Hello all,

I am trying to us an Atmel T89C51Rd2 (DIP) chip in a project but I cannot seem to get the chip to do even the simplest task.

[IMG]http://www.skynet.ie/~mikeh/SCHEMATIC.jpg [/IMG]

I have built the above circuit (wirewrapped) and I have a 0.1uF decoupling cap connected to the VCC pin of the 8051. Using the following code sometimes the LED stays on, other times it goes off, but it never keeps flashing.
Code:

PORT_PIN_LED EQU P3.4

CSEG

ORG 0000H
LJMP main

ORG 0048H

main:
CPL PORT_PIN_LED
CALL delay
JMP main

delay:
MOV R7, #200
dly1: MOV R6, #229
dly2: DJNZ R6, dly2
DJNZ R7, dly1
RET

END

(This code works perfectly using an Analog ADUC812 development board but unfortunally I cannot use this in the project.) Using a scope I have checked to ensure the crystal is oscillating, and it is, and the hardware reset is operating without any problem also.

I have also built the same circuit, using the same components, and the same power supply, on a development board and it works 90% of the time. I have tried using the circuit without the inverter present, and connecting the led to every other port pin on the chip (of course changing the code appropriately) and it makes no difference. (I have also tried changing every component and have tried 4 different T89C51RD2 chips but to no avail).

I have read all the tutorials and FAQs on this site but haven't found anything that might account for this behaviour.
The only thing I have found in the datasheet is the following but I'm not fully sure what it means to me:

The reason I think this might be important is because the only difference between the wirewrapped board and the development board is the capacitance of the boards and the capacitance of the wires used.

Any help/suggestions would be greatly appreciated because I'm well confused.

Thanks,
Ray