Hi Kai,

Thank you for the very useful ideas!

1. Check whether the oscillator is running at all!
2. Try it with a 10MHz quartz and slightly bigger burden caps.

It's nice to know that bigger caps can help. Anyway, to ensure I don't have an oscillator problem, I decided to use an external oscillator for now. It's running ok.

3. Remove the cap and pull-down resistor of reset circuit. Add a 4k7 pull-up to RST pin and let the ISP-cable control the reset line.

Ok, I've done that.

4. Do you use the updated ISP software, which knows the AT89S8253?

Yes, I'm using it.

5. ISP software wants you to input clock frequency of micro. Input the lowest frequency possible (I think 3MHz, if I remember correctly).

Ok, I've done that. This will slower the communication and help avoid noise.

6. Before connecting your micro to the cable, first initialize the ISP software. It will quit this with an error message. Then connect the target and initialize again. (At first attempt reset line isn't high and can cause malfunction. After the first initialization, even if it failed, the reset line is high and target will see a valid reset signal.)

This is very interesting. I'll pay attention to this from now on.

7. 74VHC244 isn't TTL compatible, but most PCs and laptops emit TTL-signals! This can cause malfunction, especially in a noisy environment. You can try this circuit as remedy:

Ok. In fact I was using 74VHC244 because someone posted in this forum some time ago that this would be the best option, and I didn't notice it wasn't TTL compatible. Thanks for the new schematics. Nevertheless, it seems I'm lucky my JTAG is working with my computer (it's programming another board with an AT89S8252 very well). But now I know it may not be reliable with other computers.

Now, I'm getting to this conclusion:

Oscillator is ok. Voltage levels are ok in the essencial pins. For luck my JTAG is working, and the signals are very beautiful on the scope (my LPT port is set to EPP mode). I've checked the MISO, MOSI and SCK lines, and the software is sending the instrucions to the IC, but it never responds (MISO is high all the time). The reset line is being controlled correctly, and it stays high during all the programming process.

So, as everything seems to be ok, and the micro doesn't responds, I might have a damaged IC. I'll replace it as soon as I can and give you a reply.

Thank you very much for the very good ideas and for spending your time trying to help me!!

My best regards,
Leonardo.