Hello,

Here I am again :) looking seriously for anyone's experience to find a logical answer to solve my dilemma.

First of all, let us forget my prototype and put the problem as a question:

In the very simple circuit below (pin connection of AT89S51), does the output of P0.0-P0.7 represents the first byte (at address 0000) of the 89S51 internal code, or just 0FFH (all 1's) according to the data sheet of Atmel?

Pin connections of AT89S51 in 'read' mode at address 0000 (the first code byte):

Pin 1 = P1.0 -> GND
Pin 2 = P1.1 -> GND
Pin 3 = P1.2 -> GND
Pin 4 = P1.3 -> GND
Pin 5 = P1.4 -> GND
Pin 6 = P1.5 -> GND
Pin 7 = P1.6 -> GND
Pin 8 = P1.7 -> GND
Pin 9 = RST -> +5V
Pin 10 = P3.0 -> Not connected
Pin 11 = P3.1 -> Not connected
Pin 12 = P3.2 -> Not connected
Pin 13 = P3.3 -> GND
Pin 14 = P3.4 -> Not connected
Pin 15 = P3.5 -> Not connected
Pin 16 = P3.6 -> Not connected
Pin 17 = P3.7 -> Not connected
Pin 18 = XTL2 -> Xtal 8MHz and 27 pF (to GND)
Pin 19 = XTL1 -> Xtal 8MHz and 27 pF (to GND)
Pin 20 = GND -> GND
Pin 21 = P2.0 -> GND
Pin 22 = P2.1 -> GND
Pin 23 = P2.2 -> GND
Pin 24 = P2.3 -> GND
Pin 25 = P2.4 -> Not connected
Pin 26 = P2.5 -> Not connected
Pin 27 = P2.6 -> GND
Pin 28 = P2.7 -> GND
Pin 29 = PSEN -> GND
Pin 30 = ALE/PG -> +5V
Pin 31 = EA/VP -> +5V
Pin 32 = P0.7 -> 10K (to +5V)
Pin 33 = P0.6 -> 10K (to +5V)
Pin 34 = P0.5 -> 10K (to +5V)
Pin 35 = P0.4 -> 10K (to +5V)
Pin 36 = P0.3 -> 10K (to +5V)
Pin 37 = P0.2 -> 10K (to +5V)
Pin 38 = P0.1 -> 10K (to +5V)
Pin 39 = P0.0 -> 10K (to +5V)
Pin 40 = Vcc -> +5V


In my experiment, port P0 bits of AT89S51 were all 1's even for all possible addresses (P2.3-P2.0, P1.7-P1.0). This is not the case if I read AT89C51 instead!
By the way, I borrowed today an updated programmer (Hi-Lo Sysytems, ALL-11) to check that the tested 89S51 is indeed programmed properly and also not locked.

Please, may anyone here show me what I did wrong in reading the internal code of an unlocked AT89S51? There is no problem in reading AT89C51's code!

I got its datasheet from:
http://www.atmel.com/dyn/gener...oc_id=7189

Your help will be very much appreciated.

Kerim