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Hi, I am working on a safety critical system project for railways and I am using AT89C52 micro controller.
I use a number of buffers (74ls373), decoders (74ls138) , multiplexers(74ls157) etc. During self test of the I need to check the presence of each of the components mentioned above. If any of the component is removed from the system or it has failed to function then the AT89c52 need to detect the failed or removed component and the whole system need to go to fail safe mode.
Can anyone help me with this issue. Raghu
"Loopback" is a technique where data sent out from an output is "looped" back to an input - so that the processor can verify that the complete path is working.
As well as implementing the loopback path, You also need to design the test signals carefully to ensure that they really do confirm that everything is working and, preferably, can also precisely identify the nature of any fault...
Loopback is very good to have, and to make use of. Alas, it often isn't enough. Many communication chips that have loopback support only have internal loopback, because the receive and transmit signals are on separate external pins. All logic in the chip is tested, but no actual hw pins are used.
External loopback requires that the interface is using the same signal line for both receive or transmit, or that the communications chip can listen to it's own physical transmit signals. Unless the chip does this by use of two bond wires, the signal input will not notice if the bond wire for the output pin is broken.
If external loopback is implemented using external hw, then it is hard to know if the external hw is working ok in non-loopback mode.