WE DISCUSS PORT 0,1,2,3 etc................
here we discuss the remaining pins
RST: reset inputs.A high on this pin for two machine cycle while the oscillator reset the device
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ALE/(PROG compliment ) : Address latch enable output pulse for latching the low bytes of the address during accesses to external memory.This pin is also the program pulse input(PROG compliment) during flash programming.
in normal operation ALE is emitted at a constant rate of 1/6 the oscillator frequency , and may be used for external timing or clocking purpose.Note however, that one ALE pulse is skipped during each access to external data memory
if desired , ALE operation can be disabled by setting bit "0" of SFR location 8EH . with the bit set
,ALE is active only during a MOVX or MOVC instruction.otherwise ,the pin is weakly pulled high.setting the ALE disable bit has no effect if the micro-controller in external mode.
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PSEN-->(PSEN compliment(PSEN bar)) :program store enable is the read strobe to external program, memory.
when the AT89C51 is executing code from external program memory.PSEN compliment or PSEN bar is activated twice each machine cycle , expect that two PSEN bar or PSEN compliment activation are skipped during each access to external memory.
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EA/VPP-->(read it as EA compliment or EA bar/VCC ) :External Access enable .EA bar must strapped to GND in order to enable the device to fetch code from external program memory.external memory location starting from 0000H to FFFFH.
however that if lock bit "1' is programmed ,EA bar will be internally latched on reset.
EA bar should be strapped to VCC for internal program execution.
When VCC is connected to EA bar then the device fetch code from 0000H to 0FFFH from internal ROM and remaining address 1000H to FFFFH from external program memory.
VPP : this pin also receive 12 volts programming enable voltage during flash programming , for parts that require 12 volt vpp.
XTAL 1: input to the inverting oscillator amplifier and input to the internal clock operating circuit.
XTAL 2: Output from the inverting oscillator amplifier.Labels: ATMEL 89C51
