Asok Sankar Rudra wrote:
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1)Why use AT89S8252,why not AT89C52 since for reasons of availability you decided use external RAMs so the FLASH is not going to be used anyway?
I am using AT89S52

2)I ground External Enable /EA external memory access
i.e Address 0000 starts from 0000 of device getting Y0
from '138.In this mode P0 time-multiplexes lower address and data.This lower addresed is latched out and fed as input to external RAMs,which get data input straight from P0.Thus for external memory access P0 and P2 (generating A8 to A15) are required.
I am using only external RAM

3)That does not mean P0 and P2 pins cannot be used for
any other intermittent control jobs like your buzzer and relay.
good point...

4)You are still left with P1 and P3 free.
Ports P1 and P3 have already been used up.

5)The 16x2 LCD here can be I/O mapped not memory mapped
like RAMs.Although you have not mentioned your crystal
frequency,it will not be dependable to memory map
character LCDs which are slow.I would have used 4 bit
mode of LCD where only 6 pins of P1 would suffice with
R/W grounded.So 2 pins of P1 is still free for other
jobs.P3 is free.
Initially the LCD was put on P1(only 6 pins), but later due to increase in user needs, the above mentioned design was finalised.

Rgds
Raj Shetgar