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power on reset on At89S52

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Member level 1
May 18, 2004
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Hello there!

Does anyone know the values(exactly) for the P.O.R. circuit
on that At89S52?
I have programmed it (at least that it what the flash is telling me),
but I don't get to start my circuit! The logic levels on my ports are
the same as if I weren't using a P.O.R. circuit!
Feel free to reply here or at !
Thank you for your time!

Actually my tests showed that EA had to be tied to VCC, not ALE!
What do you say about this?
Or both need VCC?





In addition to the POR circuit (R=10K from Reset to GND and C=10UF from Rese tto VCC) as Sphinx mentioned you need to make sure the pin 31 (ALE) of the micro is connected to VCC as long as you are not using external memory.



it is enough to the reset time (R=10K, C = 10U).
If don't get the start, u ought check your design.

Actually, It will not start executing code while ALE was floating. I had to tie it to VCC to see the 89S52 come to life.



I also say that make ALE hi. But the best way for POR is use a schmit triger NAND like 4093.
Use only one gate. tie the inputs togather of any one gate. connect the 100k r from input to +Vand a cap of 1U from input of gate to GND. it will work best.
I have tried it many times.And found it the best

Hi rellutzu,

The best answer for your question comes straight from the source:

You should be aware of the bidirectional status of the 89S52's reset pin : input and output, not only input as usual.

From data sheet:
A high on this pin for two machine cycles while the oscillator is running
resets the device. This pin drives high for 98 oscillator periods after the Watchdog times
out. The DISRTO bit in SFR AUXR (address 8EH) can be used to disable this feature. In
the default state of bit DISRTO, the RESET HIGH out feature is enabled.

Concerning to EA and ALE status, if EA is tied to VCC, program fetches to addresses 0000H
through 1FFFH are directed to internal memory and fetches to addresses 2000H
through FFFFH are to external memory. The ALE is input only during programing. You can reduce EMI using the 89S52' sfeature of disabling the ALE pulses during internal program fetches or let it flow. But I don't see any reasons for tied to VCC and solving your problem doing so.


you could also use a diode in teh circuitry designed in the POR that helps to remove the charge on the capacitor,

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