P89C51RD2HBA STOP GOING INTO ISP MODE

[RIZ074]

Hi...

I've been programming some P89C51RD2HBA with Flash Magic 3.33 without problems. However have noticed that after a while, I can't get them to go into ISP mode anymore. When PSEN is set low & P89C51RD2HBA is RESET, it would just continue to execute the previously programmed code. This does not happen after 1 try, but is after a number of successful ISP programming.

Have verified the following :
P2.6 is HIGH on RESET
P2.7 is HIGH on RESET
EA is kept HIGH
PSEN is LOW
ALE is floating (not connected) - but goes HIGH on RESET.

Is there a way to overcome this problem ?

[Andy Ayre]

Have you verified the pin voltages at the actual pins? If not you should do that.

Note that devices always enter ISP mode the first time they are used because they are supplied from the factory with the status byte set.

Andy