Hi,
OK, let's see if I can make this understandable:
1) The HC908QT/QY MCUs has got a small ROM monitor inside. It is only good for programming new code into the Flash EPROM.
2) To make any kind of program development possible, a special piece of code is burned into the Flash EPROM area. It sits at the interrupt vector area and is erase/write protected.
3) This code makes it very nice to do program development, but as it sits on the interrupt vectors, you can't use them. Well no, you can use them by filling in a jump vector table at the end of your user code.
This applies also to the reset vector.
4) If you buy a HC908QT/QY device, this piece of "Monitor" code has to be loaded first.
If you buy a dev. board which has a HC908QT/QY on it, I would assume that this is already burned into the Flash EPROM.
To read more about it, get the AN2305 from
5) The nice thing with this program, is that no extra pins are wasted. You don't have to worry about +9 V to the /IRQ pin and that kind of stuff. You only have to use the PTA0 pin for communication.
Regards,
Ake