Hi Forum Members,
I am facing a problem related to ISP (In-System Programming).
The same setup works good for P89C51RD2BA, but it does not work for P89C61X2BA.
Both CPUs are PLCC package, and are pin-to-pin compatible to each other.


My Setup:
P2.7, P2.6 are used as A15,A14 respectively and going to 64kB RAM. They are having
resistor pullups of 4k7 each.
I have used a Capacitor Reset circuit: C(10uF to Vcc). This holds reset pin for about 1 second.
I have brought PSEN on Jumper which can be connected to GND.
ALE is given to the Address/Data demultiplexer latch.

Method used for ISP:
1. Put the ISP Jumper. (PSEN is pulled to GND).
2. Power ON the circuit.
3. Start Flash magic.

The Problem:
The above method works OK for P89C51RD2BA, and I have delivered many products using the same scheme.
Due to unavailabilty and cost of P89C51RD2BA, I have to use P89C61X2BA. All other issues regarding compatiblity between P89C51RD2 and P89C61X2 has been taken care of; except this peculiar problem.

Tests performed:
When I place the P89C51RD2BA in this board it works OK.
When I place P89C61X2BA, it does not go into ISP mode. But by keeping The supply ON,
if I give a reset by shorting the Capacitor in the reset circuit, The P89C61X2BA goes
into ISP. Then it works like a saint, and completes the whole ISP process.
When I short the reset capacitor at power ON even for 5 seconds or more and then
release it, then ,it still does not go into ISP mode. But if I short the capacitor for a short
period of time (keeping power ON), then it goes into ISP mode.
I have tried by giving reset by using RC through Invertor (74LS04) to the CPU, The
P89C51RD2BA goes into ISP, but the P89C61X2BA does not go into ISP. But keeping the supply ON, again if I short the Reset capacitor the P89C61X2BA goes into ISP.

I have not brought the Reset pin of the CPU on a connector, and my application demands that I have to Put the device in ISP using Power ON ONLY. The application demands that I cannot put a PowerOn reset switch in the product.

Preferably I do not want to do any PCB changes, but if there is no alternative, then
I will be forced to change the PCB (and the reset circuit); which will be my Last Option.

Please give your valuable suggestions.


Thanking you,

With best regards,
from Mr. Kiran V. Sutar.