2: The 8051 Architecture

20208m

Explain the functions of the following pins:
(i) ALE
(ii) PSEN‾\overline{PSEN}
(iii) EA‾\overline{EA}
(iv) RST

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Functions of ALE, \overline{PSEN}, \overline{EA}, RST

  • ALE (30): Address Latch Enable – active-high pulse at 1/6 Fosc to latch low address A0-A7 from P0 into external latch (74LS373). Demultiplexes address/data. Also used as \overline{PROG} for Flash programming.
  • \overline{PSEN} (29): Program Store Enable – active-low output, reads external program memory (ROM) when CPU fetches outside internal. Connects to OE of external EPROM/Flash. Inactive high when executing internal.
  • \overline{EA}/VPP (31): External Access – input. EA=1 → fetch internal first (0000-0FFFH) then external after; EA=0 → all fetches external, 0H vectors external. Also VPP 21V for EPROM programming.
  • RST (9): Reset – active-high input, hold >2 machine cycles (>1µs) to reset PC=0000H, SP=07H, ports high, etc. Needs RC (10µF+8.2k) power-on.

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